Understanding HF
  Skywave Propagation
  Prediction / Forecast

Practical Approach

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1. Real-time Band Activity

Recorded hams' activity

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2. Recent propagation conditions

Real-time MUF map by KC2G

Forecasting MUF for a 3,000 Km path, i.e.
View HF Bands' Conditions at a glance

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3. MUF and f_oF₂ Real-time Maps

Recent MUF and Solar Indices

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4. Tools & Applications

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5. Recent Reports

Propagation Indices: SFI, SN, A, K
Space Weather Online ^{NOAA, ESA}
Recent Hours Blackouts
Current & Recent Solar Events

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6. Observations

Recent Solar observations
Recent Solar Wind reports
Space Weather: ACE project

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7. Predictions / Forecasts

Predicted Sunspots and 10.7cm Flux
Space Weather Predictions
Blackouts Predictions
Forecasted Space Weather

Theory-based tutorial

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  Chapter 1. HF Radio Propagation - An introduction

What is a Radio?
Radio is a form of electromagnetic (EM) radiation that propagates as a wave at the speed of light.

How are Radio waves classified?
Radio waves are classified based on their frequency and wavelength; higher frequencies have shorter wavelengths.

What is HF Radio?
HF - High Frequency radio, ranging from 3 MHz to 30 MHz (10-100 meters), often referred to as "shortwaves".

The HF Bands assigned for Radio Amateurs Hover over a line to read about band's characteristics. 160 m (1.800-2.000 MHz) is officially part of the MF range but is also referred to as HF.  80 m (3.5–3.8 MHz Region 1; 3.5–3.9 MHz Region 3; up to 4.0 MHz, in the Americas, Region 2)  60 m (5.3305-5.4069 MHz, five 2.8 KHz USB channels. In some countries unavailable or limited.  40 m (7.0-7.2 MHz Regions 1&3, up to 7.3 MHz in the Americas, Region 2  30 m (10.100-10.150 MHz) 1979 WARC (World Administrative Radio Conference) only CW and digital transmissions.  20 m (14.000-14.350 MHz) Very popular band.  17 m (18.068-18.168 MHz) 1979 WARC  15 m (21.000-21.450 MHz)  12 m (24.890-24.990 MHz) 1979 WARC  10 m (28.000-29.700 MHz, the widest HF ham band)

The Rebirth of HF Radio - Decline and Comeback
Following about 45 years of decline (1975–2020), HF Communications is making a comeback as technological advancements address associated challenges, making them less of a concern. Global HF communications relied on skywave propagation until the 1960s, when satellites gained an advantage in communication. Their dominance continues today, but they are pricey and vulnerable to numerous dangers. In some cases, satellites cannot provide complete global coverage. Given the critical importance of global data connectivity, there is broad consensus on the need for redundant infrastructure. New technologies, such as Digital Voice, ALE, and Spread Spectrum, have improved skywave communication and renewed interest in its use.

1. High Frequency radio waves can travel longer distances (compared to lower bands LW, MF, or higher bands VHF and up).
2. No Infrastructure is required.
3. Low-power wireless transmitters are sufficient over very long distances.

HF bands play a vital role in various fields, including aviation, emergency services, maritime communication, and military operations. These frequencies enable communication over vast distances, often surpassing thousands of kilometers, making them indispensable in situations where traditional communication methods are unavailable or unreliable.

During emergencies, such as natural disasters or large-scale accidents, traditional communication networks may be severely damaged or overloaded. In these situations, the HF band becomes crucial for emergency services to establish communication links and coordinate rescue efforts. HF radios can be quickly deployed to affected areas, allowing first responders to communicate and provide assistance to those in need, regardless of the distance or terrain.

How does HF radio wave propagate?
HF radio can propagate in variety of modes: Line of sight, Ground wave and Skywave.

What are HF band conditions?

This page focuses skywaves and how the ionosphere's dynamic affect them.

You may find on this website answers to questions, such as:

1. How are propagation conditions affected by time of day, season, and ionospheric state?
2. How and Why are skywaves used for long-range communication?
3. What differs ionospheric layers, and how are HF radio signals bent from the sky back to Earth?
4. There is a separate page titled HF Propagation Overview

  Chapter 2. Forecasting HF Propagation Conditions

What is Radio Propagation?
Radio propagation is the process of transmitting radio waves from one location to another.

What are Propagation Conditions?
The term "Band Conditions" refers to the quality and dependability of HF radio waves transmitted between two points on Earth.

How are propagation conditions measured?
Ionosondes are used for real-time measurements of HF propagation conditions.
WSPR can be used to produce propagation conditions maps, even without Ionosondes or beacons.

How can we predict propagation conditions?
Predicting propagation conditions for HF radio involves collecting physical parameters and using mathematical models, considering the time of day, season, Sunspots, and ionospheric conditions.

HF propagation prediction is a technique used to assess the quality of radio transmissions between Earth-specific locations via the ionosphere.

There are some general rules for predicting at what time you might find band openings and in what direction:

• Pre-dawn: 40 and 80 meters (for ambitious hams)
• Sunrise: 20 meters usually opens to ranges of 3,000-6000 km
• Late Morning: 10 and 15 meters may open to ranges over 10,000 km
• Afternoon: 20 meters may open to trans-equatorial destinations
• Evenings: 40 meters (this can be a real opportunity for DX)
• Late Evenings: 80 meters (openings can last awhile allowing for rag chewing)
• 160 meters opens even later for those with large full-size antennas

Forecast vs Prediction of HF Band Conditions

The difference between forecasting and prediction is that forecasting explicitly adds a temporal dimension.
Forecast is a time-based prediction, so it is better suited to dealing with time series data.

We need forecasts of HF radio propagation because the state of the ionosphere changes constantly and affects the performance of HF radio communication. By predicting the Maximum Usable Frequency (MUF) and other factors, we can choose the best frequency bands and times to communicate with a desired location. Forecasts can also help us avoid or prepare for situations where HF radio signals are blocked or distorted by space weather events such as solar flares or geomagnetic storms.

The forecasts are used to select the best radio communication frequencies and times, plan antenna systems, and optimize communication link coverage.

* Read below how could HF propagation conditions be estimated?

* See References orgnized by eleven (12) topics

  Chapter 3. Real-time Band Conditions
This chapter provides an overview of various methods and tools for obtaining real-time band conditions.

Propagation predictions can be based on at least one of the following options:

1. Use DX clusters, you can immediately track or trace actual radio amateur band activity.
2. Listen on the ham bands with your own rig
3. Listen using WebSDR / KiwiSDR devices
4. Monitor worldwide Beacons
   
5. Watch real-time MUF maps accesible via internet
6. Simulate the current ionospheric state, using offline and online applications and tools;
         learn its effect on HF radio waves using applications based on models
         and data collected from recent Solar Activity, Space Weather reports, and remote-sensing of the ionosphere and its layers.

In general, it's important to use a combination of such tools to get a better picture of the conditions that will impact radio communications.

No single tool will provide all of the information that is needed to make an accurate prediction of HF propgation, so it's best to use a variety of tools and compare their outputs to get a more complete understanding of the conditions.

3.1 Real-time Band Conditions - Using internet DX clusters

The following DX Clusters and Reporters show the actual ham bands activity, open bands and regions:

Popular DX Clusters:

1. Real-Time Watch of World Wide Communications Map
2. DXMAPS - shows the most recent QSOs, which may indicate a real-time DX opening.
3. DXZone - a packet cluster server collects DX and WWV spots from internet clusters and a local packet
4. DXWatch - "filter" - alerts interested hams when a specific DX station is on the air.

Reporters of digital modes:

1. PSK Reporter - amateur radio signal reporting and spotting network
2. WSPRnet^map, WSPR Rocks, WSPR Live, etc.

Real-Time Watch of World Wide Ham Activity

3.2 Assessing Band Conditions - using your own receiver

Years ago, we manually scanned HF bands using analog receivers. It took us a long time to determine the communication conditions in each band.

Nowadays, FT8 allows real-time data viewing on monitors on active stations within a specific band.
Platforms like PSKReporter provide insights into band openings.

For example, see below an online report-map, based on signals received by

Malachite DSP v3 RX conected to K-180WLA 70cm diameter Magnetic Loop antenna (MLA)

Signals were collected during an hour (15:00-16:00z) on April 2, 2023, analyzed by WSJT-X v2.6.1 software (running on a PC) and reported online to PSKReporter that generated the map shown below:

3.3 Real-Time HF Bands Monitoring using Receivers accessed via internet: WebSDR / KiwiSDR

The majority of these stations offer waterfall and spectrum displays, as well as the option to record audio.

For example watch the entire LF-MF-HF spectrum at a glance using a  Wideband WebSDR at the University of Twente, Enschede, NL

Alternatively choose a remoted receiver from WebSDR / KiwiSDR lists or the KiwiSDR Map (below)

3.4 Real-Time HF Band Monitoring using Beacons

It's a good idea to listen to NCDXF Beacon Network whenever you plan to hunt DX stations. Eighteen worldwide beacons shown on the map below share five bands 20, 17, 15, 12 and 10 m and take it in terms. All these stations use a standardized antenna and output power levels. Eighteen beacons map Click to see which beacon is now transmitting on what frequency? All 18 beacons use the same five frequencies: 14.100, 18.110, 21.150, 24.930, and 28.200 MHz. It is recommended that you spend some time on one of these frequencies, listening to transmissions from all around the world. This option may indicate where the bands are now open. The IDs of all the 18 stations are callsigns in CW and then a short carrier decreasing in four power levels: 100, 10, 1, and 0.1 Watts. If you can hear the 0.1W that means that propagation is really good or you're in a really low noise location. Put these frequencies in your receiver's memory channel and you'll be able to flick quickly between them. While you're on 28 MHz tune up between 28.2 and 28.3. There's a lot of additional beacons all on their own frequencies operating full time.

3.5 Real-time Band Conditions using online propagation maps

Real-time forecasting of HF band conditions using maps on Maximum Usable Frequencies (MUF) optimizes long-distance communication, minimizes interference, and ensures reliable and efficient utilization of the HF spectrum.

3.6 Simulate Band Conditions using offline and online applications and tools

Such apps and tools can simulate the current ionospheric condition and its effect on HF radio waves using mathematical models, data collected from recent Solar Activity, Space Weather reports, and real-time sensing of the ionosphere and its layers. All these topics are covered below.

Multiple methods and tools should be used and their outputs compared to gain a more complete understanding of the propagation conditions, because no single method or tool can provide all of the information needed to make an accurate assessment.

  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

Illustration of HF Propagation Modes

1. LOS - Line of Sight propagation.

• Line of Sight occurs when two stations are directly visible to each other.
Non-LOS-propagation could be complex reflections by conductive surfaces ^{for example: ElectronicsNotes, Wiki}.

• During the day, AM radio broadcast stations use ground wave propagation.
• A vertically polarized surface wave travels parallel to and adjacent to the Earth's surface and can cross the horizon.
• Geologic discontinuities like mountains, rivers, and deep gorges, as well as RF absorption by the earth, attenuates ground wave transmission.
• Ground waves are most effective at frequencies below 1 MHz above salty seawater or conductive ground, but are practically useless over 2 MHz.

1. Skywaves are effective at frequencies 3-30 MHz, and are used for long-range communication.
2. There are several ionospheric layers E, F1, and F2 that commonly refract HF radio waves.
3. During the day, the D layer blocks frequencies below 10 MHz, allowing higher frequencies to reach the upper layers.
4. The ionospheric layers may vary in thickness and altitude and have a non-homogenous ionization density
5. The typical ionization charge density may fluctuate in a gradient, with the highest density in the middle.
6. Ducting effects can occur at times.
7. "Skip distance" is a zone of silence (dead zone, see the illustration below) between the ground wave and sky wave where there is no reception.
   
