Radio is a type of electromagnetic (EM) energy that propagates as waves.
What is an electromagnetic (EM) wave?
An electromagnetic (EM) wave is a disturbance in electric and magnetic fields that propagates through space at the speed of light (~3×10⁸ m/s in a vacuum). These waves are generated by accelerating charges or high-frequency currents and carry energy across distances.
Figure 1.1: 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 waves can travel in different ways between a transmitter and a receiver.
See here an overview of these five wave propagation phenomena.
Figure 1.3: LightWave propagation phenomena is quite similar to Radio Wave Propagation.
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.
Additional properties of waves
Polarization: Electric field orientation.
Rays: Wave propagation direction.
Wavefronts: Surfaces of constant phase.
Field Intensity: Strength of the wave's field.
Power Density: Power per unit area.
Path Loss: Signal weakening over distance, caused by free-space loss, refraction, diffraction, reflection, aperture-medium coupling loss, and absorption. It’s also affected by terrain, environment, propagation medium, distance between transmitter and receiver, and antenna height and location.
Signal-to-Noise Ratio (SNR): A measure used to compare the level of a desired signal to the level of background noise. It's calculated as the ratio of the power of the signal to the power of the noise, often expressed in decibels (dB). The higher the SNR, the clearer and more distinguishable the signal is from the noise.
Spectrum: a property of waves describing the range of frequencies or wavelengths. Spectroscopy studies the spectra of electromagnetic radiation by measuring its wavelength or frequency with specialized equipment to understand matter's structure and properties.
The electromagnetic spectrum: Radio waves are a subset of the electromagnetic spectrum that has unique applications based on frequency and wavelength. The following display moves from long to short wavelengths, with radio waves on the left side.
Figure 1.4: The electromagnetic spectrum
The radio spectrum shown below goes from low to high frequency (long to short wavelength).
Figure 1.5: The radio spectrum divided to 11 bands
HF radio declined in the 1960s due to ever-changing Ionosphere, interference, and bandwidth limits, leading to the rise of satellite technology.
Between 1965 and 2020, satellite system issues—high costs, outages, and complex infrastructure—revived interest in HF radio. Advances like digital voice, automatic link establishment (ALE), and spread-spectrum have improved HF reliability and affordability, making it popular again for long-distance and emergency communications.
Advantages of Skywave over Satellites:
Remote Reach: Skywave covers areas without satellite access.
Infrastructure-Free: No infrastructure needed; ideal for emergencies.
How does HF radio propagate?
HF radio waves mainly propagate as skywaves, reflecting or refracting off the ionosphere, enabling long-distance communication.
For more details, see Chapter 4: HF Propagation Modes, covering line-of-sight (LOS), ground wave, and skywave propagation.
Critical Frequencies:
Higher values of foF2, MUF, and OWF suggest better HF propagation, while a higher LUF indicates communication disruptions, especially in the lower HF bands.
Ham radio activity is a reliable indicator of current band conditions. Previously, manually scanning ham bands with analog receivers was time-consuming. Today, advanced tools enable efficient global assessment of various HF bands. By combining multiple methods and tools, you can enhance your understanding of propagation conditions and ensure a more accurate assessment.
Table 2: Tools and Applications for Monitoring HF Band Conditions
The DXView map shows real-time ham band activity from the last 15 minutes, including Signal-to-Noise Ratio (SNR) levels for SSB, CW, and digital modes. This visual aid helps identify open bands and communication modes. The DXView website offers a walkthrough on interpreting the map and selecting band colors. JavaScript is required for graphics.
Figure 2.1: Real-time Ham Band Activity
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. The tool visualizes propagation paths, helping 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
Monitoring HF Propagation with WSPR, which is a weak-signal radio communication protocol used for sending and receiving low-power transmissions to test propagation paths on the ham bands. The following are useful links: WSPRnet, WSPR Rocks, WSPR Live.
Monitoring HF Propagation with FT8 is a popular digital mode that automatically decodes weak signals and provides real-time data on HF activity.
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:
Use software like WSJT-X to decode FT8 signals.
Upload your reports to PSKReporter to visualize current band conditions.
Example:
A PSKReporter chart generated by WSJT-X v2.6.1 software, illustrating global FT8 signal reception.
The following map provides near real-time data on band activity, propagation paths, and weak-signal communication conditions.
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. These stations use standardized antennas and power levels.
Figure 2.6: NCDXF beacons map
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 Watts 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.
