Understanding HF Skywave Propagation

Practical Guide

Global propagation:
17-10m band conditions.

Now and 48 hours trend

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Regional conditions:
QSOs on 11 HF Bands
during the last 15 minutes.

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Tracking digital modes
using FT8—PSKReporter.

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Online MUF map

HF conditions at a glance

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Global HF conditions

Banners and widgets

Online propagation charts

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Propagation indices now

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Global HF fadeout

Current Kp
Current Solar Flare

Recent R-S-G reports

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Forecast

Forecast Proton Flux

Forecast Solar Flare

Geomagnetic Forecast
Band Conditions Forecast

Prediction

Predict solar flux 🗗

Tools & Applications 🗗

Tutorial Chapters

  Chapter 1. Basics of HF Radio Propagation 🗗

Topics covered:
1. What is radio?
2. What is an EM wave?
3. Radio propagation properties
4. Properties of electromagnetic waves
5. The electromagnetic spectrum
6. The radio spectrum
7. The rebirth of skywave HF radio
8. The HF bands assigned to radio amateurs
9. How does HF radio propagate?
10. What are HF band conditions?
11. Key Factors Affecting HF Propagation

What is Radio

What is an electromagnetic (EM) wave

Figure 1.1: Electromagnetic Wave	Figure 1.2: A wave characterized by frequency🗗 and wavelength🗗

Frequency (f): Cycles per second (Hertz). Wavelength (λ): Distance between successive wave crests. Formula: c = f*λ, where c is light speed.

Radio signals, a type of electromagnetic radiation, typically travel in straight lines. Long-distance communication relies on waves reaching beyond the horizon. While non-linear propagation may seem complex, it can be understood with basic knowledge of electromagnetic principles, Earth's atmospheric layers, and solar-terrestrial interactions.

A comparison between Radio and Light propagation phenomena:

Figure 1.3: Radio wave propagation phenomena Radio waves can travel in different ways between a transmitter and a receiver. See here an overview of these five wave propagation phenomena. The difference between optical refraction vs. skywave refraction.

Figure 1.4: Light Wave propagation phenomena The key difference between Radio and Light is that light waves are more easily affected by obstacles and atmospheric conditions due to their shorter wavelength.

Properties of electromagnetic waves 🗗

1. Absorption 🗗: The conversion of radio wave energy into heat and electromagnetic noise through interactions with matter.
2. Amplitude 🗗: The maximum extent of a vibration or oscillation, measured from the position of equilibrium.
3. Attenuation 🗗 (Path Attenuation | Path Loss): The weakening of a signal as it travels over a distance.
4. Diffraction 🗗: Waves bend around obstacles, allowing them to spread behind them.
5. Dispersion 🗗: Separation of waves at different angles of refraction 🗗 of different frequencies/wavelengths.
6. Fading / Shadowing 🗗: Signal strength fluctuates due to obstacles and multipath propagation.
7. Electromagnetic Field | Electromagnetic Radiation 🗗: Electric and magnetic components that oscillate perpendicular to each other.
8. Field Intensity 🗗: The strength of the wave's electric or magnetic field, typically measured in (Volt/m) or (Ampere/m).
9. Frequency 🗗: The number of peaks per second.
10. Interference 🗗: Waves superpose to form a wave with different amplitudes, causing constructive or destructive interference.
11. Polarization 🗗: The orientation of the electric field of the wave, which can be linear, circular, or elliptical.
12. Power Density 🗗: The amount of power transmitted per unit area, typically measured in watts per square meter (W/m²).
13. Ray 🗗: The direction of wave propagation, often conceptualized as a line along which the energy of the wave travels.
14. Signal-to-Noise Ratio (SNR) 🗗: A measure comparing the level of a desired signal to the level of background noise, expressed in decibels (dB). A higher SNR indicates a clearer and more distinguishable signal from the noise.
15. Reflection 🗗: Waves bounce off a surface, where the angle of incidence equals the angle of reflection.
16. Refraction 🗗: Waves bend as they pass from one medium to another due to a change in wave speed, governed by Snell's law.
17. Scattering 🗗: Waves spread out in different directions due to interaction with particles or rough surfaces, leading to the diffusion of the incident wave.
18. Spectrum 🗗: The range of frequencies or wavelengths of electromagnetic waves, from radio waves to gamma rays.
19. Standing wave 🗗: A wave that oscillates in time but whose peak amplitude profile does not move in space.
20. Wave interference 🗗: Combine coherent waves by adding their intensities or displacements, considering their phase difference.
21. Wavefront 🗗: A surface of constant phase of the wave, which can be thought of as the leading edge of the wave moving through space.
22. Wavelength 🗗: The distance between consecutive peaks of a wave.

The electromagnetic spectrum 🗗: Radio waves are a subset of the electromagnetic spectrum that has unique applications based on frequency and wavelength. The following Figure 1.5 moves from long to short wavelengths, with radio waves on the left side.

Figure 1.5: The electromagnetic spectrum

The radio spectrum 🗗 shown below in Figure 1.6 goes from low to high frequency (long to short wavelength).

Figure 1.6: The radio spectrum divided into 11 bands, from 3 Hz to 3 THz.

How does HF radio propagate?
HF radio waves mainly propagate as skywaves, refracing off the ionosphere, enabling long-distance communication.

What are HF band conditions

Chapter 2. Monitoring HF Band Activity

  2.1 Real-time ham band activity using the internet 🗗

Tools like DXView and DXMAPS provide real-time visualizations of HF activity. DX clusters focuse on general band openings, while DXMAPS emphasizes specific QSOs and contests.

2.1.1 DXView map 🗗 by Jon Harder, NG0E, showing real-time ham activity in the last 15 minutes on 11 ham bands (1.8–54 MHz).

The DXView map (Figure 2.1 below) shows real-time ham band activity. This visual aid helps identify open bands and communication modes🗗.

Figure 2.1: Real-time Ham Band Activity

The DXView map helps identify open bands and communication modes🗗 based on real-time activity from the last 15 minutes. It compiles data from online sources: WSPRnet, RBn 🗗 (CW, FT4, FT8), and DX Cluster. Signal-to-Noise Ratio (SNR) 🗗 data determines if a path supports SSB (SNR > 10 dB), CW (SNR > -1 dB), or only digital modes (decoding down to about -28 dB SNR). The DXView website provides a guide on interpreting the map and selecting band colors.

While DXView focuses on band openings, the next tool (DXMAPS) focuses on specific contacts, allows users to add their info, visualize propagation paths, and analyze contest performance.

DXMAPS provides real-time charts of reported QSOs (contacts) and SWLs (shortwave listeners) across amateur bands. Visualized propagation paths may help users analyze band conditions and contest performance effectively. Registered users can send formatted DX-Spots for easier identification. Propagation mode identification is available for high bands, above 28 MHz.

Figure 2.2: QSO/SWL real time information

2.1.3 DX Clusters 🗗 are worldwide networked servers that collect messages from active radio amateurs🗗 and distribute them to all connected participants. Active radio amateurs or shortwave listeners use DX clusters to get timely information about activities on the amateur radio bands.

Figure 2.3: An illustration of DX Clusters by DALL-E AI Image Generator

Analysis of multiple DX cluster messages can be used as an indicator of propagation conditions and how they are changing. However, it’s not a perfect predictor, and local factors matter.

  2.2 Tracking digital modes 🗗

Tools:
• WSJT-X 🗗: A computer program used for weak-signal radio communication between amateur radio operators.
• PSKReporter 🗗: A global signal-reporting network that maps signal transmission and reception in near real time.
To monitor propagation conditions:
1. Use software like WSJT-X to decode FT8 signals.
2. Upload your reports to PSKReporter to visualize current band conditions.

Figure 2.4: PSKReporter Chart of Signals Received

The Receiving station

Figure 2.5: Malahite v1.3 DSP Receiver connected to
K-180WLA Magnetic Loop antenna (MLA)

  2.3 Tracking Global Beacons 🗗

Listening to the NCDXF Beacon Network is beneficial for DX station hunting. Eighteen worldwide beacons operate on five bands: 20, 17, 15, 12, and 10 meters. Figure 2.6: NCDXF beacons map—All use standardized antennas and power levels. The above is a map of the NCDXF Beacon Network, which operates on the frequencies: 14.100, 18.110, 21.150, 24.930, and 28.200 MHz. Receiving readable signals on these frequencies can indicate open bands. Beacon IDs are callsigns in CW, followed by a carrier decreasing in four power levels: 100, 10, 1, and 0.1 Watts. If you can hear the weakest 0.1-Watt signal, it suggests good propagation or a low-noise location. The NCDXF website provides further details for operators. Tune between 28.2 and 28.3 MHz for additional beacons operating full time.

  2.5 Monitor bands using remote receivers, WebSDR, and Kiwi SDR 🗗

WebSDR and Kiwi SDR offer online access to remote receivers. These platforms allow users to monitor global HF signals without local equipment. Both support multiple users and offer real-time spectrum and waterfall visualization. However, their user interfaces and functionalities differ, with each platform having unique advantages to suit various needs and preferences. The following example demonstrates the Wideband WebSDR at the University of Twente, Enschede, NL 🗗. The visual spectrum and waterfall display enable users to monitor and analyze signals from remote locations.

Figure 2.7: Real-time waterfall display for a wide radio spectrum,
frequency range of 0-29 MHz, with the ability to resize the width down to 250 KHz.

Alternatively, choose a remote receiver from the following maps 🗗:

Figure 2.8: WebSDR Global Map showing locations worldwide

Users can select a receiver to remotely monitor HF signals, access live waterfall displays, and tune into specific bands.

Figure 2.9: Global map of Kiwi SDR receivers showing locations worldwide

Chapter 3. HF Propagation Conditions: Forecasting and Prediction

1. Why Do We Need HF Propagation Forecasting?
2. Evolution of Forecasting Techniques
3. How to determine HF propagation conditions
4. Forecasting vs. Prediction
5. Forecasting and Prediction

Why Do We Need HF Propagation Forecasting?

HF propagation forecasting enables operators to select optimal frequencies and plan communication times. Key metrics such as foF2 and MUF provide real-time insights into ionospheric conditions, essential for long-distance communication.

Evolution of Forecasting Techniques

Remarkable advancements in space technology🗗, software-defined radio (SDR) 🗗, and the internet have revolutionized our understanding of radio wave propagation. Before the 1990s, propagation charts and reports were often published in amateur radio magazines. Today, real-time solar indices and computer programs provide accurate, up-to-the-minute propagation data via online tools 🗗.

How to determine HF propagation conditions

The MUF 🗗, based on ionograms 🗗, plays a key role in determining HF propagation conditions.

Forecasting vs. Prediction

The terms forecasting and prediction differ primarily in their time frames and methodologies.

• Forecasting: Short-term estimations based on current data (e.g., "Conditions will improve in the next hour").
• Prediction: Long-term estimates based on trends (e.g., "Better 40-meter conditions expected next month").

Forecasting and Prediction

Utilizing real-time propagation charts based on measured critical frequency 🗗 improves long-distance communication, reduces interference 🗗, and ensures efficient, reliable use of the HF bands 🗗.

The quickest methods to forecast HF propagation conditions over the next hour are:

1. Watch real-time DXView chart
2. Watch real-time propagation charts.

• Watch ham activity charts
• Analyze real-time charts to forecast potential propagation conditions.
• Utilize tools🗗 and software 🗗 based on solar and geomagnetic data to predict band conditions.

Chapter 4. HF Propagation Modes 🗗

This chapter reviews the primary modes of high frequency (HF) radio propagation.

There are three main modes of HF Radio Propagation:
LOS, Ground wave, and Skywave.

Figure 4.1: Overview of HF Propagation Modes In chapters 7-9, we explore the factors and conditions that influence skywave propagation.

1. Line of Sight (LOS) propagation 🗗: Short-range, direct-path communication above 30 MHz.

• Line of Sight exists when radio signals pass directly between two stations with no obstacles in between. This mode works well for short-range transmission at higher frequencies, often within a few kilometers of the visual horizon. Signals cannot follow the curvature of the globe.
• Non-LOS propagation 🗗 occurs if obstacles exist; radio waves may reflect off conductive surfaces like buildings or mountains.

2. Ground wave 🗗 or surface wave propagation: Effective below 2 MHz; influenced by terrain and conductivity.

• AM radio stations use ground wave propagation during the day.
• Vertically polarized surface waves travel parallel to the Earth's surface and can cross the horizon.
• Geologic features and RF absorption by the earth attenuate ground wave transmission.
• Ground waves are effective below 1 MHz over salty seawater or conductive ground but are ineffective above 2 MHz.

3. Skywave (or skip propagation): Long-distance propagation via ionospheric refraction (3–30 MHz).

• Ionospheric Variability: Ionization density profiles vary in thickness and altitude.
• Daytime Absorption: The lowest D region absorbs frequencies below 10 MHz, as discussed later, focusing on the LUF.
• Ducting effects: Can occur occasionally.
• The Skip Distance (Figure 4.1) is a dead zone with no reception between ground wave and skywave. It is calculated using the following formula:
  
  where D_skip is Skip Distance, h is the height, f_MUF is maximum usable frequency, and f_c denotes the critical frequency 🗗.
• Special cases:
  • Gray line (greyline): Utilizes the twilight zone around Earth separating daylight from darkness.
  • NVIS: Near Vertical Incidence Skywave operates at 2–8 MHz, using low horizontal antennas to address dead zones.
  • Sporadic E: In late spring or early fall, low VHF (30 to 150 MHz) signals can be unpredictably refracted back to Earth.

Note: Currently, this project does not cover the following propagation modes:

• Aurora 🗗 propagation
• Scatter Propagation
• Meteor Scatter propagation
• Backscatter propagation
• Moon Bounce (EME) propagation

Among these modes, skywave propagation is the most versatile for HF bands. The upcoming chapters detail the factors affecting skywaves.

  Chapter 5. How does the sun affect radio communications?

Figure 5.1: An illustration of ionosphere generation and its effect on radio waves

Knowing a bit about solar activity can help radio amateurs make better use of these effects to improve their experience or solve problems.

1. The ionosphere is a conducting region of plasma that refracts HF radio waves.
2. Global and regional propagation conditions depend on the sun's position and orientation, i.e., time of day, season, and ionospheric state above different geographical locations.
3. High solar activity increases ionization 🗗 in the ionosphere, resulting in better propagation conditions, especially in higher HF bands.
4. The sunspot number and solar flux correlate with improved global propagation conditions.
5. Solar storms 🗗 may also disrupt global communications.

  Chapter 6. The Ionosphere (preface)

This chapter serves as an introduction, laying the basis for a deeper study of the ever-changing ionosphere's influence in HF radio communication.

The term "ionosphere" 🗗 refers to the active upper region of the atmosphere 🗗 that grows and shrinks with solar energy.

The ionosphere is always changing, courtesy of NASA Goddard

Video clip: The dance of radio waves within a vibrant airglow.
Solar storms intensify the ionosphere's beauty, while
Earth's weather below adds to the unique destination.

Earth's weather and the space weather both affect the ionosphere, a spectacle of charged particles—ions and free electrons.

"Ionospheric clouds" move at different speeds and directions, with irregularities in conductivity.

The ionosphere is a series of regions in the upper atmosphere

Figure 6.1: The Ionosphere (Thermosphere) is part Earth's Atmosphere

What is the cause of the high temperatures? —Solar radiation ionizes the ionosphere, resulting in free electrons, as illustrated here.

Figure 6.2: Ionization of atoms or molecules generates free electrons

HF radio waves transmitted from Earth to the ionosphere cause these free electrons to oscillate and re-radiate, resulting in wave refractions 🗗.

The ionospheric refractive index 🗗 is analogous to that in geometrical optics 🗗. Figure 6.3 illustrates light refraction in a glass prism.

Figure 6.3: A prism bends shorter wavelengths more; this is an optical dispersion due to refraction 🗗.

A prism bends blue light more than red, creating a rainbow. Glass prisms have a higher refractive index for blue light than red (typically 1.5–1.8).
In contrast, ionospheric plasma has a refractive index slightly less than one and bends low HF bands (3–10 MHz) more than high HF bands, as shown in Figure 6.4.

Figure 6.4: The ionosphere bends lower frequencies more; this is radio wave dispersion.

The next chapter extends the explanations on the ionospheric regions and their role in skywave HF propagation.

Chapter 7: Ionospheric Influence

7.1 Ionospheric Regions

Note: People commonly use the term layers, but regions 🗗 more accurately describe the ionosphere's structure.

The D, E, and F regions form the ionospheric structure, although ionization density varies with altitude and time across the entire ionosphere.

Figure 7.1: Ionospheric regions illustration

It’s common to present the order of ionosphere regions affecting HF skywaves from the highest region downwards, as follows:

• The F region, located between 150 and 800 km above the Earth, enables long-distance HF communication in the 3.5 to 30 MHz bands.

This region consists of ionized 🗗 hydrogen (H⁺) and helium (He⁺⁺) with the highest free-electron density up to 10¹² electrons per cubic meter excited by the 10–100 nano-meter EUV 🗗. It splits during the day into two sub-regions, F₁ and F₂, which merge and slowly dissipate after sunset.

• The E region, located between 90 and 150 km above the Earth, dissipates a couple of hours after sunset.

This region consists of ions such as O₂⁺, O⁺ up to 10¹¹ electrons per cubic meter excited by the 1–10 nano-meter EUV 🗗 solar radiation.
During intense Sporadic E_(Es) 🗗 events (particularly near the equator) it sporadically reflects frequencies in the 50-144 MHz bands.

• The D region, located 50–90 km above ground, is active during daytime and dissipates at sunset.

In this region, UVC 🗗 at 121.6 nm excites nitric oxide ions (NO⁺), up to 10¹⁰ electrons per cubic meter. This causes radio frequencies to be absorbed and blocked during daylight hours, preventing frequencies lower than the lowest usable frequency (LUF) from reaching higher E and F regions (Figure 7.9).

Moreover, chaotic solar flare bursts 🗗 (X-rays with wavelengths of 0.1–1 nm) significantly enhance ionization in this region, causing blackouts that can last from minutes to hours.

Additionally, enhanced solar wind and CMEs may cause Polar Cap Absorption (PCA) events that can last up to 48 hours.

The F, E, and D regions differ in gas composition and free electron density. These regions are conceptual rather than rigidly defined. Sometimes there are plasma clouds 🗗 rich in free electrons. The average electron density affects the critical frequency of each region. Their characteristics change daily, seasonally, and throughout the solar cycle.

7.2 Long and Mid-Range Skywaves

Figure 7.3 shows skywave refractions from the F and E ionospheric regions.

Figure 7.3: Multi refractions of radio waves in the ionosphere.

The F region refracts HF (3-30 MHz); Figure 6.4 illustrates the difference between low and high HF bands refraction.
The E region sporadically refracts low VHF (50-150 MHz).

Long-range skywave propagation typically employs low transmission angles that correspond to high incident angles.

Figure 7.4: Transmission angle (α) and incident angle (θ)

A low transmission angle, which means the transmitted beam is nearly horizontal, enables refractions at higher frequencies and over longer distances. However, using real antennas at frequencies below 30 MHz to achieve low-angle radiation of less than 5 degrees can be extremely challenging.

  7.3 Skywave Multi-refractions

The ionosphere 🗗 refracts skywaves 🗗 in complex multiple modes

Figure 7.5: Complex skywave modes:

F Skip / ¹F¹E, E-F Ducted, F Chordal, E-F occasional and sporadic E^🗗.
This figure extends Fig.2.4 of ASWFC🗗.

The diagram illustrates various modes of radio wave propagation in the ionosphere, such as ionospheric tilt, chordal mode, ducted mode, sporadic E, F skip, 1F1E, and 1F1Es1F. It emphasizes how radio waves interact with the E and F regions, depicting their travel paths across long distances.

The free electrons in the ionosphere refract radio waves as they move through the ionospheric regions, where the free-electron density gradually varies; numerous refractions are what create the frequency-dependent refractions of ionosphere skywaves.

  7.4 HF Propagation Indicators: Critical Frequencies

The refraction of radio waves in the ionosphere is characterized by their critical frequency 🗗. This is the highest frequency at which radio waves refract back to Earth. Higher frequencies escape into space.

The terms (frequencies) f_oF₂, MUF, OWF, and LUF serve as indicators for HF radio propagation conditions🗗.

Figure 7.6: Vertical refraction from F₂ region

The critical frequency is dependent on the density of the free-electrons:
where f_c is the critical frequency and N_max is the free electron density.

If the transmitted frequency is higher than the plasma frequency of the ionosphere, then the electrons cannot respond fast enough, and they are not able to re-radiate the signal.

Ionosondes 🗗 determine the critical frequency, which varies significantly based on location and time.

The critical frequency varies with several factors: time of day, geographic latitude, season, solar activity, and geophysical conditions.

• Day vs. Night and Geographical Locations:
  The critical frequency varies with latitude and the day due to increased ionization from solar radiation 🗗. At night, the MUF decreases.

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The graph below shows how the critical frequency varies with latitude during the day and night.

Figure 7.7: Noon & Midnight f_oF₂ vs. Geographic Latitude, based on Australian Space Weather Service publication.
• Day Hemisphere: The red curve (F2 region) peaks around 18 degrees latitude, forming an "equatorial anomaly."
  The blue curve (E region) remains relatively flat.
• Night Hemisphere: The red curve shows a "mid-latitude trough" around 60 degrees latitude. Gradually growing towards the equator.
  The E region dissipates at night.
• Seasonal Variations: The critical frquency is higher in summer due to the sun being directly overhead and lower in winter.
• Solar Activity: High solar activity can increase the MUF by enhancing ionospheric ionization.
• Geophysical conditions: Factors such as geomagnetic activity and atmospheric tides can also have an impact.