   Where h is the height, f_MUF is maximum usable frequency and f_c denotes critical frequency.
8. NVIS is a mode of skywave with effective frequencies ranging from 2 to 8 MHz. NVIS can address the issue of "dead zone" by using very low horizontal antennas.

This page does not cover the following propagation modes:
1. Aurora and/or Meteor Scatter.
2. Backscatter
3. Low VHF 30-150 MHz - In late spring or early fall signals can be unpredictably "reflected" back to Earth via Sporadic E_(Es).
4. VHF-UHF-SHF "Scatter Propagation" due to Troposphere irregularities, offering communication between 160 km to 1600 km.

  Chapter 5. How does the sun affect radio communications?

Solar activity drives HF propagation by the ionosphere, bending radio waves thus enabling propagation beyond the horizon.

1. All forms of Solar Emissions affect "HF Propagation Conditions".
2. The communication conditions depend on the sun's position and orientation, i.e. time of day, season, and geographic location.
3. When solar activity is high, the ionosphere becomes more ionized, leading to improved propagation conditions, particularly in the higher HF bands.
4. Sunspot number and solar flux serve as indicators for assessing global propagation conditions.
5. Solar Storms, including flares and coronal mass ejections (CMEs), can cause rapid and significant changes in the ionosphere. Such disruptions can severely impact global communications.

Chapter 6. The ionosphere (preface)
This chapter serves as an introduction, laying the basis for a deeper study of the ionosphere's role in HF radio communication.

Earth's weather and space weather both affect the ionosphere - a spectacle of charged particles.

Welcome to the Ionosphere cortesy NASA Goddard

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

Ionosphere (Thermosphere) is part Earth's Atmosphere

The Thermosphere is characterized by very high temperatures ranging
from 550 to over 1100 degrees Kelvin, due to the ultraviolet solar radiation.
The correlation with SF - Solar Radio Flux F10.7 (SF=50 yields 550 K, SF=200 yields 1150 K)

How does the ionosphere affect HF Radio Propagation?

1. Upon reaching the ionosphere, EUV sunlight ionizes atoms and molecules, creating plasma, a conductive medium composed of free electrons, ions and neutral molecules.
 
2. Refraction
   Radio waves refract as they interact with the free electrons in the ionosphere quite similar to how light refracts in Geometrical Optics.

The refractive index of ionospheric plasma is frequency dependent (and complex), causing radio waves to bend away from the source until "reflections" occur, as illustrated below.



HF radio waves in the inosphere can cause free electrons to oscillate and re-radiate at the same frequency.

3. The Critical Frequency

The Critical Frequency is the highest frequency below which a radio wave is reflected back to earth and above which the signal penetrates and is lost in space.

Chapter 7 - The role of the ionosphere

The ionosphere refracts (and reflects) high-frequency (HF) radio waves, enabling long-distance communication by bouncing signals off different ionospheric layers.

Sub-topics:
  7.1 Layers   7.2 Instabilty   7.3 Skywave multi-refractions   7.4 Significant frequencies   7.5 Long range skywave   7.6 NVIS

  7.1 - Ionospheric Layers (Regions)

The ionosphere consists of four regions: layers D, E, F1 and F2.
These regions differ in terms of overall gas density, composition of the ions, and free electron densities.

Illustration of Ionospheric layers (regions)

The D layer is the lowest ionospheric layer (50-90 km), and exists only during day hours.
Higher is the E layer (90-150 km).
The highest F layer (180-600 km) splits during the day into two sub-layers called F1 and F2.

The D-E-F regions do not have sharp boundaries and there are "plasma clouds" - irregularities in the concentrations of free electrons.
The altitudes at which they occur vary during the course of a day and from season to season.

The average density of free electrons per unit volume affects the Critical Frequency of each layer.

The Absorbing Layer, D

The ionization of the D layer is due solar radiation of Hydrogen spectral line that ionizes Nitric Oxide (NO) molecules.
This layer blocks frequencies below 10 MHz, allowing higher frequencies (10-30 MHz) to reach the higher E and F layers.
Solar Flares (hard X-rays <1 nm) and/or Solar Wind Protons can significantly increase the density of free electrons in the D Layer, disrupting HF radio communication and resulting in Blackouts that can last minutes to hours.

The Reflecting Layers, E and F

The E layer reflects radio signals below 10 MHz, but during intense Sporadic E_(Es) events (especially near the equator) can reflect frequencies above 50 MHz.

The F layer has the highest free electron density due to extreme UV ionizing atomic Oxygen.
The F2 layer remains by day and night and is responsible for most skywave propagation and long distance HF radio communications.

Layer or Region	Effective height	Importance significance charactristic	When Present	Typical MUF MHz	Minimum Electron Density	Maximum Electron Density	Affected by UV Solar Wavelength	Main Ions
D	48- 90 km	Daytime Attenuation	Daytime only	2 - 6	108/m3	109/m3	121.6 nm	NO+ N2+ O2+
E*	90-150 km	Medium-Frequency and Sporadic* VHF Reflector	Negligible at night	7 - 10	109/m3	1011/m3	1-10 nm	O2+
F	180-600 km	HF Super Reflector	Splits at daytime into F1 and F2	15 - 30	1011/m3	1012/m3	10-100 nm	H+ He+

See notes about Sproadic E layer.

The typical distribution of free electrons in the ionosphere is shown below:

Why does the density of free electrons increase sharply with height?

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 layer gets most of the UV radiation compared to the lower E and D layers, while the rate of electron-ion recombination is much faster at the lowest D layer (due to the higher gas density). As a result, the free electron density of the highset F layer (at noon) is significanly higher than that of the E and D layers.

At most, only one thousandth (1/1000) of the neutral atmosphere is ionized.

  7.2 - Ionospheric Instability

The free electron density of the ionosphere is always changing. It may be regional (Ionospheric Clouds), or even global at times.
Other ionospheric disturbances include Sudden Ionospheric Disturbances (SIDs) and Traveling Ionospheric Disturbances (TIDs).

  7.3 - Skywave multi-refractions

Skywave can be bounced in a variety of modes by the ionosphere

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

See below an illustration of Complex Propagation Modes such as F Skip / ¹F¹E, E-F Ducted, F Chordal, E-F ocasional and sporadic E.

An illustration of Complex Propagation Modes D-region ignored Provided by Australian Space Weather Services

Sporadic E (Es) is an unusual form of radio propagation using a low level of the Earth's ionosphere that normally does not refract radio waves. It reflects signals off relatively small "plasma clouds" in the lower E region located at altitudes of about 95~150 km.

Equatorial sporadic E (within ±10° of the geomagnetic equator) is a regular midday local time. At polar latitudes, however, sporadic E can accompany auroras and associated disturbed magnetic conditions and is called auroral E. At mid latitudes the Es propagation often supports occasional long-distance communication during the approximately 6 weeks centered on summer solstice at VHF bands, which under normal conditions can only propagate by line-of-sight.

  7.4 - Significant frequencies relevant to skywave

f_oF₂ - The  Critical Frequency is the highest frequency below which a radio wave is reflected by an ionospheric layer at vertical incidence.
The Critical Frequency is depndent on the collision frequency of the free electrons and their density:

Where f_c is the critical frequency and N_max is the 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.
Between 2005 and 2007, the critical frequency varied from 1.8 MHz to 11 MHz, with an average of 7.5 MHz. Usually, the daily critical frequencies range from 6.8 MHz to 12 MHz.
 
MUF - The   Maximum Usable Frequency is the highest frequency at which radio communications just start to fail (at a given angle and skip distance).
MUF = f_oF₂ / cosθ; MUF factor = 1/cosθ (θ is the incident angle) is a function of the path length if the height layer is known.
As a "rule of thumb" the MUF is approximately 3-4 times the Critical Frequency .
 
OWF - The  Optimum Working Frequency  is usually 85% of the MUF.
LUF - is the  Lowest Usable Frequency below which a gradual decline in signal strength occurs.
The LUF is a soft frequency limit, as opposed to the ionospheric skip MUF, which is a sharp hard frequency limit determined by the critical angle.

See also:
• How is MUF determined?
• Regional variations (maps)
• Typical diurnal changes

  7.5 - Long range skywave

The figure below illustrates Sky-Wave reflections from ionospheric layers at various angles

Long range skywave is commonly used at low transmission angle equivalent to high incident angle

The highest MUF possible occurs at the lowest transmission angle, resulting in the longest range, i.e. the transmitted ray is almost "horizontal".

As a rule of thumb, the MUF is approximately 3 times the foF2 - Critical Frequency, as stated in section 7.3 (MUF factor = 1/cosθ).

See real-time worldwide 3000km MUF map

  7.6 - NVIS Propagation

NVIS - Near Vertical Incidence Skywave is a unique communication mode used over short distances (a few hundred kilometers).

NVIS from F layer

This method provides local coverage in hilly / jungle areas, operating 2-4 MHz at night and 4-8 MHz during day.

NVIS provides good communications within a hilly area.

NVIS requires a low dipole at 0.1-0.25 wavelengths to improve vertical radiation and reduce lower-angle radiation, thereby enhancing the signal-to-noise ratio, 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.

The latest global distribution of the critical frequency (foF2) is shown on the NVIS map.

  Chapter 8. Regional HF Conditions

Regional HF Propagation Conditions provide a detailed picture of the conditions that individual operators are likely to experience.
The regional conditions are based on the LUF, MUF and OTF between two locations.

Sub-topics:   8.1 The MUF   8.2 Ionosondes   8.3 Ionograms   8.4 Ionospheric Clouds   8.5 Diurnal changes   8.6 Current Maps of MUF and f_oF₂

  8.1 The MUF - Maximum Usable Frequency

What is MUF?

The maximum usable frequency (MUF) is the highest frequency that can be reflected from the ionosphere (at a given angle and skip distance).
It is the most effective predictor of propagation conditions between two specific locations at a given time.

  8.2 Ionosonde

An ionosonde, invented in 1925, measures and records ionosphere reflecting layers heights to determine optimal frequencies for HF communications.
It is also known as a chirpsounder.

Typical ionodonde modes are vertical and oblique:

The transmitted (Tx) frequency range from 2 to 30 MHz increases at a rate of about 100 kHz per second and is modulated digitally in increments of 25 kHz.

Matching receivers (Rx) detect and analyze the echos to determine the density of plasma at various heights (48-600 km).

The echo analysis generates an ionogram, which represent plasma density distribution in the ionospheric layers.

  8.3 Ionogram

A typical ionogram
E, F1 and F2 indicate ionospheric layers

Ionograms usually contain a dual representation:

1. A series of (more or less) horizontal lines indicating the virtual height,
   at which the (amplitude modulated) pulse will be echoed as a function of the operating frequency;


2. A curve in vertical direction representing the density of electrons per cubic centimeter,
   as a function of height.

  8.4 How do "Ionospheric Plasma Clouds" form, and how can they be detected?

The ionospheric layers are not homogeneous, as explained below:

Ionospheric layers are composed of moving "plasma clouds"
of Ions and free electrons

The ionosphere is formed by Solar EUV Radiation and is disrupted by:
(1) The chaotic Solar Activity and (2) The Tropospheric Weather from far below.
Watch the clip Welcome to the Ionosphere .
See NASA Visualization

What effect does tropospheric weather have on the ionosphere?

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.

Electrodynamical Coupling of the Troposphere with the Ionosphere

Sprites - Transient Luminous Events (TLEs)

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" detected?

The Digisonde Directogram was developed to identify ionospheric plasma "clouds".

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.

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.

  8.5 Day/Night Cycle - Diurnal changes

 
Typical diurnal changes in frequencies relevant to skywave

MUF - maximum usable frequency FOT - frequency optimum transmission (or OWF) EMUF - E layer maximum usable frequency LUF - lowest usable frequency A simulation of parameters for a 5KW transmitter link between San Francisco, CA and Honolulu, HI (Oct 2002), a presentation of Naval Postgraduate School.