2.4 Use various antennas at your station to assess HF propagation conditions
Using different antennas at your station helps assess HF propagation conditions by comparing received signal levels and signal-to-noise ratios. Switch between dipoles, verticals, and loop antennas to receive signals from beacons.
Observe variations in signal strength and clarity:
Monitor signal strength from various distant stations on different bands using different antennas (e.g., dipole, vertical, loop).
Compare reception: Note variations in signal strength across different antennas and bands.
Analyze signal quality: Observe signal quality (e.g., fading, noise levels) for each antenna.
Cross-reference data: Compare your observations with online propagation predictions and real-time propagation information.
Example: This activity requires hands-on experience and a basic understanding.
If you consistently receive strong signals from Europe on 20 meters with a vertical antenna, but weak signals with a dipole, it might indicate favorable vertical wave propagation conditions. Conversely, if 40 meters performs better with the dipole, it could suggest better horizontal wave propagation on that band.
By systematically observing these factors, you can gain valuable insights into current HF propagation conditions and optimize your antenna choices for specific bands and destinations.
WebSDR and KiwiSDR are popular SDR platforms that enable users to listen to radio waves using just a web browser, eliminating the need for a receiver and antenna. Both platforms support multiple users simultaneously and offer real-time spectrum and waterfall visualization. However, they differ in user interface features and specific functionalities. Each platform has unique advantages, making the choice dependent on your specific needs and preferences.
Figure 2.7: Real-time display of the entire LF–MF–HF spectrum
Alternatively, choose a remote receiver from the following WebSDR map:
Figure 2.8: WebSDR Global Map
Interactive map showing WebSDR locations worldwide. Users can select a WebSDR receiver to remotely monitor HF signals, access live waterfall displays, and tune into specific bands.
Or choose a remote receiver from the KiwiSDR global list, or click the following map:
Figure 2.9: KiwiSDR Global Map
Global map of KiwiSDR receivers, providing access to real-time HF signal monitoring. Users can select stations to explore propagation conditions and compare band activity at different geographic locations.
Chapter 3. HF Propagation Conditions: Forecasting and Prediction
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 real-time data (e.g., "Conditions will improve in the next hour").
Prediction: Long-term projections using historical trends (e.g., "Better 40-meter conditions expected next month").
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.
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.
HF radio waves directed from Earth to the ionosphere cause free electrons to oscillate and re-radiate, resulting in wave refractions with a refractive index similar to that in geometrical optics, explained here.
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 collisionless plasma region consists 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 dissipates after sunset.
The E region, located between 90 and 150 km above the Earth, dissipates a couple of hours after sunset.
This partly collision plasma region consists ions such as O2+, O+ up to 1011 electrons per cubic meter excited by solar radiation 1–10 nano-meter EUV. 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, dissipates at sunset.
The region's frequent collisions contribute to high radio wave absorption during the day. The D region absorbs and blocks radio frequencies below the lowest usable frequency (LUF) from reaching higher E and F regions. In this region, ultraviolet radiation at 121.6 nm (UVC) excites ions like Nitric Oxide (NO+) up to 1010 electrons per cubic meter. Solar flare bursts (0.1–1 nm X-ray) can produce blackouts for minutes to 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.
Figure 7.2 Typical Distributions of Free Electrons in the Ionosphere
The above graph is based on a review from U.C.Berkeley by Bob Brown Ph.D, NM7M (SK)
Why does the density of free electrons increase sharply with height between 50 km and 250 km?
The density of free electrons is affected by the balance of two opposing processes: ionization and recombination (the capture of a free electron by a positive ion).
The F region gets most of the UV radiation compared to the lower E and D regions, while the rate of electron-ion recombination is much faster in the lowest D region (due to the higher gas density). As a result, the free-electron density of the high-set F region (at noon) is significantly higher than that of the E and D regions.
At most, only one thousandth (1/1000) of the neutral atmosphere is ionized.
The following figure illustrates refractions of skywaves from the F and E ionospheric regions at various angles. The F region "reflects" HF (3–30 MHz) to longer ranges, and the E region sporadically "reflects" VHF (50–160 MHz) to midranges. The left two paths of signals demonstrate higher-frequency radio waves lost in space:
Figure 7.3: Refraction of radio waves in the ionosphere
Long-range skywave propagation typically uses a low transmission angle, which corresponds to a high incident angle.