See links to the online f_oF₂ maps and the recent f_oF₂ measurements at various locations around Australia.

Between the years 2005 and 2007, the global average critical frequency (f_oF₂) ranged from 1.8 MHz to 11 MHz, with an overall average of 7.5 MHz.

7.4.2   The Maximum Usable Frequency (MUF) 🗗; synonym: Highest Possible Frequency (HPF), is a fascinating concept in skywave propagation—an indicator for forecasting propagation conditions. It is the highest frequency you can use to send radio signals successfully. The MUF depends on the angle at which those signals are transmitted but is independent of the transmitting power.

Figure 7.8: MUF illustration

The MUF is calculated using the formula:

MUF = f_oF₂ × sec(θ)🗗

• f_oF₂: Critical frequency of the F2 region.
• θ: Angle of incidence relative to the vertical.
• As a rule of thumb, the MUF is approximately 3-4 times the critical frequency;
  i.e., incident angle θ = 70°-75°; transmission angle α = 15°-20°.

For vertical incidence (θ = 0), MUF equals f_oF₂. For oblique paths, MUF increases with sec(θ)🗗.

See the recent MUF charts.

7.4.3   The Optimum Working Frequency (OWF) is usually 85% of the MUF.

Synonym terms:
• Frequency of optimum traffic/transmission (FOT)
• Optimum traffic/transmission frequency (OTF)

7.4.4   The Lowest Usable Frequency (LUF) 🗗 is the lowest viable frequency for communication limited by daytime, D region absorption.

Figure 7.9: Daytime Low HF Absorption below 10 MHz

LUF, also known as the absorption-limited frequency (ALF), is a soft frequency limit, unlike the sharp cut-off of the MUF.
The D region absorbs frequencies below the LUF during the day. At night, the D region does not exist, so there is no low-frequency limit.

• During a solar flare, the LUF may rise swiftly.
• Strong solar flare 🗗 can cause blackouts lasting minutes to hours.
• See the LUF chart affected by the last M1+ solar flare.
• See the D-RAP model, which provides an online global LUF chart.

Understanding these variations is crucial for effective HF radio communication, as it helps select the optimal transmission frequency.

  7.5 NVIS Propagation

NVIS - Near Vertical Incidence Skywave 🗗 is a unique communication mode using skywaves directed almost vertically.

NVIS provides the solution for the dead zone (between ground wave and skip). It is the only solution for communication coverage in hilly and/or jungle areas over short distances of a few hundred kilometers.

Figure 7.10: How NVIS provides communications within a hilly area.

• Typical operating frequencies are 2-4 MHz at night and 4-8 MHz during day.
• NVIS requires suitable antennas (like a low dipole at hight of 0.1-0.25 wavelengths) to improve vertical radiation and reduce lower-angle radiation, contrary to what is customary for long-range communication.
• NVIS offers enhanced resistance to fading (constant signal level), and minimal attenuation, making it suitable for low transmit power levels and omnidirectional coverage, allowing flexibility in setup and placement.
• To avoid skip zones on 40 m band use NVIS when f0F2 is higher than 8.5 MHz. Switch to 80 m if the day is on the downward slope. Optimize antenna radiation pattern for the desired takeoff angle. Optimum NVIS height for horizontal dipoles: 0.18–0.22λ for TX and 0.16λ for RX🗗.

The NVIS map shows the recent global distribution of critical frequency (foF2).

  7.6 Gray line Propagation 🗗

The "gray line" (US English) is the twilight zone around the Earth separating daylight from darkness. Propagation along this zone is highly efficient because the D region, which absorbs HF signals during the day, vanishes quickly on the sunset side and hasn't formed yet on the sunrise side. Ham radio operators and shortwave listeners can optimize long-distance communications by tracking this twilight zone.

Figure 7.11: Ionospheric Regions and Gray Line The height of the F and D regions 🗗 is exaggerated in comparison to Earth dimensions.

Figure 7.12: Online gray line chart For more information click on the map.

Some radio operators use specialized gray line map 🗗 to predict when the gray line will pass over their location, as well as the best frequencies and modes of propagation to apply at that time. Overall, gray line propagation is a fascinating and useful phenomenon that has the potential to open up exciting opportunities for long-distance radio communication.

  7.7 Ionospheric conditions

The ionospheric conditions vary in geographical locations, 24-hour cycles, seasonal changes, and solar activity.

The following supplementary information is not crucial for understanding skywave propagation.

Figure 7.13: Ionospheric physical conditions

Shown on the left figure: Temperatures distribution due to low or high solar flux Free electron density Ionic compositions. Not shown on the left figure: Gas pressure and density Gas compositions Chemical reactions Winds: horizontal and vertical

The dynamics in the D region correspond to chemical reactions between the ions O⁺, N⁺, and NO⁺ with N₂, O₂, and NO.

Nutshell: This chapter examines ionospheric regions, distributions of free electrons, critical frequencies, and specific propagation modes.
The following chapter discusses regional, diurnal, and seasonal propagation conditions, including online real-time charts.

  Chapter 8. Regional HF Propagation Conditions

  8.1 Ionosonde🗗

Figure 8.1: Basic ionosonde types are vertical and oblique sounding

Every 15 minutes, ionosonde stations around the world report real-time data via the internet.

Figure 8.2: Global map of Giro digisondes as of 2017 🗗

Some stations aren't always active. Since 2021, real-time ionosonde data sharing has reduced in countries such as Russia, China, Japan, and others. Thus, significant regions of the globe are not yet covered with ionosonde stations, as shown on the above map.

Readings of foF2 from several sites can be combined to build a propagation map for foF2.

  8.2 Ionogram🗗

Ionograms typically display two key elements:

1. Horizontal Lines: These lines indicate the virtual height at which an amplitude-modulated pulse is echoed, varying with the operating frequency.
2. Vertical Curve: This curve represents the critical frequency.

Figure 8.3: A typical ionogram🗗

While this provides a simplified explanation, the reality is that the ionosphere is neither uniform nor stable, perpetually changing over time. Consequently, researchers developed the Digisonde Directogram to identify ionospheric plasma irregularities.

  8.3 Day-night: Regular diurnal cycle

The diurnal cycle on Earth occurs every 24 hours, with the sun affecting ionosphere characteristics. The figures below illustrate typical diurnal cycle: The E and F regions have larger electron densities during daylight, while the D region disappears at night. The MUF and LUF rise with the sun and diminish after sunset.

Figure 8.4: Diurnal cycle of ionospheric regions

Figure 8.5: Typical diurnal cycle based on NPS training materials🗗

FMUF: F region maximum usable frequency OWF: optimum working frequency EMUF: E region maximum usable frequency LUF: The lowest usable frequency is due to D-region absorption,   which limits the window of useful frequencies ↕

  8.4 Seasonal phenomena—variations and anomalies 🗗

Seasonal variations
Intensified solar EUV (Extreme Ultraviolet) radiation 🗗 leads to higher free-electron densities, especially during the summer months and more intensely near the equator compared to the poles.

Figure 8.6: Ionospheric region dynamics at mid-latitudes

As a result, HF propagation conditions on the bands above 10 MHz are better in the summer and closer to the equator, whereas propagation conditions on the bands below 10 MHz are better in the winter and at mid-latitudes (30° to 60°).

Summer anomalies
Summer anomalies can cause plasma irregularities in the ionosphere's mid-latitude F region in both hemispheres. Seasonal changes significantly impact ionization, with summer frequently bringing instabilities known as mid-latitude spread-F 🗗 due to increased solar radiation. The Arecibo Radio Observatory in Puerto Rico observed anomalous electron density irregularities during such an event, extending above the ionosphere's stable topside, as shown in the following figure:

Figure 8.7: Electron density anomaly at mid-latitudes 🗗

The top figure shows both the E and F regions on the same scale and the bottom figure shows E region in an expanded scale

8.5 Online real-time propagation charts🗗

   MUF
1. Online gray line chart ahows MUF at 13 stations with global propagation indices ^{updated every 3 hours; Provided by N0NBH}
2. MUF 3000 km map: HF propagation conditions at a glance ^{updated every 15 minutes; Provided by KC2G
   There is also an animated version showing the last 24 hours.}
 
   foF2
3. Online NVIS Map 🗗 shows wolrdwide distribution of f_oF₂ ^{provided by KC2G, updated every 15 minutes}
 
The following 3 NVIS maps are updated every 15 minutes by the Australian Space Weather Forecast Center (ASWFC)🗗
4. Online chart of NVIS (f_oF₂) ^ASWFC
5. Online chart of T index f_oF₂ ^ASWFC
6. Online chart of the recent f_oF₂ measurements at various locations of Australia, New Zealand and East Antarctica ^ASWFC
 
   LUF
7. Global online chart of LUF calculated by D-RAP model ^{NOAA SWPC}.
8. Online chart of LUF updates only when it detects a solar flare of magnitude M1 or higher ^ASWFC.

    Online gray line chart showing current MUF at 13 stations and global propagation indices; updated every 3 hours (by Paul L Herrman, N0NBH).

Figure 8.8: Grayline map with MUF data and some propagation indices

The above figure shows day-night, 13 local MUF reports, and the global indices: SFI 🗗, SN 🗗, A&K 🗗, 304Å 🗗, Geomag, Sig Noise.

   Online MUF 3000 km propagation map 🗗 ^{updated every 15 minutes}

This map may assist radio amateurs🗗 in finding the best times and frequencies for contacts by displaying HF propagation conditions 🗗 at a glance.

• This online map shows the calculated MUF based on ionograms.
• A radio path of 3,000 km is being considered for unification.

Figure 8.9: Online MUF 3000 km propagation map, by Andrew, KC2G
How to use this map | Notes | Animated map

How to use this map?

The colored regions of this map, defined by iso-frequency contours, illustrate the Maximum Usable Frequency 🗗 expected to refract off the ionosphere 🗗 along a 3000 km path. The map also includes the position of gray line.

The ham bands are designated by iso-frequency contours: 5.3, 7, 10.1, 14, 18, 21, 24.8, and 28 Mhz.
For example, if a given area on the map is greenish and lies between the contours labeled "10" and "14," the MUF in that location is around 12 MHz.

The raw data is MUF calculated from data collected by ionosondes, which are represented by numbered colored discs that show their location.
A number inside a disc indicates the calculated 3000km MUF from the critical ionospheric frequency 🗗, f_oF2. The information from selected stations is compiled by Mirrion 2 🗗 and GIRO🗗, and processed by the International Reference Ionosphere (IRI) model🗗 (produced by a joint task group of COSPAR🗗 and URSI🗗.

The MUF along a path between any two locations shows the possibility of long-hop DX between those points on a given band.
For example, if the MUF is 12MHz, then 30 meters band and longer will work, but 20 meters band and shorter won't.
For long multi-hop paths, the worst MUF anywhere on the path is what matters. For single-hop paths shorter than 3000 km, the usable frequency will be less than the indicated MUF. As one gets closer to vertical, i.e., NVIS 🗗, the usable frequency drops to the Critical ionospheric frequency 🗗, (f_oF2, as shown in the next map).

Notes:

1. The accuracy of the data is insufficient for commercial radio services due to several factors:
   1. Uncertainty in predicting ionospheric state:
      • Vertical sounding data introduces uncertainty when predicting the ionosphere's state.
      • The limited coverage of monitoring radio stations results in reliance on data processing.
   2. Challenges of data interpolation and extrapolation 🗗:
      • The algorithm attempts to determine the MUF (or foF2) at scattered points globally.
      • Accuracy is compromised when extrapolating from sparse data points.
      • Predictions are more reliable near measurement stations but deteriorate for distant regions.
   3. Issues with measurement stations:
      • Inconsistent or conflicting data from stations may lead to unusual results when aligning measurements.
      • Unexpected global model changes may occur due to stations going offline or reappearing, compounded by the limited initial data points.
   4. Restricted sharing of real-time data:
      • Since 2021, real-time ionosonde data sharing has reduced in countries such as Russia, China, Japan, and others.
      • Some ionosondes are accessible solely via NOAA, and GIRO outages could cause map updates to cease.
   5. Impact of geomagnetic storms and solar activity:
      • Events such as geomagnetic storms, elevated X-ray flares, and solar wind significantly affect the accuracy of MUF estimations derived from vertical sounding data.
      • While these disturbances are implicitly reflected in ionogram results, predicting band conditions remains challenging.
      • The propagation model is overly simplistic. It does not capture all the variables, such as blackouts due to D-region absorption and noise induced by geomagnetic storms.
   6. Future Development: Efforts are underway to develop geospace dynamic models to mitigate these challenges.
    
2. The "MUF(3000km)" project is the result of research and development by Andrew D Rodland - KC2G, which is based on an earlier work by Matt Smith - AF7TI. WWROF financing and data from ionosonde operators all over the world, provided by GIRO🗗 and NOAA🗗 made it feasible.
 
3. See Acknowledgments.
4. Read more about this open source project🗗.
5. Read more about the open source software and models🗗.
 
6. Roland Gafner, HB9VQQ, extended the static presentation with an animated map showing the last 24 hours in 15-minute steps.

    NVIS online live map for vertical refraction (critical frequency foF2) provided by Andrew D Rodland, KC2G ^{updated every 15 minutes}

Figure 8.11: Online NVIS Map, by Andrew, KC2G

The map's colored regions, outlined by iso-frequency contours, show the critical frequency for near-vertical ionosphere refraction. Colored discs mark ionosonde stations, with numbers representing critical frequency (foF2)—the site's raw data source.

 Another NVIS 🗗 real-time map provided by the Australian Space Weather Service🗗 is updated every 15 minutes. It displays contours of the critical ionospheric frequency 🗗 - f_oF₂. There are a few differences between this map and the KC2G map, mainly due to the choice of frequencies for the contours. The KC2G map highlights ham bands. The following map, however, is designed for commercial use.

Figure 8.12: Online NVIS map courtesy of ASWFC
Click on this online map to view the source page. There is further information.

   Online T Index Map - f_oF₂ 🗗 is provided by Australian Government Space Wheather Services🗗

The T-index forecasts high-frequency communication conditions and functions as an "equivalent sunspot number." Derived from f_oF₂ measurements, it accounts for anomalies like geomagnetic storms that may influence these readings. Typically ranging from -50 to 200, lower values indicate limited HF frequency usability (e.g., during solar minimum), while higher values correspond to optimal conditions for higher frequencies (e.g., near solar maximum).

Figure 8.13: Online T Index Map courtesy of ASWFC
T Index FAQ | T Index Map | Real-time T Indices | Forecast T indices

The recent f_oF₂ measurements at various locations of Australia, New Zealand and East Antarctica

Figure 8.14: foF2 Plots courtesy of Australian Space Weather Forecasting Centre
Click on this online chart to view the source page.

LUF (ALF) chart affected by the last M1+ solar flare

The lowest frequency at which two radio stations can connect is known as the LUF. It is dependent on ionospheric conditions due to solar flares, solar wind, and geomagnetic activity, as well as path factors (such as transmitting power and receiving SNR🗗). These variables collectively complicate mapping efforts. Figure 14.3 illustrates the attenuation resulting from solar flares and solar energetic particle (ISEP))events over the past eight hours.

The Australian Space Weather Alert System (ASWFC) provides LUF data for the recent M1+ solar flare:

Figure 8.15: LUF (ALF) chart by ASWFC🗗

This chart relies on events and updates whenever a flare of magnitude M1 or greater occurs. The top line indicates the recent flare time. The chart illustrates the LUF affected by the recent significant solar X-ray flare. As shown by the color bar, the most significant impacts occur within the inner circle. The map reflects the LUF for standard 1500 km HF circuits, where communication below the LUF is uncommon, while communication above it is generally possible. Shorter circuits may exhibit higher LUF values, enabling the use of lower frequencies. Conversely, longer circuits might still experience signal fading, even at elevated frequencies.

Chapter 9. Ionosphere Dynamics

The ionosphere has a regular daily cycle, but dramatic events cause chaotic disruptions. The atmosphere's different regions interact like a team, influencing one another in intricate ways. Weather patterns in the troposphere and activities from the Sun and Earth's magnetic field also play a role in this system. Atmospheric waves, such as gravity waves (ripples caused by air moving up and down) and planetary waves (large waves influenced by Earth's rotation and heat), along with geomagnetic activity, significantly impact the energy and dynamics in the thermosphere. This chapter delves into how these interactions affect the propagation of radio waves through the sky.

9.1 Sporadic E
9.2 Ionospheric clouds or bubbles — spread F
9.3 Ionospheric Storms cause fadeouts

  9.1 Sporadic E🗗

Sporadic E (E_s) indicates occasional refractions from highly ionized plasma clouds in the lower E region.

Figure 9.1: refraction from Sporadic E plasma cloud

Operators may use E_s for making mid-range contacts on the VHF amateur bands: 50 MHz (6 m), 70 MHz (4 m), and 144 MHz (2 m).

Sporadic E Propagation in 2 minutes courtesy of Andrew McColm, VK3FS🗗

Video clip: Equatorial sporadic E, occurring within ±10° of the geomagnetic equator, is a regular midday phenomenon. In polar latitudes, sporadic E, known as auroral E, can accompany auroras🗗 and disturbed magnetic conditions. At mid-latitudes, E_s propagation often supports occasional long-distance communication on VHF bands during the approximately six weeks centered on the summer solstice, which normally only propagate by line-of-sight.

  9.2 Ionospheric Clouds (bubbles) — spread F

All the ionospheric regions consist of plasma clouds 🗗 as illustrated below:

Figure 9.2: Ionospheric Clouds or Bubbles

The moving plasma clouds or bubbles are traveling disturbances of electron density. Ionospheric "plasma bubbles" or "clouds" are the physical cause to the observed spread F phenomenon 🗗.

How do "ionospheric clouds" affect HF propagation?

The dynamic ionosphere causes signal fading (QSB) over time. Small-scale irregularities in the ionosphere are observed at all levels, with periodic motions attributed to neutral atmospheric waves interacting with ionized components in the upper atmosphere. While understanding is limited, the research promises the ability to predict short-term changes.

Additionally the ionosphereic regions are disrupted by (1) The chaotic solar activity and (2) The tropospheric weather from far below.

What effect does tropospheric weather have on the ionosphere?

Troposphere storms, hurricanes, and strong wind patterns can all temporarily alter the TEC caused by EUV solar radiation 🗗.

In other words, the ionosphere and troposphere are coupled🗗 by a variety of mechanisms🗗.

For instance, a lightning storm can cause electrodynamic interaction.

The following figure illustrates electrodynamical coupling of the Troposphere with the Ionosphere🗗:

Figure 9.3: Ionospheric clouds due to Troposphere-Ionosphere coupling

Sprites - Transient Luminous Events (TLEs) 🗗

Figure 9.4: The different forms of Transient Luminous Events Credit: NOAA

There are other complex mechanisms that couple the troposphere to the ionosphere. We won't go into detail at this point.

In conclusion, "Ionospheric clouds"🗗 that develop as a result of the coupling 🗗 between the troposphere and ionosphere may affect skywave HF propagation.

How are ionospheric clouds or bubbles detected?

The Digisonde Directogram 🗗 may detect ionospheric plasma irregularities.

Figure 9.5: Digisonde Directogram

It consists of multi-beam ionosondes 🗗, which measure echoes coming from various locations.
Seven ionosonde🗗 beams (one vertically and six diagonally) are used to generate the ionograms🗗.
The end result is an extended ionogram of plasma clouds 🗗 as they drift over a Digisonde station 🗗.

Figure 9.6: Sample directogram
for Cachimbo station from 12 UT Oct 10 to 12 UT Oct 11, 2002.
Blue color means ionospheric motion from west to east.

  9.3 Ionospheric Storms cause fadeouts

Ionospheric storms 🗗 involve a sudden change in the density of ionized particles, usually due to solar flares. However, solar wind and tropospheric tides can also influence these storms. Below, we explain the ionospheric disturbances: SID, TID, and GRB.

9.3.1 "Sudden Ionospheric Disturbances" (SID)🗗 are any one of several ionospheric perturbations resulting from abnormally high ionization or plasma density in the D-region of the ionosphere and caused by solar flares and/or solar particle events (SPE). The SID affects HF skywave signal strengths, with lower frequencies being more heavily absorbed and resulting in a larger decrease in signal strength (see the next figure).

Figure 9.7: Fadeout signal strength vs. time
courtesy of Australian Space Weather Service

During a strong SID, the LUF will increase to a frequency higher than the MUF, thus closing the usable frequency window, an event called a fadeout or blackout.

Figure 9.8: Normal solar activity vs. SID due to flares

The current short wave fadeout—SWF event (if any):

Figure 9.9: Online SWF event
report courtesy of ASW Alert System

9.3.2 Polar cap absorption (PCA) 🗗 events, driven by solar wind, involve high-energy protons reaching Earth's atmosphere near the magnetic poles, increasing ionization in the D and E regions. These events last from an hour to several days. Coronal mass ejections (CMEs) can also release energetic protons that enhance D-region absorption in polar areas.