References:
• MUF, LUF, FOT/OWF explained
• Real-time MUF conditions

  8.6 Current Maps of MUF and f_oF₂

See below six online maps of regional propagation conditions, all based on recent Ionosonde measurements:
1. Grayline map with a few regional MUF & solar indices ^{updated every 3 hours; Provided by N0NBH}
2. MUF 3000 Km map - information about HF propagation conditions at a glance
   ^{provided by KC2G updated every 15 minutes | There is also an animated version.}
3. Map of Critical Frequency (foF2) (NVIS operation) is provided by KC2G updated every 15 minutes

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The next 3 maps for Critical MUF are provided by Australian Gov SWS, updated every 15 minutes

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4. Map of NVIS map Critical Frequency f_oF₂ at a vertical angle
5. Map of T index f_oF₂
6. Map of f_oF₂ Anomaly compared to the monthly median

    Greyline / Grayline map showing a few regional MUF values and global indices is updated every 3 hours.

It shows day/night, 13 local MUF reports, and the Global Indices: SFI, SN ^{Sunspot Number}, A&K indices, 30.4 nm, Sig Noise, Geomag.

   Real-time MUF 3000 Km HF Propagation Map - real-time worldwide MUF map ^{updated every 15 minutes}

The map below was designed for amateur radio operators, and is updated every 15 minutes.
A radio path of 3,000 Km is being considered for unification. 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 conditions at a glance. However, this tool is limited; see note 1 below

How to use this map?

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 grey 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 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 the 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).

Additional Notes:

1. The MUF(3000km) map shows the estimated MUF, calculated from ionograms. Inaccuracy can result from the limited coverage of innosonde stations, as well as the uncertainty associated with predicting the ionosphere's state using vertical sounding data. The effects of geomagnetic storms and Blackouts due to Elevated X-Ray flares and/or Proton Events are implicitly included in the results of ionograms. But it is impossible to predict band conditions for the next few hours. As a result, accuracy is insufficient for commercial radio service. Geospace dynamic models are still being developed.
 
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. See also Acknowledgments.
 
3. Read more about this open source project.
 
4. Read more about the open source software and models.
 
5. Roland Gafner, HB9VQQ extended the static presentation with an excellent Animated Map showing the last 24 hours, in 15 minutes steps.

    NVIS real-time worldwide map (critical frequency: foF2) provided by Andrew D Rodland, KC2G ^{updated every 15 minutes}

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, f_oF2.

 Another NVIS map provided the Australian Government Space Weather Services 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 (above), mainly due to the choice of frequencies for the contours. The KC2G map highlights ham bands. This map however is designed for commercial use.

Click on the map to view the source page. There is further information.

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

The T index is intended to correct inconsistencies between sunspot number and solar flux, as well as to account for geomagnetic storms that may influence the fof2.

This index can be any value, however it is usually limited to a range of -50 to 200. Low numbers suggest the usage of lower HF frequencies and vice versa.

T Index FAQ | T Index Map | Real-Time T Indices | Forecast T indices

  Current Anomaly Map of critical frequency, f_oF₂, compared to the monthly median

The plot above shows a near real-time f_oF₂ anomaly map. The anomalies are calculated by subtracting the median f_oF₂ for the last days from the currently observed f_oF₂. The current f_oF₂ and dataset are used to calculate the median f_oF₂ the identical time of day and geographical attributes. The anomaly differences are in units of MHz. The regions in red indicate significantly lower frequencies compared to the last 30-day medians.
One may choose an animated display that shows changes in anomalies for the last 1-7 days.

  Chapter 9. Total Electron Content (TEC)

TEC is an important descriptive quantity - number of electrons integrated between two points along a one-meter-squared-cross-section tube. It is calculated from real-time f_oF2 data measured by ionosondes. It is a reliable indicator of how the ionization level of the ionosphere influences the propagation conditions of radio wave transmissions. TEC is strongly affected by solar activity.

Tropospheric weather may affect TEC:
The troposphere and ionosphere are separate atmospheric regions with distinct functions. However they do interact through various processes ^{research model}.
Tropospheric lightning may induced changes in total electron content, and consequently affect propagation conditions. Thunderstorms can also worsen the signal-to-noise ratio, in particular the lower HF bands i.e., tropospheric weather may affect propagation conditions of the HF bands, especially in the tropical regions.

Thus, monitoring and modeling TEC patterns and variations allows us to better understand and prepare for the constantly changing band condtions.

TEC data is gathered from thousands of ground-based GPS receivers around the globe. It is characterized by observing carrier phase delays of received radio signals transmitted from satellites located above the ionosphere, often using GPS satellites.

TEC distributions between seasons can be compared to each other. Data analysis showed qualitative trends relating the spring and fall equinoxes as well as the summer and winter solstices.

TEC/TECU provides the number of free electrons per square meter (x1016) for a shell height of 400 km. This map is based on measurements collected from ionosonde stations around the world, it provides near real-time information and data service for the current state of the ionosphere, related forecasts, and warnings. Credit: Ionosphere Monitoring and Prediction Center (IMPC) of the European Space Agency`s network of space weather services.   Roland Gafner, HB9VQQ made an excellent Animated Map. See additional references on this topic.

Chapter 10. What is Greyline propagation?

The greyline is a narrow band around the Earth that separates day and night.
Improved long-distance communications are possible on the lower HF bands for a brief period at dawn and dusk.

Why is radio propagation better along the greyline?

The absorbing D layer is gone, but the reflecting F-layer remains. This is because the F-layer receives sunlight while the D-layer does not.
Note: Because of the denser air at lower altitudes, the D-layer dissolves before sunset, and ion recombination is faster.

The height of the F and D layers is exaggerated in comparison to Earth dimensions.

An example of a grey-line map

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

Chapter 11. Global HF & VHF Radio Propagation Conditions

Sub-topics:   11.1 Banners & Widgets   11.2 Solar Indices SN, SF   11.3 Geomagnetic Indices K, A   11.4 Predict Radio Flux at 10.7 cm

Global HF propagation conditions are the overall factors, such as Solar Activity, the ionosphere's current condition, and in particular the average global Ionization level of the F2-layer that influence HF radio waves on a global scale.

Please keep in mind that regional conditions could vary significantly from the global conditions.

  11.1 Global conditions » Banners and Widgets

Banners / Widgets may help in monitoring global variations in HF peopagation, providing users with brief messages and essential information.
Paul L Herrman (N0NBH) created the following banners:

Basic Solar indices   Additional solar indices bellow

Ham bands global conditions

Glossary of the Terms listed on the left column banner.   SFI - Solar Flux, 2800MHz (10.7cm) (SFU units) correlates with F2-layer ionization; higher value better HF conditions. SN - Daily Sunspot Number is a measure of the solar activity; correlates with better HF conditions K-Index: Geomagnetic Disturbance; good conditions K < 3   |   A -Index: 24 hours Average; good conditions A < 10 X-Ray; Scales: A, B, C, M, and  X (range from A0.0 to X9.9) affect D-layer absorption 30.4 nm: Total Solar Radiation SEM (Solar EUV Monitor) wavelenth: 30.4 nm that affects F-layer ionization Pf - Proton Flux Density   |   Ef - Electron Flux Density in the Solar Wind; both impact E-layer Aurora indicates the strength of the ionization of the F-layer in the polar regions Bz - Magnetic Field z perpendicular to Earth's Ecliptic Plane   SW - Solar Wind   Aur Lat - Aurora Latitude: Calculation from NOAA - estimate the lowest latitude auroa is observed   EsEU - Sporadic E - Europe. Updated every ½ hour. EsNA - Sporadic E - North America. Updated every ½ hour. EME Deg - Earth-Moon-Earth Degradation/attenuation. Updated every ½ hour.   MUF - Maximum Usable Frequency (MHz), updated every 15 minutes.   MS - Meteor Scatter Activity colored bar (updated every 1/4 hour).   GeoMag - Calculated - Earth`s Geomagnetic based on K-Index. Updated every 3 hours. Sig Noise - Calculated every ½ hour: S-units (Solar Wind and Geomagnetic Activity) Read more about The Current Solar Images visual comparison of the solar activity at four EUV wavelengths

Another banner with a map and extra solar-terrestrial data

  Global conditions » 11.2 - Solar Indices (SN, SF) Explained

The Extreme Ultra Violet radiation - EUV, creates the ionosphere, notably the F2-layer.

Prior to the space age, indirect markers enabled scientists to assess the F2-layer's ionization levels.
Sunspot Number and 10.7cm Solar Flux (@ 2800 MHz) have been the two Solar Indices employed.
Greater values of both may indicate better propagation conditions.

1. SSN - Sunspot Number is a count of the number of dark spots seen (electro-magnetic storms) on the sun.
   Higher SSN values indicate improved conditions on 14 MHz band and above: SSN <50 poor propagation     SSN >150 ideal propagation.
 
2. SF - F10.7 cm Solar Radio Flux

   Solar Radio Flux SF refers to (2800 MHz / 10.7 cm) radio emissions from the solar atmosphere's corona. Higher flux correlates with increased ionization of the Earth E and F layers, enhancing HF radio propagation. Typical values (from bad to good conditions):

  Global conditions » 11.3 - Geomagnetic Indices ^{K, A} Explained

Solar flux (SF) improves wave propagation conditions, but chaotic solar activity can result in disruptive geomagnetic storms, quantified by the K index.

"Making Sense of Solar Indices" - Courtesy VK2FS, https://3fs.net.au/making-sense-of-solar-indices/

Good Propagation Conditions are when SF>200 (high free electron density) and K<2 (weak geomagnetic activity).

The Geomagnetic indices K and A indicate Earth Magnetic field instability driven by chaotic solar storms that cause geomagnetic storms. Lower values of the A/K indices correlate to better propagation conditions.

The K-index is a value of Geomagnetic Activity.

1. K-index indicates a short-term (3 hours) instability of Earth Magnetic field, compared to a “quiet day”
2. Kp-index is a weighted _{place average} of K from a network of 13 mid-latitude stations.
3. A-index indicates a longer-term (24 hours) instability of Earth's geomagnetic field (eight 3-hour values of K-indices)
4. Ap-index is 24 hours average (eight 3-hour values) of Kp.

See the recent K and A indices, as provided by NOAA / NWS Space Weather Prediction Center

See the recent Terrestrial Data

  Global conditions » 11.4 - The Radio Flux at 10.7cm (SF) can be predicted

1. 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.

 
2. 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.

Chapter 12 - Solar influence

Sub-topics:   12.1 Regular Solar Emission   12.2 Solar Activity   12.3 Sunspots   12.4 Solar Storms   12.5 Solar Cycle   12.6 Current Solar Events

Two sorts of solar events are recognized: quasi-constant regular emission and stormy solar activity.

All of these solar phenomena affect HF skywave propagation conditions.

  12.1 - Regular Solar Emission

The sun emits Electromagnetic Radiation across a wide spectrum from Gama-rays to ELF (long radio waves).

The EUV (Extreme Ultra Violet) is the most significant solar emission affecting terrestrial radio communications.

see below the measured spectral lines of a "quiet Sun" at Extreme Ultra Violet - EUV:

The EUV spectrum of the Sun, as measured by the SDO flown aboard a rocket in April 2008 (solar minimum between cycles 23 and 24)

Solar Spectra in the Extreme UV (10-120 nm) are capable of ionizing molecules (of earth ionosphere) by a single-step energy transfer.
This EUV light is emitted from the solar chromosphere ^{Courtesy NASA, UCAR}.

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 Layer ionization.

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

  12.2 - Solar Activity

  12.3 - Sunspots

Sunspots are dark, cooler regions of the Sun's surface created by local magnetic activity.