Figure 7.4: Transmission angle (α) and incident angle (θ)
The highest MUF occurs at the lowesttransmission angle. This results in the longest range, implying the transmitted ray is almost horizontal. However, low-angle radiation below 5 degrees can be very difficult to achieve with practical antennas at frequencies lower than 30 MHz.
7.3 Skywave Multi-refractions
The ionosphere bounces skywaves in complex multiple modes
Figure 7.5: Complex skywave modes: F Skip / 1F1E, E-F Ducted, F Chordal, E-F ocasional and sporadic E. Diagram provided by ASWFC ibid Section 2.4, Fig.2.4.
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 refraction of radio waves in the ionosphere is characterized by their critical frequency. This is the highest frequency at which radio waves reflect back to Earth. Higher frequencies escape into space.
The frequencies foF2, MUF, OWF, and LUF serve as indicators for HF radio propagation conditions.
7.4.1 The Critical Frequency (foF2) is the highest frequency below which a radio wave is reflected by the F2-region at vertical incidence, independent of transmitter power.
Figure 7.6: Vertical reflection from F2 region
The critical frequency is dependent on the collision frequency of the free-electrons and their density: where fc is the critical frequency and Nmax 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.
Statistically, between 2005 and 2007, the global average critical frequency (foF2) varied from 1.8 MHz to 11 MHz, with an average of 7.5 MHz.
Day vs. Night and Geographical Locations: The critical frequecy varies with latitude and the day due to increased ionization from solar radiation. At night, the MUF decreases.
The graph below shows how the critical frequency varies with latitude during the day and night.
Figure 7.7: foF2 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.
7.4.2 The Maximum Usable Frequency (MUF) is a fascinating concept in skywave propagation. It is the highest radio frequency reflected by the ionosphere at a given incident angle (θ), independent of transmitter power. It is a highly effective indicator for forecasting HF radio propagation conditions.
7.4.3 The Optimum Working Frequency (OWF) is usually 85% of the MUF.
7.4.4 Lowest Usable Frequency(LUF) is where signal strength rapidly decreases due to daytime D-region absorption.
LUF, also known as the "absorption-limited frequency" (ALF or FL), is a soft frequency limit, unlike the sharp cut-off of the MUF. Frequencies below the LUF are absorbed by the D region during the day (It does not exist at night, there is no low frequency limit).
Figure 7.9: Daytime Lowest Usable Frequency (LUF)
The LUF is mostly determined by the D region, where there are few free electrons and negligible absorption occurs. The E and F regions contain fewer neutral atoms, resulting in fewer collisions and less energy loss from vibrating free electrons. This allows radio waves to be refracted and reflected rather than absorbed.
NVIS provides the solution for the dead zone (between groun dwave 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. courtesy of Chris, N6CTA.
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.
Figure 7.13 illustrates supplementary information that is not crucial for understanding skywave propagation.
The ionospheric physical conditions are: temperature distribution, free electron density, pressure, density, gas compositions, chemical reactions, and transport phenomena (horizontal and vertical winds), as illustrated below.
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.
Conclusion: Chapter 7 examines ionospheric regions, the distribution of free electrons, propagation indicators, and specific propagation modes. The following chapter discusses regional, diurnal, and seasonal propagation conditions, including online real-time charts.
The ionosonde, also known as the chirpsounder (developed in 1925), is an "HF radar" that sends short pulses of radio waves into the ionosphere to find the most optimal frequencies for HF communication. It calculates the time it takes for reflected pulses to return and then plots the height (derived from the time delay) versus frequencies to produce an ionogram. An ionosonde sweeps the HF spectrum from 2 to 30 MHz, raising the transmitted frequency (Tx) by about 100 kHz per second and digitally modulating it in 25 kHz increments. Matching receivers (Rx) detect and analyze echo signals, as seen in the next figure.
Figure 8.1: Basic ionosonde types are vertical and oblique
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, and significant regions of the globe are uncovered yet with ionosonde stations, as shown on the above map.
An ionogram is a visual representation of the height of the ionospheric reflection of a specific HF radio frequency. It shows the plasma density distribution in ionospheric regions at various altitudes (48–800 km).
Ionograms typically display two key elements:
Horizontal Lines: These lines indicate the virtual height at which an amplitude-modulated pulse is echoed, varying with the operating frequency.
The ionogram above illustrates the ionospheric E and F2 regions. The red curve shows ordinary refraction, and the green curve shows extraordinary refraction, due to the ionosphere's anisotropic nature causing double refractions (birefringence).