Figure 9.10: Illustration of Polar Cap Absorption (PCA): radio waves can't propagate over the north pole

Streams solar ejected protons increase ionization in the lower ionosphere, blocking all radio communications in polar zones. These PCA events last as long as proton energy exceeds ~10 MeV and 10 pfu at geosynchronous satellite altitudes. The resulting HF radio blackouts pose significant challenges for aviation in polar regions, especially above 82 degrees north latitude, where rerouting is necessary to maintain viable communications. See the current fadeout report.

9.3.3 Traveling Ionospheric Disturbance (TID)🗗 is a wave-like structure passing through the ionosphere that alters the altitude and angle of refraction of skywaves. TIDs travel horizontally at 5–10 km/minute, with varying phases, amplitudes, and angles of arrival. Some originate in auroral (polar) zones.

Probing traveling F region ionospheric disturbances

The Super Dual Auroral Radar Network🗗 (SuperDARN) is an international network of 35 HF radars (8 MHz–22 MHz) located in the northern and southern hemispheres.

Figure 9.11: SuperDARN site in Holmwood SDA, Saskatoon, Canada 🗗

The SuperDARN are designed to study F region ionospheric dynamics, instability, disturbances and storms. The research covers geospace phenomena, including field-aligned currents, magnetic reconnection, and mesospheric winds. It tests theories of polar cap expansion and contraction under changing IMF conditions, observing large-scale responses to substorms. The collaboration includes various institutions.🗗

9.3.4 Cosmic Gamma-ray Bursts (GRB)🗗 may also cause communications disturbances. Measurable effects are rarely observed.

On October 9, 2022, there was a cosmic gamma-ray burst that affected all ionospheric and stratospheric regions🗗. These are intense explosions observed in distant galaxies, the brightest and most extreme events in the universe. NASA describes them as the most powerful class of explosions since the Big Bang. Afterglows are longer-lived and typically emitted at longer wavelengths.

Studies are being done on this phenomenon.

  Chapter 10. Total Electron Content (TEC)🗗

What is TEC?
TEC is the total number of free electrons present along a path between two points.

Why is TEC important for HF propagation conditions?
TEC correlates with the critical frequency, f_oF₂ 🗗, and is therefore implemented in a variety of ionosphere models🗗. Moreover, the total electron content can provide additional information about the structure and dynamics of the ionosphere. It can detect and monitor ionospheric disturbances, such as those caused by solar flares or geomagnetic storms.

Units: 1 TEC Unit (TECU) is the number of free electrons per square meter (x10¹⁶) for a shell height of 400 km directly above a certain point. Values in Earth’s atmosphere can range from a few to several hundred TEC units.

How is TEC measured?
Data is gathered from GPS receivers worldwide, observing carrier phase delays in radio signals from satellites above the ionosphere, often using GPS satellites.

The effect of Tropospheric weather 🗗:
The troposphere and ionosphere are separate atmospheric layers with distinct functions. However, they do interact through various processes. Tropospheric lightning may induce changes in total electron content and consequently affect HF propagation conditions. Thunderstorms can also worsen the signal-to-noise ratio, in particular in the lower HF bands; i.e., tropospheric weather may affect these conditions, especially in tropical regions. Thus, monitoring and modeling TEC patterns and variations allows us to better understand and prepare for the constantly changing atmospheric conditions.

Online TEC maps:

Figure 10.1: Online TEC map 🗗 courtesy of the German Aerospace Center (DLR)

TEC conclusion:
Solar EUV radiation, solar wind, CMEs, and atmospheric disturbances all contribute to TEC fluctuations, which vary with time, location, seasons, geomagnetic conditions, troposphere conditions, and the solar cycle. Data analysis may reveal qualitative patterns for spring, fall, summer, and winter solstices.

Chapter 11. Global Propagation Conditions🗗

Solar activity, ionospheric conditions, and global average ionization levels in the F2 region affect HF radio waves worldwide.

The regional conditions, as explained in Chapter 8, can be very different from the global averages described in this chapter.

11.1 Banners & widgets—displaying global propagation conditions
11.2 Solar Indices
11.3 Geomagnetic Indices
11.4 Propagation Indices

  11.1 Banners and Widgets

Banners and widgets are visual aids for displaying global propagation conditions using propagation indices. They help radio operators to quickly assess current world conditions and make informed judgments about their operations.

Paul L. Herrman (N0NBH) developed the banners shown below.

Figure 11.1: Global conditions

Figure 11.2: The Basic Solar indices 🗗   SFI & SN correlate with F2-region ionization. A and K indicate geomagnetic instability. See the interpretation of these indices.

N0NBH Glossary🗗   SFI: 10.7cm Solar Flux 🗗 SN: Sunspot Number A-Index   K-Index X-Ray flare class that affects D region absorption 304Å: @SEM—Solar EUV Monitor on SOHO satellite. Pf - Proton flux | Ef - Electron flux (solar wind) Aurora 🗗 F region ionization 🗗 (polar zones) Bz - Magnetic field ↑ to ecliptic plane🗗 SW - Solar Wind speed km/s   Aur Lat - The lowest Aurora Latitude Calculated by NOAA 🗗   EsEU - Sporadic E Europe every ½ hour EsNA - Sporadic E N. America every ½ hour EME Deg - Earth-Moon-Earth attenuation every ½ hour   MUF (MHz) every 15 min MS—Meteor Scatter Activity every 15 min   GeoMag—calculated from K-Index every 3 hours. Sig Noise lvl—Background noise in S-units due to geomagnetic activity, calculated every ½ hour Current Solar Image Choose one of four EUV wavelengths, each associated with a different color of the sun disc.

Figure 11.3: Propagation conditions indices

Propagation indices displayed with views of the Sun and Earth
Figure 11.4: Solar image at 304Ångstrom	Figure 11.5: Earth view from the Moon

  11.2 Solar Indices🗗

Extreme Ultra Violet (EUV) radiation 🗗 creates the ionosphere, especially the F2-region. Since EUV is fully absorbed by the ionosphere, it doesn't reach the ground, making direct measurement impossible for ground-based devices. Before the space age, scientists relied on two indirect markers to gauge the ionization levels of the F2-region. These are the "Solar Indices":

1. SSN - Sunspot Number is a count of the number of dark spots seen on the sun.
   Higher SSN values correlate with improved conditions on 14 MHz band and above:.

See the recent SSN values.

2. SFI - Solar flux index refers to the intensity of solar radio emissions at 10.7 cm (2,800 MHz).🗗
   Higher flux correlates with increased ionization levels of the E and F regions, enhancing HF radio propagation conditions.
   The current SFI: Loading solar flux data... (solar flux units; 1sfu=10^-22 Watts per meter² per Hz).
 
3. 304Å Index measures the solar radiation strength at 304 Ångstrom (30.4 nm) EUV, emitted primarily by ionized helium in the sun's photosphere. This parameter has two measurements: one from the EVE instrument 🗗 on the Solar Dynamics Observatory (SDO) and the other from SEM instrument on the SOHO satellite 🗗. It accounts for about half of the ionization of the F region in the ionosphere and loosely correlates to the SFI. The background SFI level is typically around 134 at solar minimums and can exceed 200 or more at solar maxima. It is updated hourly.
 
4. Solar X-ray flares (1–8 Ångstrom) are measured by instruments onboard GOES satellites 🗗.
   Excessive X-ray flares can cause ionization at the D region, leading to communication disruptions and blackouts.

Understanding the Correlation between Sunspots and Solar Flux:

• Sunspot number records have been traced back to the 17th century but are often subject to interpretation. The solar flux at 10.7 cm wavelength (2,800 MHz) aligns closely with daily sunspot numbers, making both databases interchangeable.
• See a comparison table between SSN and SFI.
• The 10.7 cm Solar Flux data is more stable and reliable 🗗 compared to the Sunspot Number (SSN).
• Radio telescopes in Ottawa (from February 14, 1947, to May 31, 1991) and Penticton, British Columbia (since June 1, 1991), report solar flux density at 2,800 MHz daily at local noon (1700 GMT in Ottawa and 2000 GMT in Penticton). Corrections are made for factors like antenna gain, air absorption, solar bursts in progress, and background sky temperature.
• Due to variations in solar radiation globally, even with corrections, consistent results are challenging. Thus, readings from the Penticton Radio Observatory in British Columbia, Canada, are used as a benchmark. These numbers are crucial for predicting ionospheric radio propagation.
• The 10.7 cm radio flux consists of contributions from the undisturbed solar surface, active regions, and transient enhancements above the daily level. Levels are determined and corrected within a few percent.

  11.3 Geomagnetic Indices

Geomagnetic indices🗗 measure disturbances in Earth's magnetic field 🗗, which can disrupt HF propagation by increasing atmospheric noise and weakening radio signals. These indices are crucial for understanding the potential impacts on all communication systems, satellite operations, and even power grids.

K and A are local indices

K-index 🗗: This index represents short-term (3-hour) geomagnetic activity at a specific geomagnetic station. It quantifies disturbances in Earth’s horizontal magnetic field by comparing geomagnetic fluctuations, measured by a magnetometer🗗, to a quiet day. The K-scale is logarithmic, allowing for a more manageable representation of the wide range of geomagnetic activity magnitudes.

A-index: This index averages K values to provide a linearized view of geomagnetic activity. It is important for predicting and understanding the effects of geomagnetic storms on HF communications.

Kp and Ap are global—planetary indices.

K and A indices measure local geomagnetic activity at a single observatory. A global average of these indices is calculated from 13 mid-latitude geomagnetic observatories, marked as K_p and A_p:

• K_p: Average of K-indices from 13 observatories, indicating planetary geomagnetic activity.
• A_p: Daily planetary geomagnetic activity, derived from the Kp index.

* A comparison table between K and A indices.

* See the recent Kp and K indices.

The HPo (GFZ) indices🗗 are less commonly referenced.
This higher time resolution can be crucial for predicting and mitigating the impacts of geomagnetic storms on various technologies.

The half-hourly Hp30 and hourly Hp60, developed at GFZ (German Research Center for Geosciences)🗗, offer improved time resolutions compared to the three-hourly Kp. Together with the linear versions Ap30 and Ap60, they are collectively known as the HPo index, providing near-real-time data from about 13 geomagnetic observatories🗗.

  11.4 Skywave propagation indices🗗

HF propagation indices are essential tools for amateur radio operators to evaluate and predict radio wave propagation conditions. The key indicators include the Maximum Usable Frequency (MUF), Lowest Usable Frequency (LUF), and ionospheric noise levels. These indicators correlate with solar indices such as the Sun Spot Number (SSN), Solar Flux Index (SFI), X-ray flares, and solar wind, as well as geomagnetic indices. Understanding all these parameters is crucial for accurately estimating HF propagation conditions.

Note: The solar wind significantly influences fluctuations in the geomagnetic indices. By examining solar wind data—such as density and velocity—we can understand both the "why" and "how fast" behind these changes, allowing us to predict variations ahead of the next 3-hour K update. If you're simply determining whether the HF band is usable tonight, the local K index may suffice. However, for optimizing a specific path or timing, incorporating solar wind data becomes essential.

The global propagation indices over the recent month Figure 11.6: The recorded propagation indices over the last 30 days, provided by QRZCQ🗗 Please note the correlation between the acronyms in the title (SF, SN, AI, KI, XR) and the names of the relevant indices given below the graph: SF:=Flux index; SN:=Spot number; AI:=A index; KI:=Kp index; and XR:=X-Ray index.

Chapter 12. Solar phenomena 🗗

Solar irradiance🗗 quantifies sunlight power on a surface in watts per square meter (W/m²). On Earth, it fluctuates with location, time, and atmospheric conditions. Since 1978, space-based studies show the "solar constant" varies, influenced by cycles like the 11-year sunspot cycle. Quiet and active solar events affect space weather and HF skywave propagation.

Sub-chapters:

12.1 Quiet sun
12.2 Active Sun
12.3 Sunspots and Solar Flux
12.4 Solar storms (flares, particle events)
12.5 The Solar Cycle
12.6 Predict Solar Flux
12.7 Live Solar Activity Online
12.8 Live Solar Alerts Online (X-ray flares and solar wind protons)
12.9 Solar Radio Interference

  12.1 Quiet sun

The sun emits electromagnetic radiation 🗗 across a wide spectrum🗗 from Gama-rays to ELF (extreme long radio waves).

Figure 12.1: The solar electromagnetic spectrum, arranged left to right by wavelength from shortest to longest.

The Extreme Ultra Violet EUV 🗗 generates the ionosphere.

Figure 12.2: The EUV spectrum of the whole Sun This EUV spectrum was measured by the prototype SDO/EVE instrument flown aboard a rocket on 2008 April 14, during solar minimum between cycles 23 and 24. Ref🗗: ibid. Solar UV and X-ray spectral diagnostics, Fig. 11 on page 25 of 278.

Peak (He II) EUV radiation at a wavelength of 30.4 nm is the most important solar emission contributing to half of the Ionospheric F region ionization 🗗.

Lyman series-alpha Hydrogen-spectral-line🗗 at a wavelength of 121.6 nm ionizes Nitric Oxide (NO) at the D-region 🗗 causing mostly absorption of HF bands below 10 MHz.

12.3 Sunspots🗗 and Solar Flux🗗

• Sunspots are darker, cooler regions on the Sun's surface characterized by intense magnetic activity.
• There is a positive correlation between sunspot numbers and solar radiation intensity, including at the 10.7 cm wavelength, known as solar flux.
• Higher sunspot numbers indicate elevated solar flux levels, enhancing ionization in Earth's upper atmosphere and improving high-frequency (HF) radio wave propagation.
  Conclusion: more sunspots → higher solar flux → better HF communication.
• Sunspots vary in shape, size, and duration, lasting from a few hours to several months.
• The average number of sunspots fluctuates throughout the solar cycle, an approximate 11-year cycle of solar activity.

Left: Sunspots in visible light         Right Extreme Ultra Violet 🗗 (EUV 30.4 nm) Figure 12.3: Two images of the Sun (February 3, 2002) by Solar and Heliospheric Observatory (SOHO) satellite🗗 courtesy of European Space Agency and NASA. Q. What is the reason for analyzing sunspots in both visible and ultraviolet light? A. Observing sunspots in visible light allows us to see them directly with our eyes or telescopes. Using ultraviolet (UV) light reveals magnetic disturbances that are invisible in regular light. Studying sunspots in both visible and UV light helps us understand their features and the activities occurring on the sun.

  12.4 Solar storms (X-ray flares and particle events)🗗

For centuries, people have been observing sunspots without knowing what they are. We now understand that these are symptoms of solar storms.

Figure 12.4: Solar storms consist of solar flares 🗗 associated with CMEs🗗

Coronal Mass Ejections (CMEs) often appear as twisted ropes. Figure 12.7 presents the model connecting solar flares with CMEs.

(A) The "solar flares"🗗 are bursts of (soft X-ray and EUV, 0.1–1 nm) radiation 🗗.

Figure 12.5: A Solar flare courtesy of NOAA, May 2023

1. Solar flares enhance the ionization 🗗 of the ionosphere, specifically the D-region 🗗 at 50-90 km altitude.
2. The enhanced D region absorbs HF radio, causing radio signals to fade out. These events are known as blackouts.
3. Solar flares🗗 can last from tens of seconds to several hours.
4. Solar X-ray flares 🗗 classification: A, B, C, M, or X on a logarithmic scale.
Table 12.1: Solar flare classes

Flare Classes	B	C	M	X
Peak Irradiation 1–8 Ångstroms	< 10-6 W/m2	10-6 – 10-5 W/m2	10-5 – 10-4 W/m2	> 10-4 W/m2

5. See Table 11.2 for the correlation of flare classes with geomagnetic activity indices, and the HF band conditions.
6. See Table 11.3 for the correlation of flare classes with radio blackout scales, and the HF band conditions.
7. A link to the recent solar flares.
8. The current solar flare is .
9. The D region absorption model is used as a guide to understand fadeout events.

(B) Solar Energetic Particle Events 🗗 (CME, SEP, and SPE):

1. A coronal mass ejection (CME)🗗 is a significant ejection of plasma mass🗗 from the sun's corona into the heliosphere🗗, following solar flares. The magnetic fields of CMEs merge with the interplanetary magnetic field.
   
   Figure 12.6: LASCO C2 image 🗗, taken 8-January-2002 shows coronal mass ejection (CME)
   captured by SOlar and Heliospheric Observatory (SOHO)🗗. Credit: NASA / GSFC / SOHO / ESA

CMEs release large amounts of matter into the solar wind and interplanetary space, primarily consisting of electrons and protons.

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Coronal Mass Ejections (CMEs) occur alongside solar flares. Pre-eruption structures require magnetic energy, while post-eruption structures form magnetic flux ropes and prominences.



Figure 12.7: Model of solar flares and CMEs; enhanced diagram following Fig 1. of Shibata et al.🗗

Types of CMEs 🗗: * Halo CMEs: Appear as a halo around the Sun; often directed towards or away from Earth. * Partial Halo CMEs: CMEs: Cover part of the Sun; less impactful than full halos. * Narrow CMEs: Confined to a narrow width; less likely to impact Earth directly. * Fast CMEs: Travel faster than 500 km/s. They can cause significant geomagnetic storms. * Slow CMEs: Travel slower than 500 km/s. Generally have a lesser impact. Each type can affect Earth's magnetosphere differently, potentially causing geomagnetic storms.

Solar flares and CMEs spontaneously, disrupt the solar wind and damaging systems both near-Earth and on its surface.

The next chapter explains how space weather observations provide warnings of approaching CMEs.

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2. Solar energetic particles (SEPs) are high-energy, charged particles from the solar atmosphere and part of the solar wind. They include electrons, protons, alpha particles, and heavy ions with energies from a few tens of keV to many GeV. Solar particle events (SPEs) accelerate solar energetic particles (SEPs) either at the sites of solar flares or through shock waves generated by coronal mass ejections (CMEs). Upon reaching Earth, these high-energy particles interact with the planet's magnetosphere, influencing space weather conditions. Earth's magnetic field 🗗 guides them to the magnetic poles, causing auroras 🗗. Scott Forbush first detected SEPs as ground-level enhancements in 1942.
 
3. Solar Proton Event (SPE) occurs when the Sun emits protons that accelerate to high energies during a solar flare or coronal mass ejection (CME). These protons travel towards Earth through the solar wind or CME and are guided by interplanetary magnetic field lines.
 
4. Online report of the current solar wind heading Earth.

Sunspots, unlike flares and CMEs, are statistically predicted.
Sub-chapter 12.5 discusses the Solar Cycle.
Sub-chapter 12.6 presents long term prediction for Radio Flux at 10.7 cm.

  12.5 The Solar Cycle🗗

Sunspots change in eleven year cycles. There are many sunspots during solar maximum🗗 and few during solar minimum.

Figure 12.8: Solar Cycle: Minimum (2019) to Maximum (2024) courtesy of NASA's Goddard Space Flight Center.

Visible light images from NASA's Solar Dynamics Observatory showcase the Sun's appearance at solar minimum (left, Dec. 2019) and solar maximum (right, Aug. 2024). During solar minimum, the Sun often appears spotless. Sunspots, linked to solar activity, are used to track the solar cycle's progress.

Figure 12.9: Solar Cycle Sunspot Number Progression
Source: The International Space Environment Service (ISES)🗗

Solar magnetic flips are associated with solar maximum🗗, when the number of sunspots is near its maximum, but it is often a gradual process that can take up to 18 months. The reversal will most likely take three to four months to complete.

The sunspot cycle begins when a sunspot appears on the sun's surface at roughly 30 degrees latitude. The formation zone then travels toward the equator. At its peak intensity, the sun's global magnetic field reverses its polar regions, as if the positive and negative ends of a magnet were flipped at each of the sun's poles.

There have been 24 (11-years) solar cycles since 1749. The magnetic field of the sun totally flipped every 11 years or so. In other words, the sun's north and south poles switched places. After two reversals (22 years), the solar magnetic field returns to its former orientation. This is known as "Hale cycle".

Understanding the complex interactions between solar magnetic fields, sunspots, and the solar cycle is crucial for comprehending the Sun's dynamic behavior and its impact on Earth, specifically HF propgation conditions.

The Current 25th Cycle began in 2020. The number of sunspots observed far exceeds predictions.

July 2024 marked the peak of Solar Cycle 25, with a monthly average sunspot number of 196.5, a new high. The last time this occurred was in December 2001. Despite predictions of a similar cycle size to previous cycles, Solar Cycle 25 exceeded these expectations.

Figure 12.10: Sunspot Number progression during solar sycles 24 and 25 up to Mar 2025
Source: The International Space Environment Service (ISES)

Online chart of the recent 30-day sunspot numbers

Figure 12.11: EISN - Estimated International Sunspot Number

Figure 12.12: Solar Flux progression during solar sycle 25 up to Dec 2024
Source: The International Space Environment Service (ISES)

1. Solar Cycle Notable Events

   More than 150 years ago, the most intense geomagnetic storm was recorded on 1-2 September 1859 during solar cycle 10.
   This event is known as the Carrington Event 🗗.

   Figure 12.13: The Carrington Event

   ---

2. Sunspot cycles can vary, meaning they are not identical.
   Comparison of the recent Solar Cycles by Jan Alvestad🗗:
   The current 25th solar cycle is significantly stronger than the previous 24th cycle, but weaker than the three preceding cycles (21st-23rd).
   