These are local magnetic fields that inhibit heat transport, resulting in lower surface temperatures. Sunspots can take on a variety of shapes, change size and last from a few hours to several months.

Below see two images of the Sun, that were taken at the same time (February 3, 2002) by NASA`s Solar and Heliospheric Observatory (SOHO) satellite courtesy European Space Agency and NASA. Pictures were published by UCAR - Center for Science Education Sunspots in visible light on the left; same scene in Extreme Ultraviolet (EUV 30.4 nm) on the right. What is the reason for making observations of sunspots both in visible light and in ultraviolet light? The sunspots can be seen in visible light, while the magnetic disturbances can be seen only in ultraviolet. Both of these images have been colorized.

The Current Solar Activity

Near real-time views of the Sun shown below were taken by SOHO telescope at various EUV wavelengths.
Brighter areas show higher levels of solar surface activity, i.e. higher Solar Flux Index.

Real-time SOHO images at EUV
EIT (Extreme ultraviolet Imaging Telescope)

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

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 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). The ionospheric free electron density varies over similar magnitudes and time ranges as the EUV radiation.

  12.4 - Solar Storms

Sunspots, in contrast to flares and CMEs, are statistically predictable.
Sub-topic 11.4 presents long term prediction for Radio Flux at 10.7 cm.
Sub-topic 12.5 discusses the Solar Cycle.

  12.5 - Solar Cycle

The number of sunspots rises and falls in 11-year cycles. There are many sunspots during solar maximum and few during solar minimum.

Solar magnetic reversals occur near 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 maximal intensity, the Sun's global magnetic field has its polar regions reversed, as if a positive and negative end 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 understanding the Sun's dynamic behavior and impact on Earth.

The Current 25th Cycle

The recent Estimated International Sunspot Number (EISN)

1. Solar Cycle Notable Events

In the 19th century, more than 150 years ago, extreme events had been observed.

The Carrington Event was the most intense geomagnetic storm, recorded on 1 to 2 September 1859 during solar cycle 10.

2. Notable Events in the Recent Years

The sun is getting angry; Should you really worry about solar flares? (August 2023)

A glob of plasma and radiation that was blasted by an unexplained explosion on the sun's far side is expected to crash with Mars.
According to researchers, if the solar storm strikes the Red Planet, it may cause weak UV auroras and even shred a portion of the Martian atmosphere.

It missed us by 9 days (April 2022)


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3. Comparison of recent Solar Cycle
   

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4. 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:

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

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5. Solar Cycle - radio emissions may indicate complex processes

Multi-frequency (VHF-SHF) radio bursts superimposed on a persistent background characterize solar flares: Picture Source: Patrick McCauley Mccauley.pi, CC BY-SA 4.0

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6. Solar Radio Burst Distrubutions

Different sunspot cycles can have different radio burst distributions at 245 MHz.
That is to say that the sunspot cycles can vary and that they may not be considered identical.

See an article covering Burst Comparisons, Probabilities, and Extreme Events:
Solar Radio Burst Statistics in 8 Bands and Implications for Space Weather Effects
by O. D. Giersch, J. Kennewell, M. Lynch (2017)

---

7. Solar Interference with Terrestrial Services

Solar radio bursts from the Sun can interfere with communication, radar, and navigation systems (e.g., GPS).
The forecast of future solar events will be an underestimate of the true burst rate due to the deficiencies in the data archives.
1 Solar radio emission as a disturbance of radio mobile networks (June 2022).
2 What a Solar Flare *Sounds* Like When It Reaches Earth (2013)
3 An analysis of solar noise outbursts and their application to space communication (1971)

  12.6 - Current and Recent Solar Events

1. Effective Solar Flux / Effective Sunspot Number ^{Andrew D Rodland, KC2G | GIRO, NOAA NCEI, WWROF}
2. Current Sunspot Regions ^{Space Weather Live Belgium}
3. Solar Synoptic Map
   Forecasters at the NOAA Space Weather Prediction Center use synoptic maps to view the various characteristics of the solar surface on a daily basis. They create a snapshot of the features of the Sun each day by drawing the various phenomena they see, including active regions, coronal holes, neutral lines (the boundary between magnetic polarities), plages and filaments, and prominences. This map is a valuable tool for assessing the conditions of the sun and making the appropriate forecast for those conditions.
4. Recent Hours Solar Events:
   1. Recent Solar Watch ^NOAA
   2. Views of the Sun taken by SOHO and the Yohkoh soft-Xray telescope at various EUV wavelengths
   3. CME - Corona Mass Ejection, monitored by LASCO
   4. The sun today ^{NASA SDO's AIA - Atmospheric Imaging Assembly}
   5. Solar Data Analysis Center - serves Solar Images, Solar News, Solar Data, and Solar Research ^NASA
   6. Monitor Solar Active Regions - search by date ^{Peter Thomas Gallagher, Irland}
   7. EarthSky ^{A private initiative to advertise solar events}
5. Recent days:
   1. X-Rays Flares Scales
   2. 3 Days X-Ray Flux ^{Viewed by GOES, SWPC NOAA}
   3. 3 Days Proton Flux ^{viewed by GOES, SWPC NOAA}
   4. Solar Wind Energy ^{Rice University}
   5. Near-Earth solar wind forecasts (EUHFORIA) provided by ESA
6. Recent Month Sunspot Number ^{SILSO, Royal Observatory of Belgium}
7. Recent Month Daily Sunspot Number ^{MET Malaysia}
8. Recent Month Solar Activity Plot ^{Australian Space Weather Services}
9. Recent week, month, and year
   1. Solar Terrestrial Activity (Graphs and links) ^{Solen-Jan Alvestad}
   2. Last month Solar and geomagnetic data Table copied from Institute of Ionosphere, Kazakhstan ^{Solen-Jan Alvestad}
10. Recent Years Unique Blackouts ^NOAA
11. Archive of remarkable solar activity and space weather events (select month and year)
12. Historical innovation RHESSI NASA
13. Historic Notable Events List Of Solar Storms ^Wikipedia

  Chapter 13. Space Weather

Space weather refers to "events in space" beginning with Solar storms that affect the geospace environment and induce geomagnetic storms.

Sub-topics:
13.1 Definitions and explanations 13.2 Solar Wind 13.3 Magnetosphere 13.4 Geomagnetic Storms 13.5 Space Weather Reports 13.6 Space Weather Prediction

An illustration of the Space Weather environment
The Lagrange Mission was designed to monitor hazardous CME^{Coronal Mass Ejections} 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 is of interest because a sattlite placed at L1 can keep pace with the Earth as it orbits the Sun, always staying on a line between the Earth and the Sun. See below amimation of that satellite trapped at the L1 point of the Sun-Earth-Moon gravitational system Published by Space Weather Live

  13.1 - Space Weather Definitions and explanations

  13.2 - What is Solar Wind?

The solar wind is a stream of (quasi-constant) energetic charged particles emitted by the sun's corona, primarily electrons, protons, and alpha particles.

In addition to the quasi-constant flux of particles, the sun occasionally emits a significantly larger mass of particles known as CME (Coronal Mass Ejection).

See the recent solar wind.

The figure above is an illustration of the solar wind reaching the Earth's magnetosphere. It shows how the solar wind rearranges the magnetosphere, compressesing the magnetic field on the side facing the sun, while elongating it on the opposite (far) end.

How long does it take for Solar Wind to reach earth?
The electrons are the first to reach Earth. During a coronal mass ejection (CME), however, energetic charged particles travel at higher speed. Electrons may arrive 20 to 30 minutes after the storm begins, whereas heavier particles such as protons arrive in a day and alpha particles can take up to four days.

Variations in solar wind velocity are associated with waves and turbulence, with higher-speed streams undergoing larger fluctuations. Scientists classify solar wind based on a variety of variables, including speed, density, pressure, energy per particle, and other characteristics.

Understanding the characteristics and behavior of the solar wind is crucial for studying space weather and its effects on HF radio propagation.

  13.3 - The Magnetosphere and Earth Magnetic field

Geomagnetic conditions refer to the state of the magnetosphere created by the Earth Magnetic field as affected by the solar activity.

The magnetosphere is a "magnetic bubble" that surrounds Earth and protects us from the Solar Wind. Its shape depends on the pressure of the solar wind and the orientation of the Earth’s magnetic field.   Earth Magnetic field known also as the geomagnetic field

  13.4 - Geomagnetic Storms Affect HF propagation

Major magnetic storms can disrupt HF propagation (3-30 MHz) by changing the distribution of free electrons in the ionosphere.

Illustration of Geonagnetic storms as seen from earth close to polar regions (public domain images):

What causes geomagnetic storms?

A CME is a shock-wave of highly charged particles emitted by the sun.

When a CME enters the magnetosphere, it causes a Geomagnetic Storm

  13.5 - Online Reports of Space Weather Conditions

Real-time Solar Terrestrial Probe, provided by Rice Space Institute ^{see there "Real Time Dials"}

Real-time Space Weather Plots published by Solar Terrestrial Dispatch Click on the image below

Recent 3 days Solar X-ray flux, Proton flux, and Geomagnetic Activity Published by NOAA SWPC services Reference: Space Weather images at (US) NOAA SWPC services

Kp index provided by Europen Space Weather Service Network

Recent Solar-Terrestrial Data Provided by N0NBH, Paul L Herrman On the right last 30 days graph On the left: the recent values   SFI - Solar Flux, 2800MHz (10.7cm)higher value better HF conditions. SN - Daily Sunspot Number is a measure of the recent solar activity 30.4 nm: EUV Radiation that affects F-layer ionization EVE: SDO EUV Variability Experiment; SEM: SOHO Solar EUV Monitor K-Index: Geomagnetic Disturbance; good conditions K < 3 A -Index: 24 hours Average; good conditions A < 10 X-Ray; Scales: A, B, C, M, and  X affect D-layer absorption Ptn - Proton Flux / Elc - Electron Flux Density, both impact E-layer Aurora indicates the strength of the ionization of the F-layer in the polar regions Aur Lat - Aurora Latitude estimated lowest latitude Solar Wind speed Mag Bz - Magnetic Field z perpendicular to Earth's Ecliptic Plane S Noise S-units: This value shows how much noise, in S Units, is being generated by the interaction between the solar wind and the earth’s geomagnetic activity. The higher the numbers, the greater the noise. GeoMag Earth`s Geomagnetic Field stablility based on K-Index. Levels: Very Quiet, Quiet, Unsettled, Active, Minor Storm, Major Storm, Severe Storms, or Extreme Storms

Recent Fadeouts / Blackouts Predictions (due to D-layer absorption) as a result of X-ray Flare and/or Solar Proton Event

Click to get an animation of the recent eight hours provided by NOAA SWPC.

Explanations:

The D-Region Absorption Product was developed to address the operational impact of solar X-ray flare and Solar Energy Proton events on HF radio communication. Long-distance communications employing high frequency (HF) radio waves (3 - 30 MHz) rely on signal reflection in the ionosphere. Radio waves are frequently reflected around the peak of the F2 layer (300 km altitude), however the radio wave signal undergoes attenuation due to absorption by the intervening D-region along the path to the F2 peak and back.

1. X-ray Solar Flare
   Solar flares are radiation bursts of X-ray 1-10 Å) increase the ionization of the D-Layer at 50-90 km altitude. As a result radio signals fade out.
   The X-Ray flux levels are labeled A, B, C, M, and X on a logarithmic scale (from A0.0 to X9.9). See reports of recent flux levels.
   The D-Region Absorption model is used as a guide to understand the possible fadeouts and blackouts.
 
2. Solar Proton Event ^{SPE and SEP}
   Solar Particle Event (SPE)
   SPE refer to protons ejected by the Sun during a solar flare or a Coronal Mass Ejection shock. A major CME could cause Solar Particle Event (SPE).