While this provides a simplified explanation, the reality is that the ionosphere is neither uniform nor stable, constantly changing over time. Consequently, researchers developed the Digisonde Directogram to identify ionospheric plasma irregularities.
8.3 Day-night: 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: Typical diurnal cycle
FMUF: F region maximum usable frequency OWF: optimum working frequency EMUF: E region maximum usable frequency LUF: The lowest usable frequency due to D-region.
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 8.5: The dynamics of ionospheric regions 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 high 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 8.6: 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
The following eight online charts show HF propagation conditions, all based on recent ionosonde measurements:
MUF
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.
Online MUF 3000 Km Propagation Mapupdated every 15 minutes
This map was developed between 2018 and 2021 to assist radio amateurs in finding the best times and frequencies for contacts by displaying HF propagation conditionsat a glance.
This online map shows the estimated MUF, calculated from ionograms.
A radio path of 3,000 Km is being considered for unification.
The colored regions of this map, which are rebounded by iso-frequency contours, illustrate the Maximum Usable Frequency that is expected to bounce off of the ionosphere on a 3000 Km path. The gray line position is also included.
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, foF2. 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, (foF2, as shown in the next map).
Additional Notes:
The accuracy is insufficient for professional radio services because:
Inaccuracy results from the limited coverage of radio stations. Therefore, the map is based on data interpolation. The algorithm aims to find the MUF (or foF2) at scattered points globally, but achieving accurate extrapolation from few data points is challenging. The guessing process is better in areas close to measurement stations, but uncertainty increases in distant areas.
Stations may provide inaccurate or disagreeing data, leading to peculiar results in an attempt to align measurements. Stations may go off-line or re-appear, causing unexpected changes in the model's global picture due to the limited number of data points initially used.
Real-time ionosonde data sharing has been discontinued by countries like Russia, China, Japan, the US Space Force, and NOAA in recent years. Some ionosondes are only available through NOAA, and if GIRO outages occur, maps on this site may stop updating.
There is uncertainty associated with predicting the ionosphere's state using vertical sounding data.
Geomagnetic storms and blackouts, resulting from elevated X-ray flares and proton ejections, can significantly impact the accuracy of MUF estimation derived from vertical sounding data. This map does not refer to D region absorption or to ionospheric noise. The propagation model is too simple.
The effects of geomagnetic storms and blackouts are implicitly included in the results of ionograms. But it is impossible to predict band conditions for the next few hours. Geospace dynamic models are still being developed.
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.
Figure 8.9: Animated MUF 3000 Km Propagation Map courtey of Roland Gafner, HB9VQQ
NVIS online live map for vertical reflection (critical frequency foF2)
provided by Andrew D Rodland, KC2Gupdated every 15 minutes
Figure 8.10: Online NVIS Map, by Andrew, KC2G
The colored regions of this map, which are rebounded by iso-frequency contours, illustrate the critical frequency that is expected to bounce off of the ionosphere at near vertical angle. The ham bands (160, 80, 60, 40, 30 ,20m) are designated by iso-frequency contours: 1.8, 3.5, 5.3, 7, 10.1, and 14 Mhz.
foF2, as measured by ionosondes, is the raw data that powers the site. Colored discs indicate the location of stations. A number inside each disc represents the critical frequency, foF2.
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 - foF2. 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 8.11: Online NVIS map courtesy of ASWFC Click on this online map to view the source page. There is further information.
Online T Index Map - foF2 is provided by Australian Government Space Wheather Services
The T index indicates the highest frequencies refracted from the ionosphere, as measured by ionograms. Higher T index values indicate higher refractable frequencies. Geomagnetic activity frequently depresses the mid-latitude ionosphere, reducing the T index.
This index also measures total electron content, predicting the maximum usable frequency (MUF) for HF radio waves and reflecting ionospheric conditions. It’s derived from observed maximum ionospheric frequencies and has the same scale as sunspot numbers over several solar cycles, making it an "equivalent sunspot number."
The T index corrects anomalies between sunspot number and solar flux and takes into consideration geomagnetic storms that may alter the foF2. Values typically range between -50 and 200, with low values indicating lower HF frequencies and large ones indicating higher frequencies.
The recent foF2 measurements at various locations of Australia, New Zealand and East Antarctica
Figure 8.13: foF2 Plots courtesy of Australian Space Weather Forecasting Centre Click on this online map to view the source page.