   Figure 12.14: Comparison of the recent Solar Cycles

   ---

3. North-South Sunspot Asymmetries

Previous research has found north-south asymmetries for solar activity. These data point to some decoupling between the two hemispheres during the evolution of the solar cycle, which is consistent with dynamo theories🗗. So yet, only little data are available for the two hemispheres independently for the most important solar activity metric, sunspot numbers. Below see an example:


Figure 12.15: Sunspot Asymmetries

Hemispheric Sunsopt Number 1950-2021 provided by SIDC - Solar Influences Data Analysis Center, Royal Observatory of Belgium 🗗

  12.6 Predicting Solar Flux and Sunspot Number

The NOAA Space Weather Prediction Center forecasts the monthly sunspot number and 10.7 cm radio flux. The sunspot number represents the count of visible sunspots on the solar surface, while the 10.7 cm radio flux measures solar radio emission at 2,800 MHz. These predictions use a blend of observational data, analytical methods, and AI techniques.

1. A multi-year (2022-2040) forecast 🗗 of Sunspot number and 10.7 cm radio flux.
   The predicted values are based on the consensus of the Solar Cycle 24 Prediction Panel.
2. The 27-day Space Weather Outlook Table 🗗 offers numerical predictions for three important solar and geophysical measurements:
   2.1 10.7 cm Solar Radio Flux - This is a measure of solar activity.
   2.2 Planetary A Index - This indicates the level of geomagnetic activity.
   2.3 The Largest Daily K Values - These reflect the highest levels of geomagnetic disturbances each day.
3. Three Day Geomagnetic and Aurora Forecast by SolarHam 🗗 that relays data and images from various sources.

  12.7 Live Solar Activity Online🗗

Images of the solar activity at several wavelengths
17.1 nm Fe IX/X	19.5 nm Fe XII	28.4 nm Fe XIV	30.4 nm Helium II
Figure 12.16: Real-time SOHO 🗗 images at EUV by EIT (Extreme ultravioletImagingTelescope)🗗 Solar Images courtesy of NASA, Solar Data Analysis Center🗗 Click on a thumbnail to view a larger image (opens a new window). Sometimes you may see cluttered images (NASA CCD Bakeout explanation).

The Extreme Ultraviolet Imaging Telescope (EIT)🗗 aboard the SOHO spacecraft🗗 captures high-resolution images of the solar corona. The EIT detects EUV at certain wavelengths: 17.1, 19.5, and 28.4 nm (from ionized iron in the solar corona), as well as 30.4 nm (from helium). These four wavelengths reveal the intensity distribution originating from the solar chromosphere and the transition region 🗗. The average and local EUV intensity changes over time scales ranging from days to months due to the predictable solar rotation and from years to decades due to the predictable solar cycle. However, unpredictable X-ray flares can vary by orders of magnitude over time scales ranging from minutes to hours, as discussed in the following subchapter.

  12.8 Live Solar Alerts Online🗗

The extreme solar events like X-ray flares and high energy protons may affect space weather and HF radio propagation.

Links to Online reports and alerts

Solar X-ray flares 🗗 (bursts of radiation)				Solar wind (stream of particles)🗗
Recent Flares larger than C8	Current flare ASWFC 🗗	Flare forecast		Current Wind Rice univ.	Proton Flux Alert

12.9 Solar Radio Interference🗗

1. Solar flares and CMEs emit radio waves at various frequencies.
   • These emissions come in bursts.
   • These bursts disrupt space weather and interfere with communication systems.
   • The spectrum of radiation spans from a few kHz to several GHz.
   • Different sunspot cycles can produce distinct radio burst distributions, especially at 245 MHz.
   • Predicting future solar events is challenging due to gaps in data archives, leading to underestimated burst rates.
   • The temporal variations in the maximum solar radiation intensity at different frequencies, particularly at 245 MHz, help estimate the flow velocity in the solar corona during coronal mass ejections.
 
2. Solar radio emissions may indicate complex processes.

Below, see multi-frequency (VHF-SHF) radio bursts superimposed on a persistent background characterizing solar flares:


Figure 12.17: Multi-Radio-Frequency Observations of the Sun
Picture Source: Patrick McCauley Mccauley.pi, CC BY-SA 4.0; Author: Peijin Zhang 2022

  Chapter 13. Space Weather 🗗

Figure 13.1: Space Weather Environment; illustration based on ESA/A. Baker, CC BY-SA 3.0 IGO.

Space weather refers to conditions and events in space, primarily caused by solar activity affecting Earth and its environment. These include solar flare, solar wind, coronal mass ejection (CME), and geomagnetic storms, impacting HF (3-30 MHz) radio communication.

Wikipedia describes space weather🗗 as "a branch of space physics🗗 and aeronomy🗗, or heliophysics🗗, concerned with time-varying conditions within the Solar System🗗, emphasizing space surrounding the Earth."

Sub-chapters:

13.1 Space Weather Scales
13.2 Solar Wind Impact on Earth and HF Propagation
13.3 Earth's Magnetic Field Governs The Magnetosphere
13.4 What is Geomagnetic Activity?
13.5 Geomagnetic Storms
13.6 Space Weather Observations
13.7 Space Weather Reports
13.8 Geomagnetic forecast
13.9 Challenges in Geomagnetic Storm Forecasting

  13.2 Solar Wind Impact on Earth and HF Propagation

The solar wind 🗗 is the fundamental driver of space weather. It is a stream of charged particles🗗 emitted by the sun's corona into outer space. These particles interact with Earth’s magnetosphere and magnetic field 🗗, significantly affecting skywave propagation and triggering auroras 🗗 around the Earth’s poles.

Figure 13.2: The solar wind interacts with Earth’s magnetosphere.

The illustration above shows the solar wind reaching the magnetosphere, compressesing the magnetic field on the side facing the sun while elongating it on the opposite side.

The solar wind can vary greatly in speed, density, temperature, composition of the charged particles, and the interplanetary magnetic field (IMF). These variations are influenced by solar activity, such as coronal mass ejections (CMEs) or coronal holes🗗. Although predicting exact changes in the solar wind is challenging, there is some correlation with sunspots and solar flares. The solar wind can reach Earth within 20 to 30 minutes after a solar storm begins (relativistic electrons) and up to four days later (heavier charged particles).

The Interplanetary Magnetic Field (IMF) 🗗 extends the Sun's magnetic field into space, carried by the solar wind. It interacts with Earth's magnetosphere, affecting geomagnetic storms and auroras 🗗.

Figure 13.3: The Heliospheric Current Sheet (HCS) 🗗

The IMF originates from the Sun's corona, forming a three-dimensional plasma spiral due to the Sun's rotation, known as the Parker spiral🗗. It has radial and azimuthal components and a sector structure where the magnetic field direction can switch. Figure 13.21 shows the current prediction of plasma density and radial velocity.

  13.3 Earth's Magnetic Field Governs The Magnetosphere

Earth's magnetic field 🗗 governs the magnetosphere 🗗, the region enveloping our planet. This field protects us from the adverse effects of solar particles, X-ray flares, and cosmic radiation, all of which influence geomagnetic activity and, in turn, significantly impact skywave propagation. The strength of the magnetic field is measured in units of Gauss (G) or Tesla (T)🗗.

Figure 13.4: Earth's Magnetic field 🗗—the geomagnetic field. The orientation of Earth’s magnetic field is composed of two variables: 1. Earth's axis is tilted 23.5° to the ecliptic plane🗗 2. Earth's magnetic field is tilted 11° relative to the Earth's axis.

Figure 13.5: The magnetosphere is a "magnetic bubble" that surrounds Earth. Its shape depends on the solar wind and the orientation of the Earth’s magnetic field. Click on the figure above for additional explanations.

13.4 Geomagnetic Activity

Earth’s magnetic field is always fluctuating.

Geomagnetic activity refers to solar-induced disturbances in the Earth's magnetic field. Table 11.2 illustrates the relationship between solar activity, global geomagnetic activity (instability), and HF propagation conditions. Table 11.3 depicts the correlation between high solar activity and HF propagation conditions. Geomagnetic disturbances range from minor fluctuations to major geomagnetic storms 🗗, which are frequently associated with auroras🗗.

Auroras in polar zones result from interactions between charged solar wind particles and Earth's magnetic field, creating the glowing auroras. These interactions enhance ionization of the D-region, disrupting HF radio communications.

The following public domain images show auroras near the polar regions, known as the Northern Lights (Aurora Borealis) and Southern Lights (Aurora Australis).

Figure 13.6: Rare Red Aurora caused by oxygen at altitudes above 150 km.

Figure 13.7: Green Aurora caused by oxygen at altitudes of about 100 to 150 km.

Figure 13.8: A horizontal view of colorful auroras.
Purple and Blue caused by nitrogen molecules at lower altitudes of 90 to 100 km.

13.5 Geomagnetic Storms

Geomagnetic storms 🗗 are significant disturbances in Earth’s magnetosphere caused by solar wind shock waves or coronal mass ejections (CME).

1. Geomagnetic storms are more frequent during periods of high solar activity.
2. These storms occur one to four days after a CME, triggering auroras 🗗.

What causes geomagnetic storms?

Solar magnetic storms trigger geomagnetic storms, as illustrated in figure 13.9 below.

Figure 13.9: Interaction Between Earth's Magnetosphere and Solar Activity
When a CME enters the magnetosphere, it causes a Geomagnetic Storm

Geomagnetic Storm Dynamics

Figure 13.10: Geomagnetic Storm Dynamics based on Kakioka Magnetic Observatory, Japan 🗗
This is a typical morphology of sudden-commencement type magnetic storms (horizontal force variation).

A geomagnetic storm has three phases: initial, main, and recovery. The initial phase involves an increase in the Disturbance Storm Time (Dst) index 🗗 by 20 to 50 nano-Tesla (nT) in tens of minutes. The Dst index estimates the globally averaged change of the horizontal component of the Earth's magnetic field 🗗 at the magnetic equator based on measurements from terrestrial magnetometer stations 🗗. Dst is computed once per hour and reported in near-real-time.

13.6 Space Weather Observations Monitoring space weather involves a combination of space observations, ground-based measurements, and computer models: Space observatories: Satellites play a crucial role in predicting space weather and its impact on HF radio propagation:   ACE 🗗 (Advanced Composition Explorer): Positioned at L1 Lagrange point, provides real-time data on solar wind 🗗 and geomagnetic storms, giving up to an hour's advance warning of space weather events that can impact Earth.   GOES 🗗 (Geostationary Operational Environmental Satellites): Tracks solar flares and other space weather phenomena, aiding in timely alerts and mitigating potential impacts on HF propagation and space technology🗗.   DSCOVR 🗗 (Deep Space Climate Observatory): Positioned at L1 Lagrange point, monitors real-time solar wind, providing early warnings for geomagnetic storms. Relevant Science Focus Areas: 1. Solar wind activity. 2. Reflected and emitted radiation from the entire sunlit face of the Earth. 3. Ozone and aerosol amounts, cloud height and phase, vegetation properties, hotspot land properties and UV radiation estimates at Earth's surface.   SDO 🗗 (Solar Dynamics Observatory): Delivers detailed images of the sun divided into four spectral bands.   SOHO 🗗 (Solar and Heliospheric Observatory): Positioned at L1 Lagrange point, monitors solar activity and space weather.   STEREO 🗗 (Solar and Terrestrial Relations Observatory): Consists of STEREO-A (Ahead) and STEREO-B (Behind), which orbit the Sun near the stable Lagrange Points L4 and L5 🗗 to provide a 3D view of solar phenomena from multiple perspectives.   The Parker Solar Probe significantly contributes to the prediction of space weather. By flying closer to the Sun than any previous spacecraft, it collects unprecedented data on the solar wind and the Sun’s corona. **Note: The satellites SOHO, ACE, and DSCOVR, monitor the hazardous Coronal Mass Ejections (CMEs) at the L1 Lagrange point. Figure 13.11: Monitoring Space Weather The Lagrange Mission monitors hazardous CME headed toward Earth; A modified illustration based on ESA/A. Baker, CC BY-SA 3.0 IGO; AGU - Advanced Earth and Space Science On the right side (of the above picture), you may see an illustration of the Magnetosphere 🗗, which protects Earth from Solar Wind. The magnetosphere is a part of a dynamic, interconnected system that responds to solar, planetary, and interstellar conditions. It is disturbed when solar wind interacts with the space environment surrounding Earth. The Lagrange point L1🗗 allows a satellite to maintain a constant line with Earth as it orbits the Sun. Figure 13.12: A satellite trapped at the L1 point 🗗 of the Sun-Earth-Moon gravitational system. Published by Space Weather Live🗗 Ground-based observatories:   Ionosondes 🗗 measure the ionosphere’s electron density profile by transmitting radio waves and analyzing the returned signals. They help determine the ionospheric regions’ height and density, crucial for predicting HF radio wave propagation.   Terrestrial magnetometers 🗗 measure geomagnetic fluctuations, providing data on the Earth’s magnetic field. They help monitor geomagnetic storms and disturbances that can affect HF propagation by altering the ionosphere’s structure. See examples of terrestrial magnetomeres🗗.   Radio telescopes detect solar radio emissions, which can indicate solar flares and other disturbances. By monitoring these emissions, scientists can predict space weather events that might impact HF radio communication. Ground-based observatories, combined with satellite data, provide a comprehensive picture of space weather conditions affecting HF propagation.   13.7 Space Weather Reports For example see bellow seven online reports: Space Weather Nowcast by Serge Y. Stroobandt, ON4AA, will open a new window The current global / planetary Kp index Europen Space Weather Service The recent 3 days of Space Waether R-S-G Scales NOAA SWPC services The current K-index in Australia Austrlian Space Weather Service The recent 8-day UK — K indices and the "global" Kp British Geogolgical Survey The recent geomagnetic activity over the United States NOAA The current Solar Wind and Interplanetary Magnetic Field Rice Space Institute Kp index Nowcast provided online by The GFZ German Research Centre for Geosciences🗗 Figure 13.13: Kp index online overview The recent 3 days of Space Waether R-S-G Scales provided by NOAA SWPC services: Figure 13.14: Space weather online overview   The K-index at different regions vs Kp 🗗 Real-time K index near Australia provided by ASWFC🗗 Real-time Australia K index Figure 13.15: The current Australia K index map     The recent 8-day UK K indices and the "global" Kp provided online by British Geogolgical Survey🗗   The global Kp Figure 13.16: The recent K index map over the UK The recent geomagnetic activity over the United States K-indices and the "planetary" Kp provided online by NOAA, SWPC🗗 🗗 based on US Geomagnetic Observatories🗗: Boulder, Colorado Fredericksburg, Maryland College, Alaska Figure 13.17: 3-hour K-indices for the last 7 days over the US The last 30 days A-indices over the US provided online by NOAA Figure 13.18: Daily A-indices for the last 30 days over the US The Rice Space Institute’s provides the current solar wind🗗 data and the interplanetary magnetic field as measured by ACE 🗗. Figure 13.19: Online report of the Solar wind Speed x100 km per second Density of charged energetic particles per unit volume Pressure The force per unit area required to stop the solar wind flow; nP = nano pascals Proton's Temperature The background color reflects magnetosphere and ionosphere's status: no disruptions, potential disruptions, and severe disruptions. Figure 13.20: Online report of the Interplanetary Magnetic Field (IMF) 🗗 as measured by ACE 🗗 magnetometer🗗. Nano Tesla Potential danger to high altitude aircraft in the polar regions Impact on Magnetosphere Interactions Voltage Across the Polar Cap x10 Kv     13.8 Geomagnetic Forecast Forecasting 🗗 geomagnetic activity relies on solar and space weather observations. It is crucial for protecting power grids, communication systems, and satellites from solar storms. Knowing upcoming geomagnetic activity can help radio amateurs plan their operations effectively. Geomagnetic Warnings and Alerts 🗗     provided online by ASWPC. See below two products provided online by NOAA SWPC: Geomagnetic Activity Forecast 🗗 provided online by NOAA SWPC🗗 Ap Index: Daily global geomagnetic activity, derived from the Kp index. Geomagnetic Activity % probabilities: Observed today | Estimated 24 hours | Predicted 48 hours Kp Index Forecast: Predicts geomagnetic activity every 3 hours. This product helps predict space weather impacts on Earth, such as disruptions to communication and navigation systems. Prediction of Plasma Density and Radial Velocity 🗗 Figure 13.21: Two polar plots around the sun, provided online by NOAA SWPC: Plasma density (particles per cubic centimeter: r²×N/cm⁻³) and Radial velocity (km/s). This illustration depicts NOAA's prediction of plasma density and radial velocity from a CME originating from the Sun. The left panels (ecliptic plane and meridional slice) show spatial distribution, while the right panels show time series data for Earth and STEREO A 🗗. It may help us understand the impact of space weather on Earth. The spatial distribution plot shows the Sun as a yellow dot, Earth as a green dot, and STEREO A as a red dot. The ecliptic plane 🗗 (left vane circle) is the imaginary flat surface along which the Earth and other planets orbit the Sun. It demonstrates the plasma spreading around the Sun over time, allowing us to estimate the consequences of space weather on Earth. The meridional slice (in the middle) that intersects the Earth provides a 'side' view of the solar wind structures as they approach the planet. The Space Weather Forecast Center employs WSA-Enlil, a large-scale heliospheric model. It issues one-to-four-day warnings about solar wind structures and Earth-directed CMEs, which cause geomagnetic storms. Solar disturbances disrupt communications, harm geomagnetic systems, and jeopardize satellite operations.   13.9 Challenges in Geomagnetic Storm Forecasting Geomagnetic storm predictions are often inaccurate because only about 12% of coronal mass ejections (CMEs) actually reach Earth, leading to frequent (~88%) false warnings of potential storms🗗. Historical data shows that only a few solar storms, like the Quebec storm in 1989 and a series of storms in 2003, matched the intensity of the Carrington Event. In 2012, a powerful CME narrowly missed Earth. Physics Girl 🗗 highlighted a similar event in April 2022, where a solar storm missed Earth by just 9 days: A video clip by Dianna Cowern "Physics Girl" 🗗 Some CMEs exhibit a consistent magnetic field direction, while most show changing field directions during their passage over Earth. Generally, CMEs impacting Earth's magnetosphere will have an IMF orientation that favors geomagnetic storm generation at some point. The CME's ability to cause geomagnetic disruptions is determined by the magnetic structure of the embedded flux rope. However, existing forecasting capabilities are limited due to a scarcity of remote-sensing techniques for predicting CME deformation, rotation, and deflection.

Chapter 14. Radio blackouts or fadeouts 🗗

What are radio blackouts? A radio blackout or fadeout is a sudden signal loss induced by solar X-Ray flares, as explained here.

Observed fadeout now present

Flare alarm   Flare alarm

Predict possible fadeouts 24h ahead

During a blackout event, the drop in signal heavily affects the lower HF bands:

Figure 14.2: Typical Fadeout signal strength vs. time, courtesy of ASWS🗗

The current solar flare: relayed by ASWFCenter🗗 The recent week flare: X-ray flux by NOAA🗗

* The last significant radio blackout occurred on October 3, 2024 | The latest significant solar flares

Global Fadeout Reports

The D-RAP (D Region Absorption Predictions) model🗗 uses empirical relationships to calculate HF absorption based on space weather parameters. This model helps understand HF radio fadeouts and blackouts by providing graphical and textual information on global HF propagation conditions, as shown in the following figure:

Figure 14.3: The calculated LUF attenuation of skywaves (from 3 to 35 MHz) due to flares and SEP
Click on the figure to view an animation over the last eight hours.

The colors indicate absorption levels:
- Indigo-Blue → Lower frequencies (~5—7 MHz) are more affected.
- Red → Higher frequencies (~35 MHz) are less affected.

The graph on the right displays signal attenuation (dB) vs. frequency (MHz), showing how much radio signals weaken at different frequencies.

Electron density in the D region, which can vary within minutes, directly affects the LUF. At low latitudes, X-ray photons from solar flares lead to rapid fadeouts and blackouts. Solar wind particles cause longer-term polar cap absorption (PCA) events at high latitudes.

Chapter 15. Summary

Skywave propagation review

1. Global skywave communication depends on the ionosphere's ionization and operating frequency.
2. Ionospheric phenomena may be well understood, but they are not fully predictable.
3. Chaotic solar activity may affect skywave propagation conditions.
4. Today's technology enables better predictions of skywave propagation conditions.

Forecasting HF radio propagation: practical techniques

1. Use weak signal digital modes (FT8, JT65, WSPR) to probe the communication conditions.
2. Utilize PSKReporter for real-time feedback and strategy adjustments.
3. Monitor real-time MUF (Maximum Usable Frequency) charts to achieve optimal communication.
4. Stay adaptable: switch bands or modes as conditions change.

Key concepts

1. HF Radio Propagation Basics: Understanding the core principles of HF radio waves and ionosphere interactions.
2. Skywave Propagation: How do radio waves refract off the ionosphere for long-distance communication?
3. Critical Frequency: The Maximum Usable Frequency (MUF) influences communication quality.
4. Solar Effects: Solar phenomena influence radio communications by altering ionosphere behavior.
5. Solar X-Ray Flares: Communication can be impacted when the sun is directly overhead.
6. Solar Wind and Coronal Mass Ejections (CMEs): These events disturb communication conditions.
7. Solar Storms: These storms particularly affect the D-region, suddenly disrupting propagation.
8. Space weather and Geomagnetic activity: Geomagnetic storms and other space weather events alter communication reliability.
9. Radio Blackouts or Fadeouts: Sudden signal loss induced by solar flares.
10. Forecast Models🗗: Radio wave propagation relies on solar indices (SSN, SF), geomagnetic indices (K, A), operating frequency, time of day, and season.
11. Accuracy of Forecasting: Forecasting solar flares and geomagnetic storms often lacks accuracy.
12. Geospace Dynamic Models: These models are still being developed to forecast geomagnetic storms and blackouts, implicitly included in the results of ionograms.
13. Real-time charts: The most effective approach to quickly assess current propagation conditions, even though the accuracy is insufficient for professional radio services.