   Solar Energetic Particle (SEP)
   SEP are ejected from the Sun at high speeds, interact with Earth Magnetosphere, guided by the Earth Magnetic field, reaching the north and south poles for upper atmosphere penetration.

   When highly charged particles reach Earth Fadeouts and Blackouts (of Radio Communication) can occur.
   Fast energetic protons penetrate all the way to down the D-Layer, boosting ionozation levels at high and polar latitudes. Polar Cap Absorption (PCA) occurs because enhanced ionization significantly increases the absorption of radio signals traveling through the region. In extreme cases, HF radio signal absorption levels may reach tens of decibels, which is enough to absorb the majority (if not all) transpolar transmissions. These blackout events often last between 24 and 48 hours.

Chapter 14. Summary

1. Understanding HF radio propagation can help Radio Amateurs to plan their activities.
    
2. Solar EUV radiation ionizes the upper atmosphere.
   Free electrons in the ionosphere bend and reflect radio waves back to Earth.
   Higher electron density enables the reflection of higher frequencies.
 
3. During the day four ionospheric layers (D, E, F1, and F2) are active:

1. D-layer at 50-90 km, consists NO⁺ (ionized Nitric Oxide) up to ^~10¹⁰ electrons/m³ excited by 121.6 nano-meter UVC
   The D layer exsists only during daytime. It blocks radio wave under 10 MHz from reaching the higher layers, but enables short range NVIS operation at frequencies ranging from 2 to 8 MHz. Moreover, X-ray Solar Flares, 0.1-1 nm (X-ray), may enhance D-layer, causing Blackout events.
2. E-layer at 90-150 km, consists O₂⁺ (ionized Oxygen) up to ^~10¹¹ electrons/m³ excited by 1-10 nano-meter EUV
   Medium-Frequency (MF) and Sporadic VHF reflector. Negligible at night.
3. F-layer at 180-600 km, consists H⁺, He⁺ (ionized Hydrogen and Helium) up to ^~10¹² electrons/m³ excited by 10-100 nano-meter EUV
   Splits at daytime into F1 and F2. The F2 layer is the most important during day time.
 
4. Changes in free-electron density of each leayer occur every 24 hours due to the Earth's rotation around its axis, as well as seasonal changes.
 
5. The F layer is the dominant "reflector" of Skywaves, due to multiple refractions.
   The effective range for communication is a function of the incident angle
   
   and the MUF is affected by the free electron density in the ionosphere:
 

Ionospheric electron density - typical profiles Day/Night, and Solar Cycle Min/Max variations are based on 1987 research of Arthur D. Richmond

 
6. Local changes: The ionospheric layers are not homogeneous, rather composed of moving "plasma clouds".
   
 
7. Diurnal changes: Day/Night cycle
 
8. Seasonal effects: Electron densities are higher in the summer compared to the winter, and nearer the equator compared to the poles, due to more direct solar radiation. HF radio signals are more efficiently reflected in the summer and closer to the equator.
 
9. Regional anomalies:
   • Winter anomaly - present in the northern hemisphere, but absent in the southern hemisphere
   • Equatorial anomaly
   • Equatorial electrojet due to solar winds
   • Solar X-ray bursts cause Sudden Ionospheric Disturbances (SID)
   • P cap absorption (PCA) due to solar protons
   • Geomagnetic storms and ionospheric storms
   • Lightning storms can cause ionospheric perturbations in the D-Region
 
10. The Critical Frequency is an important characteristic that defines short wave propagation conditions. It correlates with Sunspot Number.
    The graph below illustrates variations in the Critical Frequency as a function of years and seasons.

    Critical frequency and Sunspot Number and Time, published by AGSWS
    

The left vertical axis denotes Critical Frequency - the highest frequency reflected at noon at near vertical angles from the F2, F1, and E leyers.
The information was derived from ionograms collected at noon in Canberra, Australia.
The highest F2 layer has greater relative fluctuations in electron density than the lower F1 and E layers, bacuse it is more influenced by the solar activity.

The right vertical axis shows sunspot sumber represented in the graph by the redish line.

 
11. Communication conditions can be unexpectedly disrupted due to solar storms, which affect the D Region. This layer may completely block signals in all the HF bands (3-30 MHz).
     
12. In a typical Solar Flare, X-rays penetrate to the bottom of the ionosphere (to around 80 Km) and enhance the ionization of the D layer that acts both, as a reflector of radio waves at some frequencies and an absorber of lower frequencies. The Radio Blockouts associated with Solar flares occurs in the dayside region of Earth and is most intense when the sun is directly overhead.
 
13. Solar Protons can also disrupt HF radio communication. These protons are guided by Earth Magnetic field, such that they collide with the upper atmosphere near the north and south poles. The fast-moving protons have an effect similar to the X-Ray flares and create an enhanced D-Layer thus blocking HF radio communication at high latitudes. During auroral displays, the precipitating electrons can enhance other layers of the ionosphere and have similar disrupting and blocking effects on radio communication. This occurs mainly on the night side of the polar regions of Earth where the aurora is most intense and most frequent. See Polar Cap Absorption (PCA) events.
     
14. Solar indices such as SSN, SF, and K & A are used to quantify the propagation conditions.
15. See The Current Band Conditions at a glance ☺

References

1. Monitor HF Band Activity of Radio Amateurs ^{Real-time watching of worldwide hams' activity}

   SDR - Software-Defined Radio is a technology in which analog hardware components are replaced by software on a computer or embedded system.

   1. SDR - Software Designed Radio ^Wikipedia

   There are two worldwide networks of remote public SDR receivers

   2. WebSDR list of public stations
      1. Wideband WebSDR at the University of Twente, Enschede, NL
      2. background
      3. FAQ
    
   3. KiwiSDR map of public stations
      1. KiwiSDR list of public stations
      2. Introduction to using the KiwiSDR
      3. KiwiSDR design review
    

   DX Clusters

   1. DXMAPS ^website Use DX Maps to understand HF Propagation conditions ^Youtube
   2. DXZone curation of 51 DX clusters nodes
   3. DXWatch ^filter Spot Search and Create Your Filter
   4. Sites for Checking Signal Propagation and Band Activity ^{SPARC (W6SPR)}
   5. A real-time HF DX communications Map ^{hfdxview.org by Jon Harder, NG0E}

   Reporters of digital modes

   6. PSKReporter
   7. HF Signal Propagation Reporter, PSK/JT65/FT-8/CW/JT9 ^{HamRadioConcepts KJ4YZI}
    
   APRS-IS - Automatic Packet Reporting System-Internet Service
    
   8. Find Real-Time Contacts, DX Cluster, Spotter Network, APRS  ^{HamRadioConcepts KJ4YZI}
   9. VHF Propagation Map ^{APRS-IS real-time radio propagation from stations operated near 144 MHz}

   WSPR - Weak Signal Propagation Reporter

   10. WSPRnet ^website
   11. WSPR Rocks
   12. WSPR Live
   13. Weak Signal Propagation Reporter ^Wikipedia
   14. WSPR - An Introduction for Beginners | WSJT-X Ham Radio ^{Ham Radio DX, 7-Jan-2022}
   15. WSPR Explained: How to Get Started With One-Way Ham Radio ^ExtremeTech
   16. Average propagation conditions: The recent WSPR reports on 80-10m Ham Bands up to 60 days ^{WSPR Rocks}

   Beacons

   17. NCDXF Beacon Network ^{see above}
   18. International Beacon Project ^NCDXF
   19. Beacons ^IARU
   20. International Beacon Project (IBP) ^Wikipedia
   21. High Frequency Beacons and Propagation ^VU2AWC
   22. Amateur Radio Propagation Beacon ^Wikipedia
   23. Ham Radio Beacon List ^Google
   24. Types of Radio beacon ^{HF Underground}
   25. Investigating Radio propagation using beacons ^{HF Underground}
   26. Reverse Beacon Network (RBN) | History | Online Activity
   27. Beacon monitoring programs ^DXZone

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

   28. Reverse Beacon Networks – PSK Reporter And WSPR 2013 ^{Fred Kemmerer, AB1OC}
   29. Interpreting WSPR Data for Other Communication Modes 2013 ^{Dr. Carol F. Milazzo, KP4MD}
   30. Ionospheric Sounding Using Real-Time Amateur Radio Reporting Networks 2014 ^AGU
   31. Using the WSPR Mode for Antenna Performance Evaluation and Propagation Assessment on the 160-m Band 2022 ^{Jurgen Vanhamel et al}
   32. Ham Radio Reporting Networks 2023 ^HamSCI

   Automatic link establishment (ALE)

   33. Automatic link establishment (ALE) ^Wikipedia
       ALE is a (military or commercial) feature in an HF communications radio transceiver system that enables the radio station to make contact, or initiate a circuit, between itself and another HF radio station or network of stations. The purpose is to provide a reliable rapid method of calling and connecting during constantly changing HF ionospheric propagation, reception interference, and shared spectrum use of busy or congested HF channels.
    

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2. Radio Waves Propagation

   Waves, EM Radiation, Radio waves - Basic principles

   1. Electromagnetic Radiation ^Wikipedia
   2. Electromagnetic Spectrum ^Wikipedia
   3. Lyman series-alpha hydrogen radiation at a wavelength of 121.6 nm ^Wikipedia
   4. Gama Ray ^Wikipedia
   5. Radio Waves ^Wikipedia
   6. Radio propagation ^Wikipedia
   7. Refractive index in general ^Wikipedia The refractive-index-of-the-ionosphere
   8. Introduction to RF Propagation ^{John S. Seybold}
   9. Radio Propagation Tutorial Basics ^{Electronics-Notes}

   Propagation Overviews

   10. The Rebirth of HF ^{Rohde & Schwarz}
   11. Course Overview: Atmospheric Effects on Electromagnetic Systems ^{Naval Postgraduate School}
   12. All-In-One Overview: There is nothing magic about propagation ^{José Nunes – CT1BOH (2021)}
   13. Overview: Understanding HF / VHF / UHF / SHF Propagation ^{Paul L Herrman N0NBH}
   14. Propagation of Radio Waves ^{Basu, VU2NSB} principles and methods

   Propagation Modes

   15. LOS - Line Of Sight propagation ^Wikipedia

   Ground Wave

   16. Ground Wave Propagation ^Wikipedia
   17. Ground Wave Propagation Tutorial ^{Electronics-Notes}
   18. Ground wave MF and HF propagation ^AGSWS Part of key topics within ionospheric HF propagation
   19. Ground Wave Propagation ^{BYJU’S Tuition Centre}

   Skywave / Skip

   20. Skywave or Skip Propagation ^Wikipedia
   21. Skywaves & Skip Zone ^{Electronics-Notes} Key topics within ionospheric HF propagation
   22. Path length and hop length for HF sky wave and transmitting angle ^AGSWS
   23. Skip zone ^Wikipedia
   24. Atmospheric Ducting ^Wikipedia
   25. Tropospheric Ducting ^Wikipedia
    

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3. Skywave Propagation via Ionosphere
           Propagation > Refractive Index > Ionospheric Intro > Model > Layers/Regions > MUF-LUF-OWF > Seasonal & Anomalies > Probing