The recent LUF (ALF) chart provided by the Australian Space Weather Alert System
Figure 8.14: LUF plot courtesy of the Australian Space Weather Forecasting Centre Click on this online map to view the source page.
During a solar flare, increased ionization in the D-region of the ionosphere can cause fadeout. The chart shows the LUF (ALF) for typical 1500 Km HF circuits. Communication is rare below the LUF but feasible above it. For shorter circuits, LUF levels may be too high, allowing for slightly lower frequencies. Longer circuits may still experience fadeout at higher frequencies. This real-time chart updates only when it detects a flare of magnitude M1 or higher.
Chapter 9. Ionospheric Dynamics
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.
Sporadic E (Es) indicates occasional reflections from highly ionized plasma clouds in the lower E region.
Figure 9.1: Reflection from Sporadic E plasma cloud
Operators may use Es 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, Es 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.
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, as shown in the following figure.
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.
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.
The current fadeout (SWF: short-wave fadeout) event, if any, is shown below.
Figure 9.8: Online fadeout chart courtesy of ASW Alert System
9.3.2 "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 9.9: 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.
9.3.3 "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, foF2, 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 regions 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 follow:
Figure 10.1: Online TEC map courtesy of the German Aerospace Center (DLR)
Figure 10.2: Past TEC variations (animated) courtesy of HB9VQQ
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.
Extreme Ultra Violet (EUV) radiationcreates 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":
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:.
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... SFU (Solar Flux Units); 10-22 Watts per meter² per Hz.
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 the SOHO satellite (SEM instrument). It accounts for about half of the ionization of the F region in the ionosphere and loosely correlates to the Solar Flux Index (SFI). The background level is typically around 134 at solar minimums and can exceed 200 or more at solar maxima. It is updated hourly.
Solar X-ray flares (1–8 Ångstrom) is 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.
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 Kp and Ap:
Kp: Average of K-indices from 13 observatories, indicating broad geomagnetic activity.
Ap: Daily global geomagnetic activity, derived from the Kp index.
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 A review of the 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 are correlated with solar indices such as the Sun Spot Number (SSN), Solar Flux Index (SFI), and X-ray flares, as well as geomagnetic indices like the A and Kp indices. Understanding all these parameters is crucial
for accurately estimating HF propagation conditions.
Figure 11.6: The recent solar flares (GOES) relayed by New Jersey Institute of Technology
Figure 11.7: The observed indices of propagation conditions 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.
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
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 regionionization.
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.
Sunspots vary in shape, size, and duration, lasting from hours to months.
The average number of sunspots changes throughout the solar cycle.
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. Visible light is what we see with our bare eyes. Magnetic disturbances are only detectable in EUV light.
12.4 Solar storms (X-ray flares and particle events)
In severe cases, tens of decibels of attenuation can obstruct most transpolar HF radio transmissions. Flares may cause short blackouts lasting minutes to hours, while PCA events typically last 24 to 48 hours.
For centuries, people have been observing sunspots without knowing what they are.
We now understand that these are symptoms of solar storms.
(B) Solar Energetic Particle Events (CME, SEP, and SPE):
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 12.7: 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.
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 12.8: 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 and disruptions.
Solar flares and CMEs spontaneously, disrupt the solar wind and damaging systems both near-Earth and on its surface.
Solar energetic particles (SEPs), including electrons, protons, and alpha particles, are ejected from the Sun at high speeds as part of the solar wind. Upon reaching Earth, they interact with Earth's magnetosphere. Guided by Earth's magnetic field, the charged particles are attracted to the north and south magnetic poles, causing auroras.
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.
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 12.9: 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 12.10: Solar Cycle Sunspot Number Progression Source: The International Space Environment Service (ISES)
Video clip: An animated overview of the Solar Cycle; published by NASA in May 2013
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 12.11: Sunspot Number progression during solar sycles 24 and 25 up to Dec 2024
Source: The International Space Environment Service (ISES)
Online chart of the recent 30-day sunspot numbers Figure 12.12: EISN - Estimated International Sunspot Number
Solar flux like sunspot number can be also used to show the observed and predicted Solar Cycle.
Figure 12.13: Solar Flux progression during solar sycle 25 up to Dec 2024
Source: The International Space Environment Service (ISES)
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 12.14: The Carrington Event
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 12.15: Comparison of the recent Solar Cycles
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 12.16: Sunspot Asymmetries
Hemispheric Sunsopt Number 1950-2021 provided by SIDC - Solar Influences Data Analysis Center, Royal Observatory of Belgium
12.6 Predicting Solar Flux
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.