The essay ends prematurely, but the website updates daily.

    Last but not least:

Since only a small number of amateurs operate in the SHF and higher frequencies, commercial users have begun accessing radio amateur bands🗗. However, we have gained new narrow bands in the short, medium, and long wave ranges. While these additions may be limited, they provide new opportunities for enhancing communication without dependence on commercial infrastructure.

If you have comments, questions or requests please e-mail.

73 de Doron, 4X4XM

References   Links to external sources automatically open in a new tab.

1. This page relays online data and images from the following websites:
   1. ASWFC – Space Weather Service (SWS) | Australian Space Weather Alert System
   2. British Geological Survey
   3. DLR – German Aerospace Center
   4. ESA – The European Space Agency Network
   5. NASA Solar Data Analysis Center
   6. NOAA Space Weather Prediction Center (SWPC)—index
   7. Rice Univ. Space Institute
   8. The Royal Observatory of Belgium
   9. hamqsl.com, Paul L Herrman, N0NBH
   10. hamwaves.com, Serge Stroobandt, ON4AA
   11. HFQSO.com, HF Activity Group, Tom K5VWZ et all
   12. prop.kc2g.com, Andrew D. Rodland, KC2G
   13. hb9vqq.ch, Roland Gafner, HB9VQQ
   14. hf.dxview.org, Jon Harder, NG0E
   15. qrzcq.com, QRZCQ
   16. solen.info, Jan Alvestad, retired from FMC Kongsberg Subsea AS, Norway
    
2. Monitor HF Band Activity of Radio Amateurs ^{Real-time watching of worldwide hams' activity}

   Software-Defined Radio (SDR) is a technology where analog hardware components are replaced by software.

   1. SDR - Software Designed Radio ^Wikipedia

   SDR radios

   2. List of software-defined radios ^wikipedia
   3. BEST Software Defined Radios (SDR’s) to Buy 17-Feb-2025 ^{Ham Radio DX, VK7HH, Hayden P Honeywood}
   4. Malakhite DSP portable SDR radio receiver (Russian) ^{Russian hamforum}
   5. Malahit DSP1 and DSP2 clone receivers: A YouTube playlist featuring demonstrations and explanations ^{Doron, 4X4XM}
   6. BELKA SDR Pocket RX 10 KHz - 31 MHz: A YouTube playlist featuring demonstrations and explanations ^{Doron, 4X4XM}

   WebSDR and KiwiSDR are two worldwide networks of remote public SDR receivers:

   7. List of public WebSDR stations
      • Wideband WebSDR at the University of Twente, Enschede, NL
      • WebSDR.org background information
      • FAQ about the WebSDR project
    
   8. Map of public Kiwi SDR stations
      • List of public Kiwi SDR stations
      • Introduction to using the KiwiSDR
      • KiwiSDR design review

   Activity Charts and DX Clusters

   9. Curation of 51 DX clusters nodes @DXZone Amateur Radio Internet Guide ^DXZone
   10. DXMAPS ^{Gabriel Sampol, EA6VQ}
       A video demo of DXMaps ^{OfficialSWLchannel,}
       This website displays maps and lists of recent QSOs on various ham bands (from LF to UHF) that may indicate real-time propagation conditions.
   11. DXWatch ^{custom DX filter} Spot Search and Create Your Filter ^{DXWatch—Felipe, PY1NB}
   12. Live DX Spot Reports (auto-refreshes every 60 seconds) ^{QRZ Ham Radio}
   13. The Holy Cluster ^{Israeli Association of Radio Communication, the IARC}
   14. Real-time Ham Band Activity Map ^{Jon Harder, NG0E}
   15. Sites for Checking Signal Propagation and Band Activity ^{South Pasadena Amateur Radio Club (W6SPR)}
   16. HamDXMap : MUF, foF2, live radio frequencies weather ^{Christian Furst, F5UII}
   17. Real-time propagation and band conditions ^{QRZ online}
   18. F5LEN Webcluster ^{Pascal Grandjean, F5LEN}
   19. HA8TKS DXcluster ^HA8TKS
   20. SK6AW DXcluster | Condex ^SK6AW

   Reporters of digital modes

   21. Display Reception Reports ^PSKReporter
   22. Using PSK Reporter Website as a Propagation Tool ^eHam.net
   23. HF Signal Propagation Reporter, PSK/JT65/FT-8/CW/JT9 ^{HamRadioConcepts KJ4YZI}
    
   APRS-IS —Automatic Packet Reporting System-Internet Service
    
   24. Find Real-time Contacts, DX Cluster, Spotter Network, APRS  ^{HamRadioConcepts KJ4YZI}
   25. VHF Propagation Map ^{APRS-IS real-time radio propagation from stations operated near 144 MHz}

   WSPR - Weak Signal Communication

   26. Weak Signal Communication Software ^{Joe Taylor, K1JT}

   WSPR - Weak Signal Propagation Reporter

   27. Weak Signal Propagation Reporter ^Wikipedia
   28. WSPR - An Introduction for Beginners | WSJT-X Ham Radio ^{Ham Radio DX, 7-Jan-2022}
   29. WSPR Explained: How to Get Started With One-Way Ham Radio ^ExtremeTech
   30. Weak Signal Propagation Reporter (WSPR) | Stats ^WSPRNet
   31. WSPR Rocks —An alternative map ^VK7JJ
   32. WSPR Live: Tools for the analysis of WSPR spot data.
   33. Average propagation conditions: The recent WSPR reports on 80–10m Ham Bands up to 60 days ^{WSPR Rocks}

   Beacons

   34. NCDXF Beacon Network ^{see above}
   35. International Beacon Project ^NCDXF
   36. Beacons ^IARU
   37. International Beacon Project (IBP) ^Wikipedia
   38. Worldwide List of Beacons (1.8–28 MHz) ^RSGB
   39. High Frequency Beacons and Propagation ^VU2AWC
   40. Amateur Radio Propagation Beacon ^Wikipedia
   41. Ham Radio Beacon List ^Google
   42. Radio beacon | Radio propagation using beacons ^{HF Underground}
   43. Beacon monitoring programs ^DXZone

   Detect Changes in Propagation Conditions using RBN, WSPR, PSKR etc.

   44. Reverse Beacon Network (RBN) | History | Online Activity
   45. Reverse Beacon Network on graph online ^HA8TKS
   46. Ham Radio Reporting Networks. ^HamSCI
   47. Using the WSPR Mode for Antenna Performance Evaluation and Propagation Assessment on the 160-m Band 2022 ^{Jurgen Vanhamel et al.}
   48. Ionospheric Sounding Using Real-time Amateur Radio Reporting Networks (2014) ^{Nathaniel A. Frissell, W2NAF et al.}
   49. Reverse Beacon Networks – PSK Reporter And WSPR 2013 ^{Fred Kemmerer, AB1OC}
   50. Interpreting WSPR Data for Other Communication Modes 2013 ^{Dr. Carol F. Milazzo, KP4MD}
 
3. Electromagnetic Waves Basics ► Radio Propagation

   Basic EM wave properties

   1. Absorption ^Wikipedia
   2. Amplitude ^Wikipedia
   3. Attenuation ^Wikipedia
   4. Diffraction ^Wikipedia
   5. Dispersion ^Wikipedia | Dispersion ^{Param Himalaya}
   6. Fading / Shadowing ^Wikipedia
   7. Electric field 🗗
   8. Electromagnetic field ^Wikipedia
   9. Electromagnetic radiation ^Wikipedia
   10. Field intensity | Field strength ^Wikipedia | Signal strength in telecommunications ^Wikipedia
   11. Frequency ^Wikipedia
   12. Path Attenuation—Path Loss ^Wikipedia
   13. Polarization ^Wikipedia
   14. Power Density ^Wikipedia
   15. Radio Propagation 🗗 (see below)
   16. Ray (optics) ^Wikipedia
   17. Reflection of EM waves ^Wikipedia
   18. Refraction | Refractive index ^Wikipedia
   19. Scattering ^Wikipedia
   20. Spectrum 🗗
   21. Standing wave ^Wikipedia
   22. Wave interference | Electromagnetic interference | Interference (communication) ^Wikipedia
   23. Wave Behaviors ^{NASA Science}
   24. Wavefront ^Wikipedia
   25. Wavelength ^Wikipedia

   Electromagnetic Spectrum

   26. The Electromagnetic Spectrum spans from 3 Hz (Radio Waves) to 3x10²⁴ Hz (Gama rays) ^Wikipedia
   27. The entire radio spectrum spans from 3 Hz to 3x10¹² Hz (100,000 km to 1 mm) ^Wikipedia
   28. High Frequency (HF) 3–30 megahertz (MHz) ^Wikipedia | The shortwave radio spans from 3 MHz to 30 MHz (100 m to 10 m) ^Wikipedia

   Radio Propagation

   29. Basic Radio Wave Propagation (PPt Presentation) ^{Nor Hadzfizah Mohd Radi}
   30. Introduction to RF Propagation ^{John S. Seybold}
   31. Radio EM Wave Reflection ^{Electronics-Notes, Ian Poole}
   32. Radio Propagation Tutorial Basics ^{Electronics-Notes, Ian Poole}
   33. Radio Propagation from Extremely Low Frequency (ELF) to Far infrared (FIR) ^Wikipedia
   34. Radio Wave Propagation Fundamentals Chapter 2 ^KIT.edu

   Propagation Modes

   35. Line-of-sight propagation (LOS) ^Wikipedia
   36. Non line-of-sight propagation ^Wikipedia

   Ground Wave

   37. Ground Wave Propagation ^Wikipedia
   38. Ground Wave Propagation Tutorial ^{Electronics-Notes, Ian Poole}
   39. Ground wave MF and HF propagation ^ASWFC Part of key topics within ionospheric HF propagation
   40. Ground Wave Propagation (Tutorial) ^{BYJU’S Tuition Center}
   41. Skip zone ^Wikipedia

   Skywave / Skip

   42. Skywave or Skip Propagation ^Wikipedia
   43. Skywaves & Skip Zone ^{Electronics-Notes, Ian Poole} Key topics within ionospheric HF propagation
   44. Path length and hop length for HF sky wave and transmitting angle ^ASWFC

   Skywave Propagation Tutorials

   45. HF Propagation Tutorials & Plates ^{Hamwaves - Serge Stroobandt, ON4AA}
   46. Critical frequency ^Wikipedia
   47. Ionospheric Radio Propagation A youtube playlist ^{Doron, 4X4XM}

   Skywave Propagation Overviews

   48. The Rebirth of HF ^{Rohde & Schwarz}
   49. Course Overview: Atmospheric Effects on Electromagnetic Systems ^{Naval Postgraduate School}
   50. All-In-One Overview: There is nothing magic about propagation ^{José Nunes – CT1BOH (2021)}
   51. Overview: Understanding HF / VHF / UHF / SHF Propagation (PDF) ^{Paul L Herrman N0NBH}
   52. High Frequency Communications – An Introductory Overview - Who, What, and Why? ^{Bill Foose at HIARC meeting}
   53. Propagation of Radio Waves ^{Basu, VU2NSB} principles and methods

   Complex Propagation modes

   54. Complex propagation modes of HF sky wave ^ASWFC
   55. Atmospheric Ducting ^Wikipedia
   56. Tropospheric Ducting ^Wikipedia
    
4. Propagation via the ionosphere
           Propagation ► Refractive Index ► Ionospheric Intro ► Model ► Regions ► MUF-OWF-LUF ► Seasonal & Anomalies ► Ionosphere Probing

   Ionospheric Propagation

   1. An introduction to HF propagation (2022) ^{Sean D. Gilbert Mipre, G4UCJ}
   2. An Introduction to HF propagation and the Ionosphere (1999 - 2009) ^{Murray Greenman, ZL1BPU}
   3. HF Progagation: The Basics - QST, December 1983 ^{Denis J. Lusis, W1JL/DL}
   4. Ionization (basics) ^Wikipedia
   5. Ionosphere & Radiowave Propagation ^{Electronics-Notes, Ian Poole}
   6. Introduction to Ionospheric HF Radio Propagation ^ASWFC
   7. Ionospheric propagation Basics ^{Electronics-Notes, Ian Poole}
   8. Introduction to HF Propagation (2018) ^{Rick Fletcher, W7YP}
   9. Ionospheric Propagation ^{University of Toronto}
   10. Is HF propagation reciprocal? (2006) ^{Tomas Hood, NW7US, HFRadio.org}
   11. Propagation of radio waves explained ^{Jean-Paul Suijs, PA9X}
   12. Radio Propagation 101 - Why should you be interested in propagation? ^{Dan Vanevenhoven}
   13. Regional and Long Distance Skywave Communications ^{Ken Larson, KJ6RZ}
   14. The HF Bands for Newcomers (An Overview), ARRL (2007) ^{Gary Wescom, N0GW}
   15. The Ionosphere (D, E, and F2 regions) June 2024 ^{Andrew McColm, VK3FS}
   16. The Ionosphere Part 2 (E region is critical to 50–144 MHz) Aug 2024 ^{Andrew McColm, VK3FS}
   17. The Ionosphere, Shortwave Radio, and Propagation ^{MIT Film & Video Production club}
   18. The Effects Of The Ionosphere On Radio Wave Propagation An Excellent Presentation made more than 86 years ago!!! ^{Art Bodger}
   19. Transequatorial Radio Propagation ^CO8TW
   20. Understanding HF Propagation ^{Rohde Schwarz}
   21. Understanding HF Propagation ^{Steve Nicols, G0KYA, RSGB}
   22. Ward Silver On Radio Wave Propagation ^{Ham Radio Crash Course}

   Ionospheric Research

   23. Welcome to the Ionosphere ^{NASA Goddard}
   24. Ten Things to Know About the Ionosphere ^NASA

   Ionospheric Refractions

   25. Refractive index (Optics) ^Wikipedia The refractive index of the ionosphere
   26. Refractive Index of Ionosphere Calculator ^{Calculator A to Z}
   27. Birefringence (Optics) ^Wikipedia Double refraction due to unisotropic ionosphere.
   28. The refractive index and the absorption index of the ionosphere ^{Research notes}
   29. Ionosphere and Radio Communication ^{Saradi Bora, Kamalabaria College, North Lakhimpur, Assam, India}
       The ionospheric refractive index P.126
   30. Refractive index of ionosphere ^{Plasma Physics}
   31. Ionospheric Radio Wave Propagation ^{Richard Fitzpatrick, University of Texas at Austin}
   32. The Complex Refractive Index of the Earth's Atmosphere and Ionosphere ^{Ernest K. Smith, University of Colorado}

   Ionospheric Plasma

   33. Plasma (basics) ^Wikipedia
   34. Collisionless Plasmas ^{Science Direct}
   35. Plasma recombination ^Wikipedia

   Basic Ionospheric model

   36. Ionosphere (overview) ^Wikipedia
       The section titled "Layers of ionization" should be replaced with "Regions of ionization" and it needs additional citations for verification, as explained here.
   37. Introduction to the ionosphere ^{Anita Aikio. Dept. Physics, University of Oulu, Finland}
   38. Ionospheric Radio (book 1990) ^{Kenneth Davies}

   Additional Ionospheric models

   ITU, URSI, Google search | IRI model | Neural network model | ITM Processes | Advanced Ionospheric Models

   Ionospheric Regions

   39. A clarification: Region vs. Layer: Earth's Atmosphere and Ionosphere
   40. The Ionosphere 2014 ^UCAR
   41. Mesopause ^Wikipedia
   42. Distribution of ionospheric free-electrons ^{Bob Brown, NM7M (SK), Ph.D.}
   43. The Ionosphere and the Sun ^{Naval Postgraduate School}
   44. Ionospheric D, E, F, F₁, F₂ Regions ^{Electronics-Notes, Ian Poole}
   45. Ionospheric D-region ^Britannica
   46. Sporadic E propagation ^Wikipedia
   47. Sporadic E Propagation in 2 minutes ^{Andrew McColm, VK3FS}
   48. Sporadic E Propagation ^{Andrew McColm, VK3FS}
   49. Sporadic E propagation (E_s) ^{Andrew McColm, VK3FS}
   50. Understanding Sporadic E Propagation for VHF DX ^{Ham Radio DX}
   51. Understanding Sporadic E ^{Rohde Schwarz}

   MUF, OWF, and LUF - Explanation of the concepts; see below How is MUF determined?

   52. HF Radiation - Choosing the Right Frequency ^{Naval Postgraduate School (NPS)}
   53. MUF Maximum usable frequency ^Wikipedia
   54. FOT Frequency of optimum transmission ^Wikipedia
   55. LUF Lowest usable high frequency ^Wikipedia
   56. Critical frequency, MUF, OWF, and LUF ^{Electronics-Notes, Ian Poole}
   57. How to use Ionospheric Propagation? ^{Electronics-Notes, Ian Poole}

   Ionospheric variations

   58. Sunspot Number and critical frequencies and Time (Years and Seasons) ^ASWFC
   59. Season Rollover – Why do shortwave frequencies have to change? ^{Neale Bateman, BBC}
   60. The Seasonal Behavior of the Refractive Index of the Ionosphere over the Equatorial Region ^{Turkish Journal of Science & Technology}

   Ionospheric anomalies

   61. Persistent anomalies to the idealized ionospheric model ^Wikipedia
   62. Effect of Seasonal Anomaly or Winter on The Refractive Index of in Height of The Ionospheric F2-Peak ^{International Journal of Basic & Applied Sciences}
   63. Major upwelling and overturning in the mid-latitude F region ionosphere ^{David Hysell et all, Nature}

   See complementary references on plasma bubbles/clouds and Spread F of skywaves.

   
   Ionospheric Disturbances
   64. Sudden Ionospheric Disturbance (SID) ^Wikipedia
   65. Sudden Ionospheric Disturbance (SID) Draft: WFD (23 March 2014) ^{William Denig, National Centers for Environmental Information-NOAA}
   66. Sudden Ionospheric Disturbances An overview ^{National Centers for Environmental Information-NOAA}
   67. Sudden Ionospheric Disturbances (SIDs)
   68. Travelling Ionospheric Disturbances (TIDs), ^ASWFC
   69. Ionospheric Density Irregularities, Turbulence, and Wave Disturbances during the Solar Eclipse (2017) ^{Rezy Pradipta, Endawoke Yizengaw, and Patricia H. Doherty}
   70. Modeling Amateur Radio Soundings of the Ionospheric Response to the 2017 Great American Eclipse ^{Nathaniel A. Frissell, W2NAF et al.}
   
   Ionospheric Storms
   71. Ionospheric storm ^Wikipedia
   
   Polar Cap Absorption
   72. Polar Cap Absorption (PCA) events —Explanation ^ASWFC
   73. Polar Cap Absorption — Report ^ASWFC
   74. Polar Cap Absorption Events - Massive Short Wave Communications Blackouts ^{Windows 2 Universe}
   75. What is a Polar Cap Absorption event and what signals are affected by this ^{Official SWL channel}
   76. Detailed explanation of modelling absorption due to polar cap absorption and shortwave fadeout (2020, Geological Survey of Canada) ^{R.A.D. Fiori}
   77. Effects of polar cap absorption events on geostationary satellite VHF communications systems (1970, NOAA) ^{Pope, Joseph Horace, Leinbach, H.}
   78. The Polar Cap Absorption Effects (1962, SAO NASA) ^{D.C. Rose, Syed Ziauddin}

   Ionosphere Probing Basics

   79. Radio Techniques for Probing the Terrestrial Ionosphere (book 1989) ^{R.D. Hunsucker}
   80. Introduction To Ionospheric Sounding (2006) ^{Bruce Keevers, National Geophysical Data Center, NOAA}

   Applied Ionospheric Probing Techniques ^{Ionosondes | Ionograms | Stations | Charts}

   81. Chirping Explained - Passive Ionospheric Sounding and Ranging ^{Peter Martinez, G3PLX}
   82. Chirp reception and interpretation (2013) ^{Pieter-Tjerk de Boer, PA3FWM}
   83. Software-Defined Radio Ionospheric Chirpsounder For Hf Propagation Analysis (2010) ^{Nagaraju, Melodia (NYSU); Koski (Harris Corporation)}
   84. Small Form Factor Ionosonde Antenna Development (2014) ^{Tyler Erjavec, The Ohio State University}

   Ionosondes

   85. Ionosonde ^Wikipedia
   86. Introduction to Ionospheric Sounding for Hams ^{Dr. Terry Bullett. W0ASP - University of Colorado}
   87. Ionosonde ^{HF Underground}

   Ionograms

   88. Ionogram ^Wikipedia
   89. Understanding HF Propagation and Reading Ionograms  ^{Bootstrap Workbench}
   90. Ionogram Information ^{Hamwaves - Serge Stroobandt, ON4AA}
   91. Digisonde Directogram ^{UMass Lowell Space Science Lab website, MA, US}
   92. Mirrion 2 - Real Time Ionosonde Data Mirror ^{NCEI, NOAA}
   93. Ionogram Data Info ^{GIRO, UML}
   94. The Defence Science and Technology Group High-Fidelity, Multichannel Oblique Incidence Ionosonde (2018) ^{DOI AGU}