   Skywave Propagation

   1. HF Progagation: The Basics - QST, December 1983 ^{Denis J. Lusis, W1JL/DL}
   2. The HF Bands for Newcomers (An Overview), ARRL (2007) ^{Gary Wescom, N0GW}
   3. Introduction to HF Propagation (33 pages presentation, Nov 2018) ^{Rick Fletcher, W7YP}
   4. Propagation of radio waves explained ^PA9X
      Radio waves; Earth’s atmosphere (from Troposphere to Ionosphere); Main Propagation modes; Ionospheric layers; 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.
   5. When is the best time to make an HF contact? Propagation Prediction tools Ria's Ham Shack ^{Ria Jairam, N2RJ, 7 April 2022}
      When is the ideal time to make HF contact with a specific region of the world?
      A general talk of about 18 minutes, without demonstrations or definitions of basic concepts.
   6. An Introduction to HF propagation and the Ionosphere ^ZL1BPU
   7. Ionospheric propagation Basics ^{Electronics-Notes}
   8. Introduction to Ionospheric HF Radio Propagation ^AGSWS
   9. Understanding HF Propagation ^{Rohde Schwarz}
   10. Understanding HF Propagation ^{Steve Nicols, G0KYA, RSGB}
   11. Radio Propagation 101 - Why should you be interested in propagation? ^{Dan Vanevenhoven}
   12. Ward Silver On Radio Wave Propagation ^{Ham Radio Crash Course}
   13. The Ionosphere, Shortwave Radio, and Propagation ^{MIT Film & Video Production club}
   14. The Effects Of The Ionosphere On Radio Wave Propagation An Excellent Presentation made more than 86 years ago!!! ^{Art Bodger}
   15. Ionospheric Propagation ^{University of Toronto}
   16. Regional and Long Distance Skywave Communications ^{Ken Larson, KJ6RZ}
   17. Transequatorial Radio Propagation ^CO8TW

   Ionospheric Introduction

   18. Welcome to the Ionosphere ^{NASA Goddard}
   19. Ionization (basics) ^Wikipedia
   20. Plasma (basics) ^Wikipedia
   21. Plasma recombination ^Wikipedia
   22. The Ionosphere ^UCAR
   23. Ten Things to Know About the Ionosphere ^NASA
   24. Ionosphere ^{Electronics-Notes}
   25. Refractive Index of Ionosphere Calculator ^{Calculator A to Z}
   26. The refractive index and the absorption index of the ionosphere ^{Research notes}
   27. Ionosphere and Radio Communication ^{Saradi Bora, Kamalabaria College, North Lakhimpur, Assam, India}
       The ionospheric refractive index P.126
   28. Refractive index of ionosphere ^{Plasma Physics}
   29. Ionospheric Radio Wave Propagation ^{Richard Fitzpatrick, University of Texas at Austin}
   30. The Complex Refractive Index of the Earth's Atmosphere and Ionosphere ^{Ernest K. Smith, University of Colorado}

   Ionospheric model

   31. Ionosphere (basics) ^Wikipedia
   32. Introduction to the ionosphere ^{Anita Aikio}
   33. Ionospheric model ^Wikipedia

   Layers / Regions

   34. Mesopause ^Wikipedia
   35. Distribution of ionospheric electrons ^{Bob Brown, NM7M (SK), Ph.D.}
   36. The Ionosphere and the Sun ^{Naval Postgraduate School}
   37. Layers of Ionization ^Wikipedia
   38. Ionospheric D, E, F, F1, F2 Regions ^{Electronics-Notes}
   39. D Layer ^Wikipedia
   40. Ionospheric D Region ^Britannica
   41. D-Layer absorption of radio signals ^{Ham Radio School}
   42. Day vs Night Ionospheric Layers ^{Northern Vermont University Lyndon Atmospheric Sciences}
   43. E Layer ^Wikipedia
   44. Sporadic E propagation ^Wikipedia
   45. Sporadic E Layer ^{ScienceDirect}
   46. Sporadic E Propagation in 2 minutes ^VK3FS
   47. Sporadic E Propagation ^VK3FS
   48. Understanding Sporadic E Propagation for VHF DX ^{Ham Radio DX}
   49. Understanding Sporadic E ^{Rohde Schwarz}
   50. F Layer ^Wikipedia

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

   51. HF Radiation - Choosing the Right Frequency ^{Naval Postgraduate School}
   52. MUF Maximum usable frequency ^Wikipedia
   53. The Critical frequency ^Wikipedia
       The Critical Frequency as a Function of: Free Electron Density, MUF, Plasma Frequency, Index of Refraction
   54. Critical frequency, MUF, LUF & OWF ^{Electronics-Notes}
   55. How to use Ionospheric Propagation? ^{Electronics-Notes}

   Regular Ionospheric Variations

   56. Sunspot Number and critical frequencies and Time (Years and Seasons) ^AGSWS
   57. Season Rollover – Why do shortwave frequencies have to change? ^{Neale Bateman, BBC}
   58. Persistent anomalies to the idealized ionospheric model ^Wikipedia
   59. The Seasonal Behavior of the Refractive Index of the Ionosphere over the Equatorial Region ^{Turkish Journal of Science & Technology}
   60. Effect of Seasonal Anomaly or Winter on The Refractive Index of in Height of The Ionospheric F2-Peak ^{International Journal of Basic & Applied Sciences}

   Ionosphere Probing ^{Principles | Ionosondes | Ionograms | Stations | Charts | R & D}

   61. Introduction To Ionospheric Sounding (2006) ^{Bruce Keevers, National Geophysical Data Center, NOAA}

   Principles - Theoretical and Methodolical Aspects

   62. Chirping Explained - Passive Ionospheric Sounding and Ranging ^{Peter Martinez, G3PLX}
   63. Chirp reception and interpretation (2013) ^{Pieter-Tjerk de Boer, PA3FWM}
   64. Software-Defined Radio Ionospheric Chirpsounder For Hf Propagation Analysis (2010) ^{Nagaraju, Melodia (NY State Univ); Koski (Harris Corporation)}
       A prototype description of Software Defined Radio (SDR) chirpsounder system, based on a commercially-available SDR platform, that dynamically select the best channels to be used in an HF link, to maximize communications capacity and reliability.

   Ionosondes

   65. Introduction to Ionospheric Sounding for Hams ^{Dr. Terry Bullett. W0ASP - University of Colorado}
   66. Ionosonde ^Wikipedia
   67. Ionosonde ^{HF Underground}

   Ionograms

   68. Ionogram ^Wikipedia
   69. Understanding HF Propagation and Reading Ionograms  ^{Bootstrap Workbench}
   70. Digisonde Directogram ^{UMass Lowell Space Science Lab website, MA, US}
   71. Digital Ionogram DataBase ^{Global Ionosphere Radio Observatory (GIRO)}
   72. Simultaneous Ionospheric Observations Around The Globe ^{Lowell Digisonde International (LDI)}
   73. Mirrion 2 - Real Time Ionosonde Data Mirror ^{Space Weather Service at NOAA}
   74. Ionogram Data Info ^{GIRO, UML}
   75. The DST Group High-Fidelity, Multichannel Oblique Incidence Ionosonde (2018) ^{DOI AGU}
   76. Remote sensing of the ionosphere ^{Google Search}
   77. ICON - Ionospheric Connection Explorer ^Wikipedia
    

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4. 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}
   5. Near-Vertical Incidence Sky-Wave Propagation 36 pages Presentation for radio hams ^{Gerald Schuler, DU1GS / DL3KGS}
   6. Near Vertical Incidence Skywave (NVIS) ^{by 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, W0STU}
   12. NVIS Explained - part 1 ^NCSCOUT
       NVIS Explained - part 2
       NVIS Explained - part 3
       NVIS EXPLAINED citing the above 3-parts publication ^AmRRON
   13. NVIS Antennas ^{Dale Hunt, WB6BYU}

   Extended Research papers

   14. Radio communication via Near Vertical Incidence Skywave propagation: an overview ^{Telecommun Syst (2017) DOI, Ben A. Witvliet, Rosa Ma Alsina-Pagès}
   15. Analysis of the Ordinary and Extraordinary Ionospheric Modes for NVIS Digital Communications Channels ^{Sensors (Basel)}
   16. NVIS HF signal propagation in ionosphere using calculus of variations ^{Geodesy and Geodynamics, Umut Sezen, Feza Arikan, Orhan Arikan}

---

5. Greyline Propagation
    
   1. Grey Line HF Radio Propagation ^{Electronic Notes}
   2. Identifying Gray-Line Propagation Openings ^DXLab
   3. Greyline Propagation ^G0KYA
   4. Gray-line Propagation Explained ^{Radio Hobbyist}
   5. Round the world echoes ^G3CWI
   6. An introduction to gray-line DXing ^{Rob Kalmeije}
   7. Greyline Map ^{DX QSL Net}
   8. Greyline Map ^DXFUN
       

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6. Solar Phenomena affecting HF Propagation

   Steady Electromagnetic Radiation

   1. Solar Radiation / Sunlight ^Wikipedia
   2. (Solar) Extreme Ultraviolet (EUV) ^Wikipedia

   Solar Activity

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

   Sunspots

   5. Sunspots ^Wikipedia
   6. Sunspot Number ^AGSWS
   7. The Lifetime of a Sunspot Group ^AGSWS
   8. Effective sunspot number: A tool for ionospheric mapping and modelling ^{URSI General Assembly 2008}

   Solar Cycle

   9. Solar Cycle ^Wikipedia
   10. Solar Cycle ^NASA
   11. Solar Cycle Progression ^NOAA
   12. Sunspot number series: latest update ^{SILSO, Royal Observatory of Belgium}
   13. North-South Asymetry of Monthly Hemispheric Sunspot Numbers ^{SILSO, Royal Observatory of Belgium}
   14. Solar Cycle ^AGSWS

   Solar Magnetic Storms (referred as "solar radiation storms")

   15. Solar Raditaion storms Proton Events ^NOAA
   16. Solar Radiation Storm ^{Space Weather Live}
   17. Solar Flares ^Wikipedia
   18. Classification of X-ray Solar Flares or Solar Flare Alphabet Soup ^{Spaceweather.Com}
   19. Understanding how solar flares affect radio communications ^{Barrett Communications, Australia}
   20. Solar Particle Event (SPE) ^Wikipedia
   21. Solar energetic particles (SEP) ^Wikipedia
   22. Solar Proton Events Affecting the Earth Environment 1976 - present ^NASA
   23. Next-Generation Solar Proton Monitors for Space Weather ^Eos
   24. The Difference Between CMEs and Solar Flares ^NASA

   CME

   25. What is Coronal Mass Ejection ^Wikipedia
   26. Coronal Mass Ejections - CME ^NOAA

   The result: Particle Precipitation

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

   Solar Observation reports

   30. SDO Mission ^{NASA - The Solar Dynamics Observatory}
   31. The Active Sun from SDO: 30.4 nm ^{NASA - The Solar Dynamics Observatory}
   32. 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.
       The article is an Overview of Science Objectives, Instrument Design, Data Products, and Model Developments.
       Published in Solar Phys. - DOI 10.1007/s11207-009-9487-6}

   Analysis

   33. Responses and Periodic Variations of Cosmic Ray Intensity and Solar Wind Speed to Sunspot Numbers (2020) ^{Hindawi - Collaborative work} Advances in Astronomy: Relationship of sunspot numbers with cosmic ray intensity and solar wind speed.
       _{Article ID 3527570 | https://doi.org/10.1155/2020/3527570}
 

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7. Solar Indices as a measure of Global HF & VHF Radio Propagation
    
   1. The history of the 10.7 cm solar flux ^{Government of Canada}
   2. The 10.7 cm solar radio flux ^{K. F. Tapping, AGU}
   3. Penticton/Ottawa 2800 MHz Solar Flux ^NOAA
   4. Solar Indices: SFI, SN, A, K, Kp ^{Electronics-Notes}
   5. K-index ^Wikipedia
   6. Understanding Solar Indices ^ARRL
   7. Solar Indices - Glossary of Terms ^{HamQSL , Paul L Herrman, N0NBH}
   8. What are Solar Flux, Ap, and Kp Indices? ^VK3FS
   9. Solar Index and Propagation Made Easy - HF Ham Radio ^{The Smokin Ape}
      
   10. Current Ham Radio Propagation Conditions ^{HR4NT - Ham Radio for Non-Techies}
   11. Solar Resource Page ^{Mark A. Downing, WM7D}
   12. Beginners Guide to Propagation Forecasting ^{Ed Poccia, KC2LM}
    

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8. Space Weather What impact does it have on HF radio propagation?
    