There are two recommended sources:
Predicted sunspot number and Radio Flux at 10.7 cm
are provided by NOAA / NWS Space Weather Prediction Center
This is a multi-year (2022-2040) forecast of the monthly Sunspot Number and the monthly F10.7 cm radio flux.
The Predicted values are based on the consensus of the Solar Cycle 24 Prediction Panel.
27-Day Outlook of 10.7 cm Sun Radio Flux and the Earth Geomagnetic Indices prepared by the US Dept. of Commerce, NOAA, Space Weather Prediction Center.
The 27-day Space Weather Outlook Table , issued Mondays by 1500 UTC, is a numerical forecast of three key solar-geophysical indices; the 10.7 cm solar radio flux, the planetary A index, and the largest daily K values. A complete summary of weekly activity and 27-day forecasts since 1997, plus an extensive descriptive, are online as the weekly.
3 Day Geomagnetic and Aurora Forecast by SolarHam. SolarHam website relays data and images from various sources.
12.7 Live Solar Activity Online
Near real-time views of the Sun shown below were taken by SOHO telescope at four EUV wavelengths, each associated with a different color of the sun disc. Brighter areas show higher levels of solar surface activity, i.e. higher Solar Flux Index.
Images of the solar activity at several wavelengths
17.1nm Fe IX/X
19.5nm Fe XII
28.4nm Fe XIV
30.4nm Helium II
Figure 12.17: Real-time SOHO images at EUV
by EIT (Extreme ultraviolet Imaging Telescope)
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 the text "CCD Bakeout" instead of the images.
For a technical explanation, read NASA CCD Bakeout explanation.
The Extreme ultraviolet Imaging Telescope (EIT) is an instrument on the SOHO spacecraft, used to obtain high-resolution images of the solar corona. The EIT is sensitive to EUV light at different wavelengths: 17.1, 19.5, 28.4 nm produced by ionized Iron, and 30.4 nm produced by Helium. The four images show the intensity distribution at these wavelengths, originating in the solar chromosphere and the transition region.
The average and local intensity may vary by orders of magnitude on time scales of minutes to hours (unpredictable solar flares), days to months (predictable solar rotation), and years to decades (predictable Solar Cycle).
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.
Solar radio emissions may indicate complex processes.
Below, see multi-frequency (VHF-SHF) radio bursts superimposed on a persistent background characterizing solar flares:
Figure 12.18: Multi-Radio-Frequency Observations of the Sun
Picture Source: Patrick McCauley Mccauley.pi, CC BY-SA 4.0; Author: Peijin Zhang 2022
Understanding space weather is crucial for predicting its impact on HF radio propagation and other technologies.
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."
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.
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 Interplanetary Magnetic Field (IMF) is the magnetic field carried by the solar wind. It interacts with Earth’s magnetosphere, and when aligned with Earth’s magnetic field, it can cause magnetic reconnection events, leading to geomagnetic storms.
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). Read an extended explanation here.
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 conditions and, in turn, significantly impact HF skywave propagation.
Figure 13.3: 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 13.4: 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
Geomagnetic activity refers to disturbances in Earth’s magnetic field caused by solar wind and other solar phenomena. These disturbances can range from minor fluctuations to major geomagnetic storms, often associated with auroras.
The following public domain images show auroras near the polar regions, known as the Northern Lights (Aurora Borealis) and Southern Lights (Aurora Australis). These phenomena occur when charged particles from the solar wind interact with Earth's magnetic field and atmosphere, exciting atoms and molecules to emit light.
Figure 13.5: Rare Red Aurora caused by oxygen at altitudes above 150 km.
Figure 13.6: Green Aurora caused by oxygen at altitudes of about 100 to 150 km.
Figure 13.7: A horizontal view of colorful auroras. Purple and Blue caused by nitrogen molecules, usually appearing at lower altitudes of 90 to 100 km.
Geomagnetic storms are more frequent during periods of high solar activity.
Geomagnetic storms occur one to four days after a CME.
These storms alter the normal conditions of the magnetosphere and ionosphere.
High absorption levels in the lower HF bands near the equator can cause a complete fadeout of HF signals.