   Global Ionosphere Radio Observatory (GIRO)

   95. Station map | Station list | Fast Station List | Calendar list | Ionogram-Scaled Characteristics ^GIRO
   96. Station Map: Global Digisonde Stations ^{Lowell Digisonde International (LDI)}
   97. DIGISONDE®: Simultaneous Ionospheric Observations Around The Globe ^LDI

   Probing ionospheric disturbances by Auroral Radar Network

   98. Super Dual Auroral Radar Network (SuperDARN) ^Wikipedia
   99. Super Dual Auroral Radar Network (SuperDARN) ^JHU/APL
   100. First Observations of LSTID Using Automated Amateur Radio Receiving Networks (2022) ^{Nathaniel A. Frissell, W2NAF et al.}
 
5. NVIS ^{a unique mode of a skywave: real-time map, explanation}
    
   1. Understanding NVIS  ^{Rohde Schwarz}
   2. HF NVIS  ^{Military HF Radio}
   3. NVIS ^Wikipedia
   4. NVIS Propagation: Near Vertical Incidence Skywave ^{Electronics-Notes, Ian Poole}
   5. Near-Vertical Incidence Sky-Wave Propagation 36 pages Presentation for radio hams ^{Gerald Schuler, DU1GS / DL3KGS}
   6. Near Vertical Incidence Skywave (NVIS) ^{W8BYH, Fayette ARES}
   7. Near Vertical incidence Skywave Propagation NVIS Antennas  80, 60, 40m bands ^{KB9VBR Antennas}
   8. NVIS Overview  ^{David Casler, KE0OG}
   9. Ham Radio NVIS for Regional Communications  ^{Radio Prepper}
   10. NVIS - Near Vertical Incidence Skywave ^{_{What is it? advantages; antennas; links} Jim Glover, KX0U (ex WB5UDE)}
   11. Near Vertical Incidence Skywave (NVIS) ^{Ham Radio School, WØSTU}
   12. NVIS propagation ^{Dave Lawrence, VA3ORP (2007)}
   13. NVIS explained - part 1, part 2, part 3 ^NCSCOUT —NVIS explained citing these 3-parts publication ^AmRRON
   14. NVIS Antennas ^{Dale Hunt, WB6BYU}

   NVIS Extended Research Papers

   15. Radio communication via NVIS propagation: an overview ^{Telecom Sys (2017) DOI, Ben A. Witvliet, Rosa Ma Alsina-Pagès}
   16. Analysis of the Ordinary and Extraordinary Ionospheric Modes for NVIS Digital Communications Channels ^{Sensors (Basel)}
   17. NVIS HF signal propagation in ionosphere using calculus of variations ^{Geodesy and Geodynamics, Umut Sezen, Feza Arikan, Orhan Arikan}
 
6. Gray line Propagation
    
   1. An introduction to gray-line DXing ^{Rob Kalmeije}
   2. Grey Line HF Radio Propagation ^{Electronic Notes}
   3. Grey line Map ^{Doug Brandon, N6RT @ DX QSL Net}
   4. Gray line Map ^DXFUN
   5. Gray line Propagation ^G0KYA
   6. Gray-line Propagation Explained ^{Radio Hobbyist}
   7. Identifying Gray-Line Propagation Openings ^DXLab

---

 
7. Propagation Indices (Indexes) include Solar Indices and Geomagnetic Indices
           They are used as indicators of Global Propagation Conditions
    
   1. Beginner's Guide for Radio Propagation Indexes May 2024 ^{Greg Lane, N4KGL}
   2. Beginners Guide to Propagation Forecasting ^{Ed Poccia, KC2LM}
   3. Circular of Basic Indices for Ionospheric Propagation ^ITU
   4. Global Indices - Glossary of Terms ^{HamQSL, Paul L Herrman, N0NBH}
   5. Making sense of Solar Indices ^{Andrew McColm, VK3FS}
   6. Quick Guide to HF Propagation Using Solar Indices ^{W2VTM; N2LVI}
      Quick Guide to HF Propagation Using Solar Indices ^{Kingsport amateur Radio Club 2020}
   7. Understanding HF propagation reports Amateur Radio views and reviews for Beginners ^{Rich, VE2XIP}
   8. What are Solar Flux, Ap, and Kp Indices? ^{Andrew McColm, VK3FS}
   9. What exactly are the key Indicies? ^{Andrew McColm, VK3FS}

   Focus on Solar Indices

   10. Solar Index and Propagation Made Easy - HF Ham Radio Jan 2021 ^{TheSmokinApe, Steve E McGrane, KT0ADS, Luck WI}
   11. Solar induced indices: SFI, SN, A, K, Kp ^{Electronics-Notes, Ian Poole}
   12. The history of the 10.7 cm solar flux ^{Government of Canada}
   13. The 10.7 cm solar radio flux ^{K. F. Tapping, AGU}
   14. Penticton/Ottawa 2,800 MHz Solar Flux ^NOAA

   Focus on Geomagnetic Indices

   15. Planetary K-index ^{NOAA / NWS Space Weather Prediction Center}
   16. K-index – Definition & Detailed Explanation ^{Sentinel Mission}
   17. K-index ^Wikipedia
   18. Hp30 and Hp60 vs. Kp index ^{GFZ (German Research Center for Geosciences)}
 
8. Observations
               Terrestrial | Solar ► Space weather ► TEC ► MUF ► Propagation Charts
   

   Current Geomagnetic Activity measured by terrestrial magnetometer stations

   1. US Geomagnetic Observatories ^{U.S. Geological Survey (USGS)}
   2. Current Global Geomagnetic Activity ^{British Geological Survey}
   3. Recent 7 days: K and A indices by station ^NOAA
   4. K-index distribution —low, middle and high latitude ^{Space Weather Live}
   5. The Kp index ^{Space Weather Live}

   Earth Observations

   6. Deep Space Climate Observatory ^Wikipedia
   7. DSCOVR: Deep Space Climate Observatory ^NOAA
   8. Deep Space Climate Observatory (DSCOVR) ^NASA
   9. DSCOVR: Earth Polychromatic Imaging Camera (EPIC) ^NASA

   Solar Observations

   10. Views of the Sun taken by SOHO at EUV wavelengths
   11. Solar Flare Activity Explained ^ASWFC
   12. Solar Flare Forecast ^{ASWAS: Australian Space Weather Alert System}
   13. CME - Corona Mass Ejection, monitored by LASCO Chronograph ^{NOAA SWPC}
   14. Current Solar Images ^{Solar Data Analysis Center (SDAC), NASA Goddard Space Flight Center}
   15. Current Sunspot Regions ^{Space Weather Live Belgium}
   16. Solar Data Analysis Center - serves Solar Images, Solar News, Solar Data, and Solar Research ^NASA
   17. Solar Resource Page ^{Mark A. Downing, WM7D}
   18. Yohkoh Soft X-Ray Telescope (1991-2001) ^Wikipedia

   SDO

   19. SDO Mission ^{NASA - The Solar Dynamics Observatory}
   20. SDO guide ^NASA
   21. Highlights From SDO's 10 Years of Solar Observation ^NASA
   22. The Active Sun from SDO: 30.4 nm ^{NASA - The Solar Dynamics Observatory}
   23. Solar Demon Flare Detection running in real time on SDO/AIA ^{Royal Observatory of Belgium}
   24. Sun In Time ^{AIA (Atmospheric Imaging Assembly), relays SDO images courtesy of NASA}
   25. EVE Overview ^{Solar Phys. - The Solar Dynamics Observatory
       The EVE project (real-time high-resolution EUV measurements) was designed to improve understanding of the evolution of solar flares
       and extend the related mathematical models 🗗 used to analyze solar flare events.}
   26. Solar UV and X-ray spectral diagnostics ^{Giulio Del Zanna, Helen E. Mason - Living Reviews in Solar Physics (2018) 15:5}

   Solar storms and space weather

   27. Dr. Tamitha Skov - Space Weather Woman; Wikipedia, Youtube channel, facebook, Homesite
       Dr. Tamitha Skov is a space weather physicist that reviews solar storms and anlyzes how they affect spacewetaher. She specializes in forecasting and analyzing space weather processes in the heliosphere and exosphere 🗗. Her work extends to both traditional media and social platforms. As a credentialed space weather forecaster, she helps the public understand the effects of space weather.
   28. Monitor Solar Active Regions - search by date ^{Peter Thomas Gallagher, Irland}

   Space Weather Observations

   29. Current Space Weather Parameters Online report updated every 2 minutes ^{Solar Terrestrial Dispatch
       Solar Wind, X-ray flares, Auroral Storm Potential, Current Magnetic Indices}
   30. R6 Army MARS: Consolidated Solar Weather—Real-time Terrestrial indices (D-Rap, Kp and GOES)— Online report ^{Region 6 Army MARS}
   31. ACE Solar Wind in the last 24 hours—Online report ^{ACE–NOAA SWPC}

   ---

   Solar wind (particles reaching Earth measured by ACE and GOES)
   32. GOES: Geostationary Operational Environmental Satellite ^Wikipedia
   33. GOES: Geostationary Operational Environmental Satellite Network ^NASA
   34. Solar Proton Flux from 6 hours to 7 days ^{GOES—NOAA SWPC}
   35. Near-Earth solar wind forecast (EUHFORIA) provided by ESA
   36. Real-time forecast of Solar Energetic Proton Events ^{Prof. Dr. Marlon Núñez (Universidad de Málaga, Spain)}
   37. Forecasting Solar Energetic Proton events (E > 10 MeV) ^{Prof. Dr. Marlon Núñez (Universidad de Málaga, Spain)}
   Solar wind Magnetospheric Multiscale (MMS)
   Four Magnetospheric Multiscale (MMS) spacecraft, flying in a tetrahedral formation, detect charged particles and magnetic fields in space, helping scientists understand how the solar wind interacts with Earth’s magnetosphere. This mission, involving Rice University, studies magnetic reconnection, acceleration, and turbulence in space.
   38. Magnetospheric Multi Scale (MMS) See also the index of /spw/realtime ^{Rice University}
   39. Magnetic reconnection ^Wikipedia
   40. Magnetospheric Multiscale (MMS) Mission ^{National Aeronautics and Space Administration (NASA), Goddard Space Flight Center}
   41. Magnetospheric Multiscale Mission ^Wikipedia

   Index of NOAA images

   42. Space weather prediction center: index of images ^NOAA

   Recent Days Geomagnetic Indices

   43. Recent 3 days: X-ray, proton flux, and geomagnetic activity ^NOAA
   44. Latest events (recent Solar Watch) GOES ^{Lockheed Martin Solar & Astrophysics Laboratory (LMSAL)}

   ---

   X-Ray flares—EM Radiation
   45. Space Weather Alerts, Watches and Warnings ^{SWPC, NOAA}
   46. Recent GOES X-ray Flux up to 3 days ^{GOES, NOAA SWPC}
   47. Recent flare @ 13.1 nm wavelength ^{NOAA SWPC}
   48. Solar and Heliospheric Observatory - SOHO ^{ESA & NASA}
   49. Extreme ultraviolet Imaging Telescope (EIT) aboard SOHO ^Wikipedia
   50. X-Ray Flares review of the last 3 days from SDO, SOHO, GOES, and STEREO ^{Relayed by Spaceweather-live, Belgium}

   Recent month solar observations

   51. Recent Month Sunspot Number ^{SILSO, Royal Observatory of Belgium}
   52. Recent Month Daily Sunspot Number ^{MET Malaysia}
   53. Recent Month Solar Activity Plot ^{Australian Space Weather Service}
   54. Recent month Solar and geomagnetic data—Table copied from Institute of Ionosphere, Kazakhstan ^{Solen-Jan Alvestad}

   Reviews and comparisons of past Solar observations

   55. May 2024 Solar storms ^Wikipedia
   56. Geomagnetic storms May 2024 ^Duckduckgo
   57. Solar Terrestrial Activity Reports ^{Solen-Jan Alvestad}
   58. What does the sun’s X-ray flux tell us? ^Earthsky
   59. The aurora and solar activity archive (select month and year) ^{Space Weather Live}
   60. RHESSI: The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is a retired project that watched solar flares daily from 2002 to 2023. ^NASA
   61. Notable "List of solar storms" ^Wikipedia

   ---

   Real-time TEC - Total Electron Content (calculated)

   62. Total Electron Content (TEC) ^Wikipedia
   63. TEC - Recent theories, methods and models 🗗
   64. Near-real-time TEC maps ^{ESA - Europen Space Weather Service}
   65. Animated TEC maps ^{Roland Gafner, HB9VQQ}
   66. TEC at Ionosphere Monitoring and Prediction Center ^ESA
   67. One-hour Forecast Global TEC Map ^{DLR (ESA)}
   68. Station list ^{DLR (ESA)}
   69. Archive of TEC ^{DLR (ESA)}
   70. North American TEC ^NOAA
   71. Near real-time global TEC Map ^ASWFC
   72. Global Ionosphere Map (GIM) ^SpringerLink

   Real-time Ionograms

   73. Recent ionograms (Cyprus) ^{University of Twente, Enschede, Netherlands}
   74. Animated ionograms Latest 24-Hour ^GIRO
   75. Ionosonde stations connected to NOAA ^{NGDC, NOAA}
   76. Real-time ionogram near your location ^{Hamwaves - Serge Stroobandt, ON4AA}

   Real-time MUF estimations using ionograms at different locations

   77. Ionosonde station list ^{UML - University of Massachusetts Lowell}
   78. GIRO - Instrumentaion ^{GIRO, UML}
   79. About GIRO ^{UML, Center for Atmospheric Research}
   80. Real-time foF2 - Plots for Today, Yesterday and the past 5 days (more than 100 links to Inonosonde stations)^NOAA

   HF Propagation Charts from Critical Frequency Data

   81. Current f_oF2 (NVIS) Propagation Map, updated every 15 minutes ^{Andrew D Rodland, KC2G}
   82. Current MUF 3000 km propagation map, updated every 15 minutes ^{Andrew D Rodland, KC2G}
   83. Ionospheric Maps - Current f_oF2 Plots (Global) ^ASWFC
   84. Hourly Area Predictions (HAP) Charts of selected regions ^ASWFC
   85. Current f_oF2 Plots (Asia & Australia) ^ASWFC
   86. Amateur Radio Usable HF Frequencies & Forecast refreshed every 20 minutes ^{Remarkable Technologies, Inc.}
   87. Global HF Propagation ^{Andy Smith, G7IZU}

---

9. Solar Phenomena

   Solar Radiation

   1. Sunlight ^Wikipedia
   2. Extreme Ultraviolet (EUV) ^Wikipedia
   3. Solar irradiance ^Wikipedia

   Solar Physics

   4. Solar Physics (Heliophysics) ^{Youtube playlist}
   5. Heliophysics ^Wikipedia
   6. Heliophysics ^NASA
   7. Heliophysics and amateur radio: citizen science collaborations . . . ^{Nathaniel A. Frissell, W2NAF et al.}
   8. Heliosphere ^Wikipedia
   9. Chromosphere ^Wikipedia
   10. Solar transition region ^Wikipedia
   11. Transition region ^NASA

   Solar Wind

   12. Solar Wind Phenomena ^NOAA
   13. Solar Wind ^Wikipedia

   Active Sun

   14. Overview of Solar phenomena ^Wikipedia - Sunspots (Solar Cycle), flux (SF), solar wind, particle events, flares, CME
   15. Links to types of Solar storms ^Wikipedia

   Sunspots

   16. Sunspots ^Wikipedia
   17. Sunspot Number ^ASWFC
   18. The Lifetime of a Sunspot Group ^ASWFC
   19. Effective sunspot number: A tool for ionospheric mapping and modelling ^{URSI General Assembly 2008}

   Coronal Holes

   20. Coronal hole ^Wikipedia
   21. Coronal Holes ^NOAA
   22. What is a Coronal Hole? ^ASWFC

   Solar Cycle

   23. Carrington Event ^Wikipedia
   24. Solar Cycle ^Wikipedia
   25. Solar Cycle ^ASWFC
   26. Solar Cycle Progression ^NOAA
   27. Solar maximum ^Wikipedia
   28. Solar minimum ^Wikipedia
   29. Progression of solar cycle 25 ^Helio4Cast
   30. Sunspot number series: latest update ^{SILSO, Royal Observatory of Belgium}
   31. The Sun Has Reached the Solar Maximum Period October 15, 2024 ^{NASA's Goddard Space Flight Center}
   32. North-South Asymetry of Monthly Hemispheric Sunspot Numbers ^{SILSO, Royal Observatory of Belgium}
   33. Understanding the Magnetic Sun ^NASA

   The Solar Dynamo

   34. The solar dynamo begins near the surface 22 May 2024 ^{Nature by Geoffrey M. Vasil et al}
   35. Evolution of Solar and Stellar Dynamo Theory ^{Paul Charbonneau & Dmitry Sokoloff @ Space Science Reviews, Springer 2023}
   36. An overlooked piece of the solar dynamo puzzle 2019 ^HZDR
   37. The Solar Dynamo: The Physical Basis of the Solar Cycle and the Sun’s Magnetic Field 2017 ^{Credible Hulk}
   38. The Solar Dynamo: Toroidal and Radial Magnetic Fields 2008 ^NASA
   39. The Solar Dynamo: Plasma Flows ^{Tom Bridgman @ NASA, August 19, 2008}
   40. The Sun as a Dynamo ^{High Altitude Observatory @ NCAR (2001-2008)}

   Solar Radiation Storms

     Solar X-ray Flares
   41. Classification of X-ray solar flares or solar flare alphabet soup ^{Spaceweather.Com}
   42. Radio blackouts R-scale ^NOAA
   43. Flare class table ^ASWFC
   44. Solar flare ^Wikipedia
   45. Solar flares (radio blackouts) ^{NOAA SWPC}
   46. Solar Radiation Storm ^{NOAA SWPC}
   47. Solar Radiation Storm ^{Space Weather Live}
   48. Understanding how solar flares affect radio communications ^{Barrett Communications, Australia}
   49. Hot-plasma ejections associated with compact-loop solar flares ^{Kazunari Shibata et al. Astrophys. J. Lett. 451 L83 (1995)}
   50. X-ray and gamma-ray emission of solar flares 2019 ^{Alexandra Lysenko, Dmitry Frederiks, Rafail L. Aptekar}
    
     Solar Particle Events
   51. Solar Particle Event (SPE) ^Wikipedia
   52. Solar energetic particles (SEP) ^Wikipedia
   53. Solar Radiation Stroms S-scale ^NOAA
   54. Solar Proton Events Affecting the Earth Environment: Historical list, 1976 - present ^NASA
   55. Next-Generation Solar Proton Monitors for Space Weather ^Eos
   56. The Difference Between CMEs and solar flares ^NASA

   CME

   57. What is Coronal Mass Ejection ^Wikipedia
   58. Coronal Mass Ejections - CME ^NOAA
   59. Coronal mass ejection orientation ^{Google search}

   Particle Precipitation

   60. Particle Precipitation ^{ScienceDirect}
   61. Particle Precipitation in the Earth and Other Planetary Systems: Sources and Impacts ^Frontiers
   62. Energetic particle precipitation ^{Laboratory for Atmospheric and Space Physics, Univ. of Colorado}

   Solar Radio Emissions

   63. Solar radio ^Wikipedia
   64. Radio bursts from the Sun ^Wikipedia
   65. The Effect of Solar Radio Bursts on GNSS Signals (2022) ^{Xinan Yue et al., Science Direct}
   66. Solar radio emission as a disturbance of radio mobile networks at 2.6GHz (June 2022) ^{Giuliano Muratore, Teresa Giannini & Davide Micheli}
   67. Solar Radio Burst Statistics and Implications for Space Weather Effects at 8 frequencies:
       245; 410; 610; 1,415; 2,696; 4,995; 8,800; and 15,400 MHz (2017) ^{O. D. Giersch, J. Kennewell, M. Lynch}
   68. Radio ’screams’ from the Sun (below 10 MHz) warn of radiation storms (2007) ^{Bill Steigerwald, NASA; Bernhard Fleck; ESA}
   69. Distributions of Radio emissions at 245 MHz during flares (2005) ^{Yury Yasyukevich, Researchgate}
   70. Multi-wavelength analysis of CME-driven shock and Type II solar radio burst band-splitting (2001) ^{Shirsh Lata Soni, et al}
   71. An analysis of solar noise outbursts and their application to space communication (1971) ^{Marion Francis Moen}

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10. Space weather phenomena
          ^{Definition & concepts | Impact on HF radio Propagation | Geomagnetic storms & Aurora | Prediction | SID TID | Space Weather Agencies & Services}
    
    Space Weather Definition and Concepts
    1. Space Weather ^Wikipedia
    2. Five Questions About Space Weather and Its Effects on Earth, Answered ^NASA
    3. What Is Space Weather? ^NOAA
    4. What is Space Weather? ^ASWFC
    5. Solar-terrestrial science ^{Canadian Space Agency}
    6. Definition of Space Weather ^{Instituto Nacional de Pesquisas Espaciais (INPE), Brazil}
    7. Answering five key questions about space weather ^NASA
    8. Space Weather ^{Naval Postgraduate School}
    9. Space Weather Highlights ^AGU
    10. Space Weather Scales Explanation | PDF format ^NOAA
    11. The Space Weather Forecast Explained ^{British Geological Survey}
    12. Solar storms: a new challenge on the horizon? November 2023 ^{Counsel of the European Union}
    13. A Media Primer for the Solar Cycle and Space Weather ^NESDIS
     