   1. Space Weather ^Wikipedia
   2. A Media Primer for the Solar Cycle and Space Weather ^NESDIS
   3. Solar-terrestrial science ^CSA
   4. Solar Wind ^Wikipedia
   5. Space Weather ^{Naval Postgraduate School}
   6. The Magnetosphere ^Wikipedia
   7. Magnetosphere (MS) ^NASA
   8. The Interplanetary Magnetic Field (IMF) - Sun’s magnetic field, B(t)x,y,z, Earth’s magnetosphere ^{Space Weather Live}
   9. Mathematical Models of Space Weather ^NASA
   10. How does Space Weather impact HF radio communication? ^NOAA
   11. Space Weather and Radio Communications ^AGSWS
   12. Sudden Ionospheric Disturbances - SID, ^AGSWS
   13. A presentation: Solar Activity and HF Propagation ^{Paul Harden, NA5N © QRP-ARCI – 2005}
   14. NWS Space Weather Prediction Center ^NOAA
   15. Space weather: What is it and how is it predicted? ^SpaceCom
   16. How to Improve Space Weather Forecasting (2020) ^{Eos, AGU}
   17. How to Assess the Quality of Space Weather Forecasts? (2021) ^{Eos, AGU}
   18. Space Weather Highlights ^AGU
   19. Presentation: Space Weather and Propagation(2019) ^{Martin Buehring, KB4MG}
   20. The Sun and HF radio propagation ^{Electronic Notes}

   Geomagnetic Storms

   21. Geomagnetic Storms ^Wikipedia
   22. Geomagnetic Storms ^NOAA
   23. Geomagnetic Storms ^{Maine Emergency Management Agency}
   24. 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
   25. The impact of geomagnetic storms on HF propagation ^{Bing Search}
   26. Space weather impact on radio wave propagation ^{Norbert Jakowski, German Aerospace Center (DLR), Institute for Solar-Terrestrial Physics 2 Feb 2023}
   27. Monitoring and forecasting of ionospheric space weather - effects of geomagnetic storms ^{J. Lastovicka, Institute of Atmospheric Physics, Czech Republic}
   28. Effect of magnetic storms (substorms) on HF propagation: A review ^{D. V. Blagoveshchenskii, Geomagnetism and Aeronomy volume 53, pages 409–423 (July 2013)}
   29. High-Frequency Communications Response to Solar Activity in September 2017 as Observed by Amateur Radio Networks ^AGU
   30. Effect of Weak Magnetic Storms on the Propagation of HF Radio Waves ^{Kurkin, V. I. ; Polekh, N. M. ; Zolotukhina, N. A. (Feb 2022)}
   31. HF Propagation during geomagnetic storms at a low latitude station ^{Physics & Astronomy International Journal 2020}
   32. Enhanced Trans-Equatorial Propagation following Geomagnetic Storms ^{Oliver P. Ferrell, Nature volume 167, pages 811–812 (1951)}

   Space Weather Agencies & Services

   33. The International Space Environment Service ^ISES
   34. National Oceanic and Atmospheric Administration ^NOAA
   35. International Service Providers ^NOAA
   36. NOAA / NWS Space Weather Prediction Center ^NOAA
   37. Space Weather Prediction Center ^Wikipedia
   38. Canadian Space Agency ^CSA
   39. Space Weather Canada ^SWC
   40. The Embrace Program ^Brazil
   41. European Space Agency - Space Weather Service ^ESA
   42. Space Weather - Met Office ^UK
   43. Solar Influence Data Analysis Center ^{Royal Observatory of Belgium}
   44. Australian Space Weather Forecasting Center - Space Weather Services ^AGSWS
   45. Overview of the Australian Space Weather Alert System 2022 ^AGSWS
   46. Australian Bureau of Meteorology, Space Weather Services ^AGSWS
   47. Space Weather - Met Office ^UK
   48. South African National Space Agency (SANSA) ^SANSA
   49. Mission Space ^LEO
   50. Space Weather Canada
   51. World Meteorological Organiztion ^WMO
   52. American Commercial Space Weather Association ^ACSWA
   53. Space Weather Forecast Japan ^{NICT, ISES, RWC}
   54. Korean Space Weather Center ^RRA/KSWC
   55. China-Russia Consortium Global Space Weather Center
    
   

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9. Recent Observations
               Solar > Space > Terrestrial ^{Geomagnetic Indices}, TEC ^{Total Electron Content} > Propagation Conditions
   

   Recent Solar Observations

   1. Solar and Heliospheric Observatory - SOHO ^{ESA & NASA}
   2. Extreme ultraviolet Imaging Telescope (EIT) ^Wikipedia
   3. Yohkoh Soft X-Ray Telescope ^Wikipedia
   4. Recent Blackout Events ^NOAA
   5. ACE Real-Time Solar Wind ^NOAA
   6. Recent 3 days: X-ray, Proton Flux, and Geomagnetic Activity ^NOAA
   7. R6 Army MARS: Consolidated Solar Weather Real-time Terrestrial indices due to Solar Weather ^{Region 6 Army MARS}
      Compare fof2 today, yesterday, 5 fays ago, at Austin TX, Eglin AFB, and Boulder, CO
      Unusual D-Region Absorption Patterms
      Recent 3 days K-index. Solar X-Ray Flux, GOES Magnetometer, Electgron and Proton Flux
   8. Space Weather Prediction Center - Index of images ^NOAA

   Recent Radio Ham Records of Solar Data

   9. Recent records of eSSN / eSFI ( 1y / 3m / 1m / 1w / 1d ) ^{Andrew D Rodland, KC2G courtesy GIRO, NOAA NCEI, WWROF}
   10. Live Ionospheric Data ^{Paul L. Herrman, N0NBH presented by Meteorscan.com}
       EVE - SDO EUV Variability Experiment; SEM: SOHO Solar EUV Monitor
   11. Live Solar Events ^{Andy Smith, G7IZU}
   12. K7RA Solar Update ^{ARRL (Google Search)}
   13. Current Solar Conditions and Ham Radio Propagation ^W5MMW
   14. SolarHam Latest Imagery of Solar Watch and Alerts of Space Weather ^VE3EN
   15. Today Sun data and propagation ^QRZCQ Response may be very slow
       • Effects of Sun Activity (SFI, SN, X-ray, A, Kp) in the last 36 hours
       • Sun Data History in the last 30 days
       • Current Propagation based on WSPRNet
       • Current Propagation based on DXCluster
       • Propagation data in the last 30 days
   16. Ionogram Information ^{Hamwaves - Serge Stroobandt, ON4AA}

   Real-time Space Weather Conditions

   17. Space Weather Conditions ^NOAA
   18. Space Weather Conditions ^AGSWS
   19. Live Space Weather ^{Andy Smith, G7IZU}

   Terrestrial - Geomagnetic Indices

   20. Current Space Weather Parameters ^{Solar Terrestrial Dispatch}
   21. Station K and A Indices for the last 30 days ^NOAA
   22. Magnetospheric MultiScale (MMS) ^Rice

   Real-time TEC - Total Electron Content (calculated)

   23. Total Electron Content (TEC) ^Wikipedia
   24. Near-real-time TEC maps ^{ESA - Europen Space Weather Service}
   25. TEC at Ionosphere Monitoring and Prediction Center ^ESA
   26. One-hour Forecast Global TEC Map ^{DLR (ESA)}
   27. Station List ^{DLR (ESA)}
   28. Archive of TEC ^{DLR (ESA)}
   29. North American TEC ^NOAA
   30. Near Real-Time Global TEC Map ^AGSWS
   31. TEC - Recent Theories, Methods and Models

   Real-time MUF estimations using ionograms at different locations

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

   Real-time Ionograms

   36. Recent Ionograms (Cyprus) ^{University of Twente, Enschede, Netherlands}
   37. Animated Ionograms Latest 24-Hour ^GIRO
   38. Ionosonde Stations Connected to NOAA ^{NGDC, NOAA}
   39. Ionogram Information ^{Hamwaves - Serge Stroobandt, ON4AA}

   Ionograms Research Development

   40. Small Form Factor Ionosonde Antenna Development ^{Tyler Erjavec, The Ohio State University}
   41. Observations of pole-to-pole, stratosphere-to-ionosphere connection ^{MIT Haystack Observatory}
   42. Ionospheric Density Irregularities, Turbulence, and Wave Disturbances during the Solar Eclipse over North America 21 August 2017 ^{MIT Haystack Observatory}

   HF Propagation Charts (Products of Ionograms)

   43. Current f_oF2 (NVIS) Propagation Map updated every 15 minutes ^{Andrew D Rodland, KC2G}
   44. Current MUF 3000 Km Propagation Map updated every 15 minutes ^{Andrew D Rodland, KC2G}
   45. Ionospheric Maps - Current f_oF2 Plots (Global) ^AGSWS
   46. Hourly Area Predictions (HAP) Charts of selected regions ^AGSWS
   47. Current f_oF2 Plots (Asia & Australia) ^AGSWS
   48. Usable HF Frequencies for US Amateur Radio Charts ^{Remarkable Technologies, Inc.}
   49. Global HF Propagation ^{Andy Smith, G7IZU}
    

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10. Predicted Solar Flux, Sunspot Number & Space Weather Indices
    
    1. Radio Communications Dashboard ^{SWPC NOAA}
    2. D Region Absorption Prediction (D-RAP) - Blackout Prediction ^{SWPC NOAA}
    3. Predicted sunspot number and Radio Flux at 10.7 cm ^{NOAA / NWS Space Weather Prediction Center}
    4. 27-Day Outlook of 10.7 cm Sun Radio Flux and the Earth Geomagnetic Indices ^NOAA
    5. Current Solar Indices from wwv | Space Weather the last and next 24 hours ^{Doug Brandon, N6RT}
    6. Add Solar-Terrestrial Data to your Website ^{HamQSL , Paul L Herrman, N0NBH}
    7. Solar Flare Probabilities ^VE3EN
    8. 3 Day Geomagnetic and Aurora Forecast ^VE3EN
    9. Propagation Links ^{eham.net
       NOAA Alerts, Observations, Scales Activity; DX Cluster WWV announcements, Propagation Links}
     

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11. Tools and Applications for analysing and forecasting HF propagation

    Apps Categories: Real-Time Activity / Band Monitoring, Real-Time Maps & Charts, Prediction Software, Mathematical models, etc.

    1. App-Category: Online Activity / Band Monitoring - gathering information of Real-time Hams' Activity and/or beacons
        
       1. Real-Time Ham Band Activity Map ^{hfdxview.org by Jon Harder, NG0E}
       2. Analyzing Propagation From Active DX Stations ^DXLab
       3. Radio Propagation Maps Based on established contacts ^{Andy Smith, G7IZU}
     
    2. App-Category: Online Tools for HF Propagation Real-Time Conditions (Charts, Raw Data)

       Real-Time HF Propagation Tools

       1. HF-START - HF Simulator Targeting of All-users, Regional Telecommunications ^NICT
          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
       2. Real-Time HF Propagation (up-to-date) ^{Hamwaves - Serge Stroobandt, ON4AA}
          Real-time online dashboard of solar activity influencing HF propagation on Earth

       Real-Time Maps & Charts

       3. 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 extended this project with an excellent Animated Map viewing the last 24 hours, in 15 minutes steps.
        