There are two main effects on radio communications: storms can reduce the MUF in equatorial regions, worsening conditions for HF bands, while increasing the MUF in polar regions, improving low VHF contacts.
Figure 13.8: Geomagnetic Storm Dynamics courtesy of 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 a few magnetometer stations. Dst is computed once per hour and reported in near-real-time.
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): 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): Monitors real-time solar wind, providing early warnings for geomagnetic storms.
SDO (Solar Dynamics Observatory): Delivers detailed images of the sun divided into four spectral bands.
SOHO (Solar and Heliospheric Observatory): 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.
Several satellites (SOHO, ACE, and DSCOVR) monitor the hazardous Coronal Mass Ejections (CMEs).
These sattalites are positioned at the L1 Lagrange point, where Earth’s and Sun’s gravitational forces balance.
Figure 13.10: Monitoring Space Weather
The Lagrange Mission monitors hazardous CME headed toward Earth.
Credit: European Space Agency. Baker, CC BY-SA 3.0 IGO AGU - Advanced Earth and Space Science;
Titles added by webmaster (4x4xm)
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 sattelite to maintain a constant line with Earth as it orbits the Sun.
Figure 13.11: 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.
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.
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.
See below two products provided online by NOAA SWPC, Geomagnetic Activity Forecast and Plasma Density and Radial Velocity Prediction:
Geomagnetic Forecast provided online by NOAA SWPC
Ap Index: Daily global geomagnetic activity, derived from the Kp index.
Geomagnetic Activity % probabilities: Observed / Estimated / Predicted
Kp Index Forecast: Predicts geomagnetic activity every 3 hours.
This report helps predict space weather impacts on Earth, such as disruptions to communication and navigation systems.
Figure 13.20: Prediction of Plasma Density and Radial Velocity provided online by NOAA SWPC
This illustration shows NOAA's prediction of plasma density and radial velocity from a CME originating from the Sun. The left panels depict the spatial distribution, while the right panels display time series data for Earth and STEREO A, which is useful for understanding space weather's impact on Earth. The center of the spatial distribution plot represents the Sun. The plane of events is the ecliptic plane, the imaginary flat surface that Earth and other planets orbit around the Sun. This data visually represents how plasma density and radial velocity from a CME are distributed around the Sun and throughout the solar system over time. It helps predict how space weather might affect Earth and other celestial objects.
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
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.
Figure 14.1: Current and predicted fadeouts as reported online by ASWFC
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
Table 10: Correlation between band conditions, radio blackout scale, and solar flare class
X-ray flares and proton flux elevate D region absorption, affecting HF communications. The D-RAP model discussed below helps understand HF radio degradation and blackouts by providing graphical and text information on global HF propagation conditions. Electron density in the D region, which can vary within minutes, directly influences the Lowest Usable Frequency (LUF). The D-RAP model uses empirical relationships to compute HF absorption and Maximum Usable Frequency (MUF) based on space weather parameters.
Prediction Model
The D region absorption Prediction (D-RAP) Model (by SWPC-NOAA) analyzes how solar phenomena affect HF radio communication, as illustrated below:
Figure 14.3: The predicted attenuation of skywaves (from 3 to 35 MHz) due to D region absorption by flares or SEP Click on the figure to view an animation over the last eight hours.
Geospace Dynamic Models: These models are still being developed to forecast geomagnetic storms and blackouts, implicitly included in the results of ionograms.
We did, however, gain new narrow bands in the short, medium, and long wave bands. It may not be enough, but it opens up new avenues for communication improvement that do not rely on commercial infrastructure.
If you have comments, questions or requests please e-mail.
73 de Doron, 4X4XM
References Links to external references open in a new tab.
The references below are organized by topic, as follows:
Map for amateur radio uses (HamRadio) : In the shape of a terrestrial world globe, the MUF and Aurora Borealis layer. This gives the distances and directions of antennas between amateur radio stations, the position on the Maidenhead Locator grid.
The Ionosphere June 2024 Andrew McColm, VK3FS The ionosphere, generated by solar radiation, plays a crucial role in radio communication with the D, E, and F2 regions outlined in this video.
The Ionosphere Part 2 Aug 2024 Andrew McColm, VK3FS Ionized oxygen atoms in the E region are critical to good DX in the 50–144 MHz bands. EUV and activity from our sun affect the MUF and LUF in the 3–30 MHz bands.