    The Impact of Space Weather on Radio Communication
    14. Space Weather Indices * Comparison tables * ^ASWFC
    15. How does Space Weather impact HF radio communication? ^NOAA
    16. Space Weather and Radio Communications ^ASWFC
    17. Ionospheric conditions - Space Weather ^{Space Weather Canada}
    18. Mastering HF Communication: Decoding Space Weather Data August 2023 ^{Chris, N6CTA}
        The article explains how radio amateurs can use real-time space weather data to optimize HF communication
    19. The Sun and HF radio propagation ^{Electronic Notes}
    20. Space Weather and Propagation (A presentation 2019) ^{Martin Buehring, KB4MG}
    21. Solar Activity and HF Propagation (A presentation) ^{Paul Harden, NA5N © QRP-ARCI – 2005}
    22. Ionospheric Disturbances and Their Impacts on HF Radio Wave Propagation ^URSI
    23. Effect of magnetic storms (substorms) on HF propagation: A review ^{D. V. Blagoveshchenskii}

    Geomagnetic storms

    24. Geomagnetic storms ^Wikipedia
    25. Geomagnetic storms ^NOAA
    
    The Impact of Geomagnetic storms on Radio Communication
    26. Geomagnetic storm G-scale ^NOAA
    27. Geomagnetic storms ^{Maine Emergency Management Agency}
    28. A presentation: Solar Activity and HF Propagation ^{Paul Harden, NA5N © QRP-ARCI – 2005} Pages 85-88 focus on the impact of geomagnetic storms on HF propagation
    29. The impact of geomagnetic storms on HF propagation ^{Bing Search}
    30. Space weather impact on radio wave propagation Feb 2023 ^{Norbert Jakowski, German Aerospace Center (DLR), Institute for Solar-Terrestrial Physics}
    31. Monitoring and forecasting of ionospheric space weather - effects of geomagnetic storms 2002 ^{J. Lastovicka, Institute of Atmospheric Physics, Czech Republic}
    32. Effect of magnetic storms (substorms) on HF propagation: A review ^{D. V. Blagoveshchenskii, Geomagnetism 🗗 and Aeronomy volume 53, pages 409–423 (July 2013)}
    33. Effect of Weak Magnetic Storms on the Propagation of HF Radio Waves ^{Kurkin, V. I. ; Polekh, N. M. ; Zolotukhina, N. A. (Feb 2022)}
    34. HF Propagation during geomagnetic storms at a low latitude station ^{Physics & Astronomy International Journal 2020}
    35. Enhanced Trans-Equatorial Propagation following Geomagnetic storms ^{Oliver P. Ferrell, Nature volume 167, pages 811–812 (1951)}
    
    Geomagnetic storms dynamics
    36. The Disturbance Storm Time (Dst) Index ^NOAA
    37. Geomagnetic storm overview ^{Kakioka Magnetic Observatory, Japan}
    38. Geomagnetically Induced Currents (GICs) in Equatorial Region ^{Jusoh M. Huzaimy}
    39. Evolution and Consequences of Coronal Mass Ejections in the Heliosphere (April 2022) ^{Wageesh Mishra, Insia}
        9 Different phases of a typical geomagnetic storm are shown. The sudden commencement, initial, main and recovery phases are characterized by a sudden rise, constant, fast decrease, and slow recovery in the horizontal components of Earth's magnetic field, respectively.
    40. Compare Geomagnetic storms during solar maximum vs. solar minimum ^{Finn Soraas}
    41. Classifying and bounding geomagnetic storms based on the SYM-H and ASY-H indices (2024) ^{Armando Collado-Villaverde, Pablo Muñoz & Consuelo Cid}
    42. Distribution and Recovery Phase of Geomagnetic Storms During Solar Cycles 23 and 24 ^{Wageesh Mishra et al}
    43. Geomagnetic storm main phase effect on the equatorial ionosphere over Ile–Ife as measured from GPS observations (2020) ^{Ayomide O. Olabode, Emmanuel A. Ariyibi}
    44. Book: Ring Current Investigations The Quest for Space Weather Prediction (2020) ^{Vania K. Jordanova, Raluca Ilie, Margaret W. Chen}
    
    Notable geomagnetic storms
    45. 5 geomagnetic storms that reshaped society ^USGS.gov
    46. High-Frequency Communications Response to Solar Activity in September 2017 as Observed by Amateur Radio Networks ^AGU
    47. Influence of 31 August – 1 September, 2019 ionospheric storm on HF 2 radio wave propagation ^{Yiyang Luo et al}
    48. Strong geomagnetic storm reaches Earth, continues through weekend May 2024 ^NOAA

    Aurora

    49. Aurora ^Wikipedia
    50. Astronomy Picture of the Day Search Results for "aurora" ^NASA
    51. Aurora ^{NOAA SWPC}
    52. The Science, Beauty, and Mystery of Auroras ^{NOAA SWPC}
    53. The Auroral E-region is a Source for Ionospheric Scintillation ^EOS
    54. Diffuse Auroral Electron and Ion Precipitation Effects on RCM-E Comparisons With Satellite Data During the 17 March 2013 Storm ^{JGR Space Physics 2019 - Chen, Lemon, Hecht, Sazykin, Wolf, Boyd, Valek}
    55. Impacts of Auroral Precipitation on HF Propagation: A Hypothetical Over-the-Horizon Radar Case Study ^{Joshua J. Ruck, David R. Themens}
    56. Auroral Propagation ^RSGB
    57. Radio Auroras ^{Ham Radio Engineering: GM8JBJ}
    58. Aurora Event Propagation ^{Gregory A Sarratt, W4DGH}
    59. Using Auroral Propagation for Ham Radio ^{Electronics notes}
    60. Auroras & Radio Propagation including Auroral Backscatter ^{Electronics notes}
    61. Aurora Prediction North Pole ^NOAA
    62. Aurora Prediction South Pole ^NOAA
    63. Tonights Static Viewline Forecast Aurora Prediction North Pole ^NOAA
    64. Tomorrow Static Viewline Forecast Aurora Prediction North Pole ^NOAA
    65. 3 Day Geomagnetic and Aurora Forecast ^{SolarHam, Kevin, VE3EN}
    66. At what Kp index can I see aurora? ^{Doron, 4X4XM}
    67. The Magnetosphere ^Wikipedia
    68. The Magnetosphere Illustaration ^{Hamwaves - Serge Stroobandt, ON4AA}
    69. Magnetospheres ^NASA

    Interplanetary magnetic field

    70. Interplanetary magnetic field IMF ^Wikipedia
    71. The Interplanetary Magnetic Field (IMF) - Sun’s magnetic field, B(t)x,y,z, Earth’s magnetosphere ^{Space Weather Live}
    72. Dynamics of the Interplanetary Gas and Magnetic Fields (1958) ^{Parker, E. N}
    73. Relating 27-Day Averages of Solar, Interplanetary Medium Parameters, and Geomagnetic Activity Proxies in Solar Cycle 24 2021
    74. Do Intrinsic Magnetic Fields Protect Planetary Atmospheres from Stellar Winds?
    75. Investigation of the relationship between geomagnetic activity and solar wind parameters based on a novel neural network (potential learning)
    76. Artist's Conception of the Heliospheric Current Sheet (HCS) 1969-1980 ^{Schatten, Wilcox, Ness, Svalgaard, Hoeksema, and Scherrer}
    77. The 3-D Shape of the HCS (1978) ^{Smith, Tsurutani, and Rosenberg}
    78. Analysis of the heliospheric current sheet’s local structure based on a magnetic model 2022 ^{D. Arrazola, J. J. Blanco and M. A. Hidalgo}
    79. The Schatten current sheet October 2024 ^{Kalman Knizhnik}

    Gama rays | See Cosmic rays

    80. Gamma-ray burst ^{Wikipedia
        Gamma-ray bursts are the most intense explosions in the universe, observed in distant galaxies, with longer-lived afterglows and longer wavelengths emitted.}
    81. Evidence of an upper ionospheric electric field perturbation correlated with a gamma ray burst (GRB), ^{Mirko Piersanti et al, November 2023
        Analyzing the October 9, 2022, gamma-ray burst (GRB221009A).}
    82. Disturbance of Geophysical Fields and the Ionosphere during a Strong Geomagnetic Storm on April 23, 2023 ^{V. V. Adushkin, A. A. Spivak et al}

    Correlation between Cosmic rays 🗗 and sunspot numbers

    83. Periodic Variations of Cosmic Ray Intensity and Solar Wind Speed to Sunspot Numbers (2020) ^{Hindawi - Collaborative work}
    84. Cosmic Ray Showers (2016) ^NOAA
    85. Cosmic Rays and the Solar Cycle (2005) ^{University of Delaware}

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11. Space weather agencies and their services
    Only national agencies that provide worldwide services in English are included.
     
    1. World Meteorological Organiztion ^WMO
    2. ISES: The International Space Environment Service; ISES 23 members
    3. A list of 14 International Service Providers ^NOAA
    4. European Space Agency - Space Weather Service (ESA)
    5. NOAA Space Weather Prediction Center (SWPC) services:
       • Real-time Space Weather Conditions ^{R-S-G, EUV 🗗, CME, Aurora, GOES Flux (X-Ray, Proton), K-index}
       • Index of NOAA SWPC services: products, experimental, images, text ^{data format}, and json ^{data format}
       • Solar Wind predicted, 3-hours to 7-days
       • 3 day forecast ^{R, S, G - Space Weather Scales}
       • Space weather products and data ^{NOAA, SWPC}
       • Geophysical Alert Message The next 24 hours based on recent indices; WWV.txt ^{NOAA Alrets}
       • National Centers for Environmental Information (NCEI) ^NOAA
    6. Space Weather Prediction Center (SWPC) ^Wikipedia
    7. American Commercial Space Weather Association of 19 companies ^ACSWA
    8. Australian Space Weather Forecasting Centre ASWFC | Alert System 2022
    9. Belgium: Solar Influence Data Analysis Center (SIDC) ^{Royal Observatory of Belgium}
    10. Brazil: The Embrace Program ^{Instituto Nacional de Pesquisas Espaciais (INPE), Brazil}
    11. Britain: (BGS; MOSWOC; UKMO) Meteorological Office | Geological Survey
    12. Canada: Space Agency (CSA) | Space Weather (SWC)
    13. Chaina: Space Environment Prediction Center (SEPC)—National Space Science Center (NSSC)
    14. Germany: The GFZ German Research Centre for Geosciences ^GFZ
    15. Japan: Space Weather Forecast National Institute of Information and Communications Technology ^{NICT, ISES, RWC}
    16. Korea: Space Weather Center ^RRA/KSWC
    17. South Africa: National Space Agency (SANSA) ^SANSA
    18. Taiwan: Space Weather Operational Office ^{Central Weather Administration (CWA)}

---

12. Forecasting and prediction

    Forecasting and Prediction of Solar Activity

    1. Predicted long term sunspot number and Radio Flux at 10.7 cm ^{NOAA / NWS Space Weather Prediction Center}
    2. 27-Day Outlook of 10.7 cm Sun Radio Flux and the Earth Geomagnetic Indices ^NOAA
    3. Solar Flare Forecast ^ASWFC
    4. Solar flare probabilities ^{SolarHam, Kevin, VE3EN}
    5. Solar Synoptic Map ^NOAA
    6. Weekly Highlights and Forecasts of Solar and Geomagnetic Activity ^NOAA
    7. Sun news activity, Solar flare, CME, Aurora ^EarthSky
    8. Flashes on the Sun Could Help Scientists Predict Solar Flares Jan 17, 2023 ^NASA
    9. We can now predict dangerous solar flares a day before they happen 30 July 2020 ^{Jonathan O’Callaghan, Newscientist}
    10. A tech-destroying solar flare could hit Earth within 100 years 16 October 2017 ^{Leah Crane, Newscientist}

    Space Weather Prediction

    11. Space weather: What is it and how is it predicted? ^SpaceCom

    Space Weather Forecasting

    12. Space weather forecast ^NOAA
    13. Radio Communications Dashboard ^{SWPC NOAA}
    14. Past, Current, and 3-day forecast of R-S-G ^ASWFC
    15. Space Weather Forecast Discussion ^{SWPC NOAA}
    16. How to Improve Space Weather Forecasting (2020) ^{Eos, AGU}
    17. How to Assess the Quality of Space Weather Forecasts? (2021) ^{Eos, AGU}
    18. HF Radio & Space Weather Dashboard ^{Ismael PELLEJERO IBAÑEZ, EA4FSI}

    Forecast Geomagnetic Activity

    19. 3-Day Geomagnetic Forecast (text) ^NOAA
    20. Importance and challenges of geomagnetic storm forecasting ^{Frontiers in Astronomy and Space Sciences}

    Geomagnetic Warnings and Alerts

    21. Geomagnetic Warning ^ASWFC
    22. Geomagnetic Alert ^ASWFC

    Low-accuracy Geomagnetic Storm Predictions

    23. Is a solar flare the same thing as a CME? ^EarthSky
    24. The Difference Between CMEs and solar flares ^NASA
    25. Solar Storms: Odds, Fractions and Percentages ^NASA
    26. Near Miss: The Solar Superstorm of July 2012 ^NASA
    27. It missed us by 9 days 2022 April, 18 ^{Dianna Cowern known as "Physics Girl"}
    28. Coronal Mass Ejections: Models and Their Observational Basis ^{P. F. Chen}

    Blackout and SID

    29. Communications blackout ^Wikipedia
    30. Radio blackouta R-scale ^NOAA
    31. The D-RAP model | Global D-Region Absorption Prediction Documentation ^{SWPC NOAA}
    32. A dynamic collection of propagation information gathered from many different sources ^{Doug Brandon, N6RT}
    33. Propagation Links ^{eHam.net Team}

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13. Tools and Applications for analysis, prediction, and forecasting HF propagation

    Apps Categories: Real-time Activity / Band Monitoring, real-time maps & Charts, Prediction Software, Mathematical models 🗗, etc.

    Online tools

    Online Activity and Band Monitoring

    Gathering information of real-time activity on the ham bands 🗗
     
    1. Real-time Ham Band Activity Map ^{Jon Harder, NG0E}
    2. Analyzing Propagation From Active DX Stations Band Activity by (1) Time of Day, (2) Continent ^DXLab
    3. Radio Propagation Maps Based on established contacts; Choose a propagation map from the menu ^{Andy Smith, G7IZU}

    Online tools, charts and raw data

    4. Propagation Data and Tools ^{HF Underground}
    5. When is the best time to make an HF contact? Propagation Prediction tools Ria's Ham Shack, 7 April 2022 ^{Ria Jairam, N2RJ}

    Real-time HF Propagation Tools

    6. HF-START - HF Simulator Targeting of All-users, Regional Telecommunications ^{NICT, Japan}
       HF-START - High Frequency Simulator Targeting for All-users’ Regional Telecommunications - is HF propagation simulator that is developed to meet the needs of space weather users for, but not limit to telecommunications: real-time info, web tools, about
    7. HF Propagation Tools ^{Hamwaves - Serge Stroobandt, ON4AA}
       Real-time online dashboard of solar activity influencing HF propagation on Earth.
    8. Real-time HF propagation space weather ^{Hamwaves - Serge Stroobandt, ON4AA}
       Real-time online dashboard of solar activity influencing HF propagation on Earth.

    Propagation Banners

    9. Add Solar-Terrestrial Data to your Website ^{HamQSL , Paul L Herrman, N0NBH}

    Real-time Maps & Charts 🗗

    10. MUF 3000 km map based on Real-time measurements ^{Andrew D Rodland, KC2G}
        * Read more about the MUF (3000 km) project
        * Read a review titled: "Developing an Open-Source HF Propagation Prediction Tool".
        Roland Gafner, HB9VQQ, provides an animated map view of the last 24 hours in 15-minute steps.
    11. HamDXMap for the DXer, radio propagation concepts ^{Christian Furst, F5UII}

    Forecasting and Prediction Software

    Forecasting Software

    12. An Open-Source IRI-based Nowcasting Tool for Ionospheric Electron Density and HF Propagation ^{Andrew D Rodland (2022 Harvard Abstracts)}
        An overview of the software and the models behind prop.kc2g.com, a website using the IRI-2016 model, conditioned on near-real-time ionosonde data, to provide global maps of MUF(3000) and foF2. While primarily designed for radio amateur use, this system is useful for nowcasting of F region ionospheric density and mesoscale low elevation HF propagation characteristics.
    13. The Advanced Stand Alone Prediction System (ASAPS) ^ASWFC
        Australian Space Weather Forecasting Centre offer three software products to predict HF propagation:
        1. GWPS - designed for HF operators working in defence and emergency services
        2. ASAPS Kernel - The Advanced Stand Alone Prediction System designed for government, defence and emergency services
        3. Consultancies - designed for industry, defence and emergency services
    14. S/N HF Propagation Forecast Calculator for the current month ^DL0NOT

    Prediction Software

    "Proppy"
    15. Proppy Online - HF Propagation Prediction ^{James Watson, M0DNS}
    16. Proppy HF Circuit Prediction: NCDXF/IARU Beacons ^{James Watson, M0DNS}
    17. Proppy HF Circuit Prediction: RadCom's monthly propagation predictions ^{James Watson, M0DNS}
    
    "DR2W"
    18. DR2W - Predict Propagation Conditions ^{DK9IP (Winfried), DH3WO (Wolfgang), DJ2BQ (Ewald), ZS1AO/DJ2HD (Mathew)}
        A Long-term forecasting cannot take into account unpredicted ionospheric and magnetic disturbances or anomalies.
    
    "VOACAP"
    19. VOACAP Primer ^{James (Jim) Coleman, KA6A}
    20. VOACAP Online Application for Ham Radio ^{Jari Perkiömäki, OH6BG / OG6G}
        VOACAP forecasts monthly average of the expected reliability with diurnal and seasonal variations.
        A Long-term forecasting cannot take into account unpredicted ionospheric and magnetic disturbances or anomalies.
    21. VOACAP Quick Guide ^{Jari Perkiömäki, OH6BG / OG6G}
    22. VOACAP Shortwave Prediction Software ^{Rob Wagner VK3BVW}
    23. How to use VOACAP - Part 1: Overview, Part 2, Part 3 ^{Jari OH6BG & OH7BG Raisa}
    24. VOACAP DX Charts ^VOACAP
    25. VOACAP Charts for RadCom ^VOACAP
    26. RadCom online Propagation Prediction Tools ^RSGB
    
    "IOCAP"
    27. Ionospheric Characterisation Analysis and Prediction tool (IOCAP) ^SANSA
    28. IOCAP Application Introduction Video ^SANSA
        The South African National Space Agency (SANSA) created i/o cap Primary Work Surface, an operational HF communication solution.
        It's a modern, user-friendly HF frequency prediction tool that's simple to use and accurate. In a software program, it blends space weather research and practical HF experience.

    Misc.

    29. DX Toolbox - Shortwave / Ham Radio / HF Radio Propagation ^{Black Cat Systems}
        This is a software application that provides a range of tools for HF radio operators, including propagation forecast based on the Solar Terrestrial Dispatch (STL) model. It also includes a real-time solar data display and a gray line map.
    30. HF Propagation (Microsoft Apps) ^{Stefan Heesch, HB9TWS}
    31. PROPHF v1.8, HF Propagation predictions ^{Christian, F6GQK}
    32. W6ELProp (2002) ^{Sheldon C. Shallon, W6EL}
        Predicts skywave propagation between any two locations on the earth on frequencies between 3 and 30 MHz
    33. HamCAP (VOACAP interface) by Alex Shovkoplyas, VE3NEA. Rated 8.93 by DxZone
    34. The Propagation Software Pages A collection of links ^AC6V

    HF Propagation Software Review

    35. Review of HF Propagation analysis & prediction programs Research Oriented ^{Luxorion, LX4SKY}
        Amateur propagation programs, accessible via the internet, provide graphical solutions and simulate ionospheric effects using near-real-time data or well-known functions, achieving high accuracy.
    36. Review of Propagation prediction programs - VOACAP-based ^{Luxorion, LX4SKY}
        VOACAP, a US government-funded HF propagation prediction engine, has been continuously improved over since the 1980s.
    37. Predicting and Monitoring Propagation ^DXLab
        * Solar terminator display and prediction - shows gray line at any specified date and time.
        * Propagation prediction - provides a graphical view of openings by frequency and time using your choice of the included VOACAP, ICEPAC, and IONCAP forecasting engines.
    38. PropView ^DXLab
        PropView forecasts LUF and MUF between two locations over a 24-hour period using VOACAP, ICEPAC, and IONCAP engines. It can specify locations via latitude/longitude entry or DXCC prefix entry. PropView can build schedules for the IARU/HF beacon network and monitor the NCDXF/IARU International Beacon Network. It interoperates with Commander and DXView for automatic monitoring and location display.
    39. RF prop, Radio Propagation & Diffraction Calculator, W6ELProp, PropView, HamCAP ^DxZone
    40. Radio Propagation Forecasting (2019) ^{Basu, VU2NSB} Beacons, VOACAP, CCIR and URSI Models

    HFTA - High Frequency Terrain Assessment

    41. Introduction to HFTA - High Frequency Terrain Assessment ^{Nashua Area Radio Society, N1FD}
    42. Operating Instructions for HFTA, Version 1.04 (2013) ^ARRL
    43. HF Terrain Analysis Using HFTA (2015) ^{Stan Gibbs, K0RV}
    44. HFTA and Take Off Angles ~ 01/20/2021 ^{RATPAC Amateur Radio}
    45. Maximizing Performance of HF Antennas with Irregular Terrain ^{Jim Breakall, WA3FET}
    46. Introduction to HFTA – high frequency terrain assessment and more | Request an Azimuthal Map ^{Tom, NS6T}

    Space weather models

    47. Mathematical Models of Space Weather ^NASA
    48. Space Weather Modeling Framework (SWMF)

    Ionospheric models

    49. Ionosphere modeling ^{Google Search}
    50. HF radio wave propagation ionosphere models ^{Google Search}
    51. Semi-Empirical ionosphere models ^{Google Search}
    52. Full Wave ionosphere models HF propagation ^{Google Search}
    53. Solar activity ionosphere models ^{Google Search}
    54. International Reference Ionosphere model (2020) ^IRI
        IRI, an international project, established a Working Group in the late 1960s to create an empirical standard model of the ionosphere. It is sponsored by the Committee on Space Research ^COSPAR and the International Union of Radio Science ^URSI
    55. The radio refractive index: its formula and refractivity data—including maps (updated May 2016) ^ITU
    56. Ionospheric Characteristics And Methods Of Basic MUF, Operational MUF AND Ray-Path Prediction updated 25 Feb 2004 ^{Recommendation ITU-R P.434-6}
    57. HF propagation prediction method updated 11 July 2001 ^{ITU-R P.533 model}
    58. Propagation Factors Affecting Frequency Sharing In HF Terrestrial Systems updated 12 Mar 2001 ^ITU
    59. Global Assimilation of Ionospheric Measurements (GAIM) model
    60. Advanced D region Ionosphere Prediction System (ADIPS)
    61. What can we expect from a HF propagation model? ^{Luxorion, LX4SKY}
        Mathematical models 🗗 and numerical procedures simulate dynamic processes in HF radio propagation, considering interactions between the Sun's and Earth's surfaces, sun, space weather, ionosphere, and atmosphere.