Propagation of radio waves explainedJean-Paul Suijs, PA9X Radio waves; Earth’s atmosphere (from Troposphere to Ionosphere); Main Propagation modes; Ionospheric regions; Solar Activity; Sunspots and Solar Flux; Solar Wind; Earth’s Geomagnetic Field; Solar flares; Coronal holes; CME; The 27-day cycle; The sunspot cycle; The Earth’s seasons; How HF propagation is affected by solar activity: Flares, Coronal holes, CME; Unique propagation effects: Sporadic-E, Backscatter, Aurora, Meteor scatter, Trans-Equatorial, Field Aligned Irregularities.
EVE OverviewSolar 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.
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.
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 solar wind interacts with Earth’s magnetosphere. This mission, involving Rice University, studies magnetic reconnection, acceleration, and turbulence in space.
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. In particular it mentions the Hp30 index, MUF , f0F2, and eSFI. These metrics are crucial for selecting the right frequency bands and forecasting propagation conditions. It provides practical insights and essential links to help enthusiasts determine the best conditions for different types of HF operations, including high-band DX, low-band DX, and NVIS operations. Additionally, it includes tips on antenna optimization to enhance HF communication effectiveness.
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.
Gamma-ray burstWikipedia Gamma-ray bursts are the most intense explosions in the universe, observed in distant galaxies, with longer-lived afterglows and longer wavelengths emitted.
Space weather effects on communications systems June 2020 Mark MacAleste, CISA–Cybersecurity and Infrastructure Security Agency Effects, Operational Impacts, Mitigations, and Research Gaps for Communications Systems
Forecast tools
Online Activity and Band Monitoring
Gathering information of real-time activity on the ham bands
HF Propagation ToolsHamwaves - Serge Stroobandt, ON4AA Real-time online dashboard of solar activity influencing HF propagation on Earth.
Real-time HF propagation space weatherHamwaves - Serge Stroobandt, ON4AA Real-time online dashboard of solar activity influencing HF propagation on Earth.
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.
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
DR2W - Predict Propagation ConditionsDK9IP (Winfried), DH3WO (Wolfgang), DJ2BQ (Ewald), ZS1AO/DJ2HD (Mathew)
A Long-term forecasting cannot take into account unpredicted ionospheric and magnetic disturbances or anomalies.
VOACAP Online Application for Ham RadioJari Perkiömäki, OH6BG / OG6G
VOACAP forecats 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.
IOCAP Application Introduction VideoSANSA 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.
DX Toolbox - Shortwave / Ham Radio / HF Radio PropagationBlack 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.
Proplab-Pro v3: RevieweHamManualspacew.com
Proplab-Pro 3.2 (Build 45, March 2023) Three-dimensional ray-tracing ionosphere; can run as standalone; not free.
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.
VOACAP, a US government-funded HF propagation prediction engine, has been continuously improved over since the 1980s.
Predicting and Monitoring PropagationDXLab * 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.
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.
Comparison of observed and predicted MUF(3000)F2 in the polar cap regionRadio 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.
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.
ITU-R DirectoryITU Software, Data and Validation examples for ionospheric and tropospheric radio wave propagation and radio noise
ITU-R P.533 model This is an ITU table links to Software, Data and Validation examples for ionospheric and tropospheric radio wave propagation and radio noise in a wide range of propagation conditions. The ITUR HF Prop experimental software, was written by G4FKH and HZ1JB, and is based on the ITU-R P.533 method.
It uses a probabilistic approach to estimate radio coverage with algorithms that are supposed to be more accurate than other similar programs.
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.
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.
Amateur RadioWikipedia
The name of the hobbyAmateur Radio or Ham Radio refers to a non-commercial communication, wireless experimentation, self-training, private recreation,
radiosport, contesting, and emergency communications activity that may use radio transmitters and receivers.
Radio AmateurWikipedia Radio Amateur or Radio Ham is the person usualy a licensed operator who communicates with other radio amateurs on amateur radio frequencies.
Amateur radio stationWikipedia Read about different types of stations used by amateur radio operators.
The Basics of Radio Wave PropagationEdwin 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.
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.
Ham Radio Science Citizen InvestigationHamSCI 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.
Understanding HF Skywave Propagation
to explore HF skywave propagation. It enhances amateur radio activities using tutorials on indices, diagrams, charts, online reports, and banners and includes a table of contents, references
Table of contents
References
where fc is the critical frequency and Nmax is the free electron density.
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
A YouTube playlist featuring demonstrations and explanations Doron, 4X4XM