    Validation of models

    62. ITU-R Directory (2025) ^{ITU
        Software, Data and Validation examples for ionospheric and tropospheric radio wave propagation and radio noise}
    63. HF Propagation modeling validation for Earth to space transmission by multiple bounces links 2021 ^{S. Rougerie et al}
    64. Comparison of observed and predicted MUF(3000)F2 in the polar cap region ^{Radio Science AGU (2015)}
        Comparison of ICEPAC, VOACAP, and REC533 models reveal diurnal and seasonal variations. Summer diurnal variation is not represented by the VOACAP or ICEPAC models. REC533 surpasses VOACAP during the winter and equinox months. ICEPAC performs poorly during periods of low solar activity.
    65. Validation of High Frequency (HF) Propagation Prediction Models in the Arctic region 2014 ^{Athieno, R., Jayachandran, P. T.}
    66. Evaluation of ICEPAC model for HF propagation prediction 2016 ^{SANSA Space Science}
    67. An attempt to validate HF propagation prediction conditions over Sub-Saharan Africa 2011 ^{Mpho Tshisaphungo et al}
    68. Experimental verification of a generalized multivariate propagation model for ionospheric HF signals 1996 ^{Y. Abramovicht et al}

    Ray-tracing models

    69. Ray Tracing ionosphere models HF propagation ^{Google Search}
    70. VOACAP–Voice of America Coverage Analysis Program is a professional hf system performance prediction tool
        VOACAP predicts monthly average expected reliability, considering diurnal and seasonal variations, but does not account for unpredicted ionospheric and magnetic disturbances or anomalies.
    71. General information on the ICEPAC propagation prediction model ^{Jari Perkiömäki, OH6BG}
    72. ITUR HF Prop
        Prediction of HF circuits based on Recommendation ITU-R P.533 model - an improved (2017) point-to-point propagation prediction tool, based on an ITU engine, developed by Gwyn Williams, G4FKH.

    Ionospheric models that simulate a neural network

    73. Neural Network Ionospheric Model (NNIM)

    Hybrid ionospheric models

    74. Application of Machine Learning Techniques to HF Propagation Prediction ^{Richard Buckley, William N. Furman - Rochester, NY}

---

 
14. Supplementary references

    Our hobby

    1. Amateur Radio ^Wikipedia
       Amateur Radio, also known as Ham Radio, is a hobby involving non-commercial communication, wireless experimentation, self-training, private recreation, radiosport, contesting, and emergency communications. This activity utilizes radio transmitters and receivers.
    2. Radio Amateur ^Wikipedia
       Radio Amateur or Radio Ham is the person usualy a licensed operator who communicates with other radio amateurs on amateur radio frequencies.
    3. Amateur radio station ^Wikipedia Read about different types of stations used by amateur radio operators.
    4. History of Amateur Radio ^Wikipedia
    5. Etymology of ham radio ^Wikipedia
    6. Why is it called ham radio? ^{Field Radio}
    7. Status Summary of Radio Amateurs & Amateur Stations of The World 2000 ^{IARU (archived)}
    8. Number of radio amateurs by country from 2000 to 2022 ^{Ham Radio DX, VK7HH, Hayden P Honeywood
       The IARU officially reported worldwide figures for hams in 2000. The statistics for 2022 are an estimate based on a prior pattern of growth.}
    9. Shortwave listening (SWL) ^Wikipedia

    The Bands assigned for Radio Amateurs

    10. Nomenclature of the frequency and wavelength bands used in telecommunications (2022) ^{ITU Radiocommunication Bureau}
    11. Amateur radio frequency allocations ^Wikipedia
    12. WARC bands ^Wikipedia
    13. Authorized frequency bands ^ARRL
    14. HF bands ^RSGB
    15. Ham Radio Frequencies ^{The DXZone}
    16. Amateur Radio Band Characteristics ^{N4UJW, Ham Universe}

    Ham radio propagation websites

    17. Historical charts of past events eSFI (Solar-flux-index) and eSSN(Sunspot-number) courtesy of Andrew D Rodland, KC2G.
    18. Live Ionospheric Data ^{Paul L. Herrman, N0NBH presented by Meteorscan.com}
    19. Current global HF Score ^{HF Activity Group (hfqso.com), Tom K5VWZ—Palmetto Tech Network LLC}
    20. Sun data and propagation—The last 36 hours—The last 30 days—WSPEnet—DxCluster ^QRZCQ
    21. Solar Conditions & Ham Radio Propagation (indices) ^W5MMW
    22. SolarHam—Real-time Space Weather—Latest Solar Imagery and Alerts ^{SolarHam, Kevin, VE3EN}
    23. Live Solar Events—Radio Reflection Detection ^{Andy Smith, G7IZU}
    24. The Basics of Radio Wave Propagation ^{Edwin C. Jones, MD, PhD (AE4TM) Knoxville, TN}
        That page provides an overview of radio wave transmission processes, including a glossary of solar and propagation terms.

    Communication Modes and Techniques

    25. List of amateur radio modes ^Wikipedia

    Signal Processing and Communication

    26. Analog transmission ^Wikipedia
    27. Analog to digital converter ^Wikipedia
    28. Data communication ^Wikipedia
    29. Digital to analog converter ^Wikipedia
    30. Modulation ^Wikipedia
    31. Radio ^Wikipedia
    32. Signal transmission ^Wikipedia

    FT8

    33. FT8 ^Wikipedia
    34. FT8 Frequency Chart: Navigating the Digital Mode Landscape ^{Thehamshack, Jerry L Withers, KD7OKK}

    Digital Voice (DV)

    35. Digital Voice the Easy Way 2023 ^QST
    36. FreeDV: Open Source Amateur Digital Voice 2023^FreeDV
    37. A Guide to Digital Voice on Amateur Radio April 2021 ^{Andrew McColm, VK3FS}
    38. How to Use FreeDV Digital Voice Over HF Ham Radio Dec 2020^{Ham Radio Crash Course}
    39. Using FreeDV To Talk On Digital HF 80M Oct 2019 ^{Tech Minds}
    40. RSGB 2018 Convention lecture: FreeDV - Digital Voice for HF and other low SNR channels Sept 2019 ^RSGB
    41. Digital Voice on HF 2013 ^G4ILO
    42. Will digital voice (on HF) ever be a thing? 2018 ^{Dan, KB6NU}
    43. International Digital Audio Broadcasting Standards: Voice Coding and Amateur Radio Applications 2003 ^QEX
    44. Practical HF Digital Voice June 2000 ^{G4GUO, G4JNT , QEX}

    Automatic link establishment (ALE)

    Automatic Link Establishment (ALE) is a feature that enables a radio station to select the best frequency to establish a connection with another HF radio station or network of stations. It replaces traditional prediction techniques and reliance on trained operators.

    45. Automatic link establishment (ALE) ^Wikipedia
    46. Youtube clips about ALE:
        • ALE overview 14-June-2021 ^{Sal, 9K2GV}
        • ALE ION2G software 26-Sept-2021 ^{Ham Radio Crash Course, Josh, KI6NAZ}
        • ALE SCS P4Dragon Modem Promo 5-Oct-2021 ^Commsprepper
        • ALE SCS P4Dragon Modem Demo 6-Nov-2021 ^Commsprepper
    47. Free and paid software for ALE:
        • ALE 2G Intelligent Standards-Based HF Communications Software 2023 ^ION2G
        • PC-ALE / PCALE support site Nov 2020 ^{Steve Hajducek, N2CKH}
        • PC-ALE 1.602; Latest version Feb 22, 2012. Only trial is free!
    48. Automatic Link Establishment Overview 2018 ^{COMMS Working Group}
    49. HF Automatic Link Establishment (ALE) 2009 ^{Kingston Amateur Radio Club}
    50. ALE HF Network Ham Radio Amateur Radio 2007 ^{Bonnie Crystal, KQ6XA, HFLINK}
    51. ALE - The coming of Automatic Link Establishment, QST 1995 ^{Ronald E. Menold, AD4TB}

    Spread Spectrum

    Spread Spectrum ^Wikipedia

    Frequency-hopping spread spectrum ^Wikipedia

    Technological concepts

    52. Satellite ^Wikipedia
    53. Lagrange points (Google Search)
    54. The Lagrange Mission ^Wikipedia

    Scientific answers to radio amateurs' questions:

    55. Ham Radio Science Citizen Investigation ^{HamSCI
        HamSCI promotes collaboration between researchers and radio operators, supports the development of standards and agreements, and advances projects with the following goals: * Advance scientific research through amateur radio. * Encourage the development of new technologies. * Provide educational opportunities for amateurs and the public.}
    56. High-frequency Active Auroral Research Program HAARP ^Wikipedia
    57. Benefits of the HAARP Project for Radio Hams
    58. Study of HF Radio Propagation Using HAARP and the Ham WSPR Network (2018) ^{Citizen Space Science, Fallen}
    59. HAARP (The High Frequency Active Auroral Research Program) ^{DARA Hamvention}

    HF Propagation Research 1958-1990

    60. Basic Radio Propagation Predictions for September 1958, Three Months in Advance ^{National Bureau Of Standards}
    61. An Introduction to Ionosphere and Magnetosphere 1972 isbn: 9780521083416 ^{J. A. Ratcliffe}
        * Abstract ^IAEA * Selected pages ^{Google books} * Borrowing From The Lending Library
         
    62. Solar-Terrestrial Prediction Proceedings | Solar-Terrestrial Prediction Proceedings 1979 ^{Richard F. Donnelly, Space Environment Lab, NOAA}
    63. The Earth's Ionosphere (book 1989) Plasma Physics and Electrodynamics ^{Michael C. Kelley}
    64. Ionospheric Radio 🗗 (book 1990) ^{Kenneth Davies}

    Special articles by Bob Brown, NM7M (SK), Ph.D. U.C.Berkeley

    65. The Little Pistol's Guide to HF Propagation (1996) ^{Bob Brown}
    66. The Big Gun's Guide to Low-Band Propagation | text format (2002) ^{Bob Brown}
    67. HF Propagation Tutorial ^{Bob Brown (SK), NM7M}

    The AI tools used to improve the presentation of this website

    68. Quillbot, since September 2022
    69. Initial chats with ChatGPT 3.5, since April 2023
    70. ChatGTP 4o, since May 2024
    71. Copilot Microsoft, since June 2024
    72. Gemini Google, since November 2024
    73. Grok 3 (X.com), since December 2024
 

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15. Misc. References

    Physical concepts

    1. Physics ^Wikipedia
    2. Physical quantity ^Wikipedia
    3. Dimensional analysis ^Wikipedia
    4. Field (physics) ^Wikipedia
    5. Electric field ^Wikipedia
    6. Magnetic field ^Wikipedia
    7. Radio Waves ^NASA
    8. Signal-to-noise ratio (SNR or S/N) ^Wikipedia
    9. Flux ^Wikipedia
    10. Storm What are storms? ^Wikipedia
    11. Physical Coupling ^Wikipedia
    12. Collision frequency ^Wikipedia
    13. Collision frequency ^{Physical Chemistry}
    14. Spectroscopy ^Wikipedia
    15. Spectroscopy: A YouTube playlist featuring demonstrations and explanations ^{Doron, 4X4XM}
    16. Lyman series-alpha hydrogen radiation at a wavelength of 121.6 nm [nm = nano-meter 10^-9meter] ^Wikipedia

    Mathematical and numerical concepts

    17. Interpolation ^Wikipedia
    18. Extrapolation ^Wikipedia

    Geophysical concepts

    19. The atmosphere of Earth ^Wikipedia
    20. Exosphere ^Wikipedia
    21. Definition of Aeronomy ^UMich
    22. Earth's magnetic field ^Wikipedia Gauss (unit) ^Wikipedia Tesla (unit) ^Wikipedia

    Geomagnetism: All the aspects of the Earth's magnetic field that surrounds Earth in the form of the magnetosphere.

    23. Origin of Earth’s Magnetic Field ^Earth.com
    24. Geomagnetism - an overview ^{Science Direct}
    25. Global-GMDs: The global map of geomagnetic disturbances ^{Hongyi Hu, Zhonghua Xu}
    26. Sustaining Earth’s magnetic dynamo ^Nature
    27. Understand Earth's geomagnetic field through the dynamo effect principle (video) ^Britanica
    28. Magnetometer ^Wikipedia
    29. Magnetometers A Comprehensive Guide
    30. Magnetometry ^{D. Waller & B. E. Strauss}

    Astromonomical concepts

    31. The Solar System ^Wikipedia

    Geometrical concepts

    32. Ecplictic Plane | Plane of the Solar System ^Wikipedia
    33. Geometrical Optics ^Wikipedia
    34. Secant Trigonometry term ^Wikipedia

    Deterministic Chaos

    35. Deterministic Chaos ^{The Exploratorium, 1996}
    36. Deterministic Chaos ^{Principia Cybernetica 2000}
    37. Concepts: Chaos ^{New England Complex Systems Institute}

    HF Propagation - Novel Research and Analysis

    38. Short and long term prediction of ionospheric HF radio propagation ^{J. Mielich und J. Bremer (2010)}
    39. Spread-F occurrences and relationships with foF2 and h′F at low and mid-latitudes in China (2018) ^{Wang, Guo, Zhao, Ding & Lin (Chaina)}
    40. Long-Term Changes in Ionospheric Climate in Terms of foF2 ^{Jan Lastovicka (2022)}
    41. Ionospheric Monitoring and Modeling Applicable to Coastal and Marine Environments ^{Ljiljana R. Cander and Bruno Zolesi (2019)}
    42. Statistically analyzing the ionospheric irregularity effect on radio occultation ^{M. Li and X. Yue, Atmos. Meas. Tech., 14, 3003–3013, 2021}
    43. Analysis of Ionospheric Disturbance Response to the Heavy Rain Event ^{Jian Kong, Lulu Shan, Xiao Yan, Youkun Wang - Remote Sens. 2022, 14(3), 510}
    44. A simplified HF radio channel forecasting model ^{E.V. Moskaleva, N.Y. Zaalov, Advances in Space Research}
    45. Ionospheric current ^{Upper Atmospheric Science Division of the British Antarctic Survey}
    46. Radio Propagation Prediction for HF Communications (2018) ^{Dept. of Appl/ Physics & Tel., Midlands State Univ., Gweru, Zimbabwe}
    47. The influence of high latitude off-great circle propagation effects on HF communication systems and radiolocation ^{M. Warrington, A.J. Stocker, N. Zaalov (2002)}
    48. Analyzing the current ionospheric conditions ^{Google search}

    Recent Theories, Methods and Models

    49. Develop ionosphere computer models to enhance HF radio propagation ^{Military Aerospace 2022}
    50. Investigation of Two Prediction Models of Maximum Usable Frequency for HF Communication
        Based on Oblique- and Vertical-Incidence Sounding Data (2022) ^{atmosphere MDPI}

    ITM Processes

    General explanation, physical phenomena (plasma bubbles or clouds), Spread F of skywaves

    51. Terrestrial Atmosphere ITM (Ionosphere, Thermosphere, Mesosphere) Processes ^{NASA Visualization (2018)}
    52. Ionosonde Data 19-7-2020 ^{Larisa Goncharenko
        Stratosphere-to-ionosphere couplings; Pole-to-pole Observations; Sudden Stratospheric Warming induce global disturbances}

    Ionospheric clouds or bubbles

    53. Detection of Rapidly Moving Ionospheric Clouds ^{H. Wells, J. M. Watts, D. George (1946)}
    54. Three-dimensional simulation study of ionospheric plasma clouds ^{S. Zalesak, J. Drake, J. Huba (1990)}
    55. Nonlinear 3-D Simulations of the Gradient Drift and Secondary Kelvin–Helmholtz Instabilities in Ionospheric Plasma Clouds 2003 ^{Almarhabi et al)}
    56. Articles about "Ionospheric Plasma Bubbles" ^{Google search}
    57. Articles about "Ionospheric Plasma Clouds" ^{Google search}

    Spread F phenomenon

    58. Speard-F Definition 2024 ^Ametsoc.org
    59. Spread F Radio Propagation ^{Ian Poole, Electronics Notes}
    60. Automatic classification of spread‐F types in ionogram images using support vector machine and convolutional neural network April 2024 ^{Earth, Planets and Space}
    61. Resolution of the equatorial spread F problem: Revisited 2023 ^{J. D. Huba}
    62. Ionosonde Observations of Spread F and Spread Es at Low and Middle Latitudes during the Recovery Phase of the 7–9 September 2017 Geomagnetic Storm 2021 ^{Lehui Wei et al}
    63. Spread F - an overview | ScienceDirect Topics Book 2020, The Dynamical Ionosphere ^{ScienceDirect}
    64. Multi-station investigation of spread F over Europe during low to high solar activity J. Space Weather Space Clim. 2018 ^{Krishnendu Sekhar Paul et al}
    65. A theoretical analysis of global characteristics of spread-F 2001 ^{Zuo Xiao & Tianhua Zhang}
    66. A Review of Equatorial Spread F 1999 ^{Rick McDaniel}
    67. Spread-F theories—a review 1985 ^{Journal of Atmospheric and Terrestrial Physics}
    68. Equatorial Spread F 1962 ^{Wynne Calvert}
    69. A Survey of Spread F 1960 ^{F.N. Glover}

    Vertical Coupling (Troposphere - Ionosphere)

    70. Sprite (lightning) ^Wikipedia
    71. ICON - Ionospheric Connection Explorer ^Wikipedia
    72. Upper-atmospheric lightning ^Wikipedia
    73. Transient Luminous Events: Lightning above our atmosphere ^AccuWeather
    74. NASA ScienceCasts: Observing Lightning from the International Space Station ^NASA
    75. Severe Weather 101: Lightning Types ^NOAA
    76. Transient Luminous Events (TLEs) ^SKYbrary
    77. Investigations of the Transient Luminous Events with the small satellites, balloons and ground-based instruments ^{Safura Mirzayeva 2022 Master Thesis}
    78. Solar cycle changes to planetary wave propagation and their influence on the middle atmosphere circulation (1997) ^{Arnold & Robinson}
    79. Electrodynamical Coupling of Earth's Atmosphere and Ionosphere: An Overview (2011) ^{A. K. Singh, Devendraa Siingh, R. P. Singh, Sandhya Mishra}
    80. A review of vertical coupling in the Atmosphere-Ionosphere system:
        Effects of waves, sudden stratospheric warmings, space weather, and of solar activity (2015) ^{Erdal Yiğit, Petra Koucká Knížová, Katya Georgieva, William Ward}
    81. Electrodynamical Coupling of Earth's Atmosphere and Ionosphere: An Overview (2020) ^{Prof. Ashok K. Singh et al, University of Lucknow}
    82. A Review of Low Frequency Electromagnetic Wave Phenomena Related to Tropospheric-Ionospheric Coupling Mechanisms (2012) ^NASA
    83. TEC variations detected over southern Africa due to lightning storms ^{M M Amin, Inggs, P J Cilliers; South African National Space Agency}

    Advanced Ionospheric Models

    84. NET vs. IRI ionospheric models April 2025 ^{Review on this website}
    85. 3-D Characterization of Global Ionospheric Disturbances During the 15 January 2022 Tonga Volcanic Eruption January 2025 ^{Changzhi Zhai et al}
    86. Next-decade needs for 3-D ionosphere imaging May 2023 ^Frontiers
    87. In-space measurements could enhance high-frequency radio capabilities April 2022 ^DARPA