        
    3. App-Category: Prediction Software (Calculators using various models)
       1. 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.
       2. The Advanced Stand Alone Prediction System (ASAPS) ^AGSWS
          Australian Space Weather Forecasting Center 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
       3. S/N HF Propagation Forecast Calculator for the current month ^DL0NOT
       4. 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
       5. VOACAP Online Application for Ham Radio ^{Jari 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.
       6. VOACAP Quick Guide ^{Jari Perkiömäki, OH6BG / OG6G}
       7. VOACAP Shortwave Prediction Software ^{Rob Wagner VK3BVW}
       8. How to use VOACAP - Part 1: Overview, Part 2, Part 3 ^{Jari OH6BG & OH7BG Raisa}
       9. VOACAP Charts for RadCom ^VOACAP
       10. Proppy Online - HF Propagation Prediction (2022) ^{James Watson, M0DNS}
       11. RadCom online Propagation Prediction Tools ^RSGB
       12. Ionospheric Characterisation Analysis and Prediction tool (IOCAP) ^SANSA
       13. 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.
       14. 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 grayline map.
       15. HF Propagation (Google Play) ^{Android Package Kit}
       16. HF Propagation (Microsoft Apps) ^{Stefan Heesch, HB9TWS}
       17. Proplab-Pro v3: Review ^eHam Manual ^spacew.com
           Proplab-Pro 3.2 (Build 45, March 2023) Three-dimensional ray-tracing ionosphere; can run as standalone; not free.
       18. DXPROP 1.4 (2010) ^{Christian RAMADE (F6GQK)} Rated 6.10 by DxZone
           DXprop freeware (developped for US Navy) is a propagation forecast for radio amateurs that can predict propagation on 12 frequencies.
       19. W6ELProp (2002) ^W6EL Rated: 7.56 by DxZone
           Predicts skywave propagation between any two locations on the earth on frequencies between 3 and 30 MHz
       20. HamCAP (VOACAP interface) by Alex Shovkoplyas, VE3NEA. Rated 8.93 by DxZone
       21. The Propagation Software Pages A collection of links ^AC6V
        
    4. App-Category: Overviews and Reviews of prediction software
        
       1. What can we expect from a HF propagation model? ^Luxorion Dynamic processes relevant to HF radio propagation are simulated using mathematical models, and numerical procedures.
          Interactions between the Sun's surface and the Earth's surface are considered using sun, space weather, ionosphere,
          and atmosphere models, all of which can aid in the prediction of HF radio propagation.
       2. Evaluation of various models for HF propagation prediction ^{SANSA Space Science}
       3. Review of HF Propagation analysis & prediction programs Research Oriented ^Luxorion Some of these propagation programs are only accessible via the Internet via a web interface and provide graphical solutions.
          Amateurs have also created small applications that simulate various ionospheric effects.
          Using either near-real-time data or well-known functions, the majority of them achieve extremely high accuracy.
       4. Review of Propagation prediction programs - VOACAP-based ^Luxorion The VOACAP propagation prediction engine is the result of decades of US government-funded HF propagation research
          stretching back to the dawn of computing. While VOACAP's forecasting capability has been continuously improved
          as knowledge about HF propagation has increased, its software technology is firmly rooted in the 1980s.
       5. Predicting and Monitoring Propagation ^DXLab
          * Solar terminator display and prediction - shows greyline 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.
        
       6. PropView ^DXLab Rated 8.27 by The DXZone PropView uses the included VOACAP, ICEPAC, and IONCAP propagation prediction engines to forecast
          the LUF and MUF between two locations over a specified 24 hour period.
          Results are rendered in an easy-to-understand color-graphic display.
          You can specify locations via direct latitude/longitude entry.
          Alternatively, PropView interoperates with DXView to allow location selection via DXCC prefix entry
          or by clicking on locations on a world map. It can:
          (1) build schedules for the IARU/HF beacon network and automatically QSY your transceiver to monitor each scheduled beacon.
          (2) monitor the NCDXF/IARU International Beacon Network to assess actual propagation and compare it with forecast propagation.
          Beacon schedules can be assembled by band, by location, or by bearing from your QTH.
          PropView interoperates with Commander and DXView to automatically QSY your transceiver
          to hear each beacon in your schedule, and to display the location of the current beacon.
       7. Propagation prediction software for ham radio ^DxZone A review RF prop, Radio Propagation & Diffraction Calculator, W6ELProp, PropView, HamCAP
       8. Radio Propagation Forecasting (2019) ^{Basu, VU2NSB} Beacons, VOACAP, CCIR and URSI Models
     
    5. App-Category: Mathematical models / Numerical procedures based on:
       ^{solar activity, space wather, Earth Magnetic field, ionospheric models, ray-tracying taking into account the time of day}.
       1. ITU-R Directory
       2. 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.
       3. Mathematical Models of Space Weather ^NASA
       4. Space Weather Modeling Framework (SWMF)
       5. Global Assimilation of Ionospheric Measurements (GAIM) model
       6. Advanced D-layer Ionosphere Prediction System (ADIPS)
     
    6. App-Category: Ray-tracing models based on frequency, angle of incidence, and electron density profiles of the ionosphere.
        
       1. IONCAP - Ionospheric Communications Analysis and Prediction HF transmission prediction program for US military and other applications since 1986.
          It was based on Automatic Link Establishment (ALE) Frequency Selection for a Ten-Node Australian High-Frequency Network.
          This program was clumsy, slow, and complicated, as it only allowed users with a sufficient background in ionospheric physics
          and computer data entry experience to use it. A new version was developed to fix these flaws while also improving capability
          to the point where it could be used by a layperson.
       2. 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.
       3. VOACAP (Voice of America Coverage Analysis Program)
          VOACAP forecats monthly average of the expected reliability with diurnal and seasonal variations,
          but it does not account for unpredicted ionospheric and magnetic disturbances or anomalies,
          i.e. what are the expected variations of A-index, K-index, and energy densities of solar proton / electron flux, etc.
     
    7. App-Category: Neural network models:
       These models use machine learning techniques to predict the behavior of radio waves based on input data.
       They may take into account factors such as solar activity, geomagnetic conditions, and the time of day.
       The HF Ionospheric Prediction and Solar Terrestrial Data Center uses machine learning algorithms to predict ionospheric conditions.
       1. Neural Network Ionospheric Model (NNIM)
        
    8. App-Category: Hybrid methods: Integration of several methods to provide predictions
       1. Application of Machine Learning Techniques to HF Propagation Prediction ^{Richard Buckley, William N. Furman - Rochester, NY}

---

 
12. Misc. References ^{Definitions, cross-disciplinary research etc.}

    Our hobby

    1. Amateur Radio ^Wikipedia
       The name of the hobby "Amateur 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.
    2. Amateur radio station ^Wikipedia
       Read about different types of stations used by amateur radio operators.
    3. Radio Amateur ^Wikipedia
       "Radio Amateur" is the person usualy a licensed operator who communicates with other radio amateurs on amateur radio frequencies.
    4. Shortwave listening (SWL) ^Wikipedia
       Shortwave listening, or SWLing, is the hobby of listening to shortwave radio. See for example OfficialSWLchannel a dedicated youtube channel for shortwave listeners.

    The HF Bands assigned for Radio Amateurs

    5. Amateur Radio Band Characteristics ^{Ham Universe}
    6. Ham Radio Bands ^DXZone

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

    7. The Little Pistol's Guide to HF Propagation (1996) ^{Bob Brown}
    8. HF Propagation Tutorial ^{Bob Brown (SK), NM7M
       "There is a lot of information out there on the Internet; but what about understanding?"}

    Communication Modes and Techniques

    FT8

    9. FT8 ^Wikipedia

    Digital Voice (DV)

    10. Digital Voice the Easy Way 2023 ^QST
    11. FreeDV: Open Source Amateur Digital Voice 2023^FreeDV
    12. A Guide to Digital Voice on Amateur Radio April 2021 ^VK3FS
    13. How to Use FreeDV Digital Voice Over HF Ham Radio Dec 2020^{Ham Radio Crash Course}
    14. Using FreeDV To Talk On Digital HF 80M Oct 2019 ^{Tech Minds}
    15. RSGB 2018 Convention lecture: FreeDV - Digital Voice for HF and other low SNR channels Sept 2019 ^RSGB
    16. Digital Voice on HF 2013 ^G4ILO
    17. Will digital voice (on HF) ever be a thing? 2018^{Dan, KB6NU}
    18. International Digital Audio Broadcasting Standards: Voice Coding and Amateur Radio Applications 2003 ^QEX
    19. Practical HF Digital Voice June 2000 ^{G4GUO, G4JNT , QEX}

    ALE

    20. ALE - Automatic Link Establishment ^Wikipedia
    21. Automatic Link Establishment (ALE) Demo with SCS P4Dragon Modem Commsprepper (Nov 2021)^Wikipedia
    22. Automatic Link Establishment Overview 2018
    23. HF Automatic Link Establishment (ALE) ^{Kingston Amateur Radio Club (2009)}
    24. ALE HF Network Ham Radio Amateur Radio ^{Bonnie Crystal, KQ6XA, HFLINK (2007)}
    25. ALE - The Coming Of Automatic Link Establishment ^{Ronald E. Menold, AD4TB, QST 1995}

    Spread Spectrum

    26. Spread Spectrum ^Wikipedia
    27. Frequency-hopping spread spectrum ^Wikipedia

    Science and technology-related terms

    Ecplictic Plane ^Wikipedia
    Geometrical Optics ^Wikipedia
    Secant ^Wikipedia
    Earth Magnetic field ^Wikipedia
    Physical Coupling ^Wikipedia
    Satellite ^Wikipedia
    The Lagrange Mission ^Wikipedia
    Lagrange points (Google Search)
    Aurora
    Aurora ^Wikipedia
    The Auroral E-region is a Source for Ionospheric Scintillation ^EOS
    The auroral E-layer ionization and the auroral luminosity ^{Omholt, A. (1955)}
    Auroral Effects on the Ionospheric E-Layer ^{Omholt, A. (1965)}
    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}
    7 Magical Places to View Auroras ^{National Geographic}
    Deterministic Chaos
    Deterministic Chaos ^{The Exploratorium, 1996}
    Deterministic chaos refers in the world of dynamics to the generation of random, unpredictable behavior from a simple, but nonlinear rule. The rule has no "noise", randomness, or probabilities built in. Instead, through the rule's repeated application the long-term behavior becomes quite complicated. In this sense, the unpredictability "emerges" over time.
    Deterministic Chaos ^{Principia Cybernetica 2000}
    A system is chaotic if its trajectory through state space is sensitively dependent on the initial conditions, that is, if unobservably small causes can produce large effects.
    Concepts: Chaos ^{New England Complex Systems Institute}
    A chaotic system is a deterministic system that is difficult to predict. A deterministic system is defined as one whose state at one time completely determines its state for all future times.

    HF Propagation Research 1958-1979

    28. Basic Radio Propagation Predictions for September 1958, Three Months in Advance ^{National Bureau Of Standards}
    29. Ionospheric Radio Propagation 1965 (replaced an obsolete pubication of 1948) ^{Kenneth Davies, National Bureau Of Standards}
    30. Solar-Terrestrial Prediction Proceedings | Solar-Terrestrial Prediction Proceedings 1979 ^{Richard F. Donnelly, Space Environment Lab, NOAA}

    HF Propagation - Novel Research and Analysis

    31. Monitoring and forecasting of ionospheric space weather - effects of geomagnetic storms ^{Jan Lastovicka (2002)}
    32. Short and long term prediction of ionospheric HF radio propagation ^{J. Mielich und J. Bremer (2010)}
        A modified ionospheric activity index AI has been developed on the basis of ionospheric f_oF₂ observations.
        Such index can be helpful for an interested user to get information about the current state of the ionosphere. Using ionosonde data.
    33. Spread-F occurrences and relationships with foF2 and h′F at low and mid-latitudes in China (2018) ^{Wang, Guo, Zhao, Ding & Lin (Chaina)}
        Ionospheric irregularities appear as scattered echoes in high-frequency (HF) band ionograms that are known as spread-F events that include frequency spread-F (FSF), range spread-F (RSF), and mixed spread-F (MSF) events.
    34. Long-Term Changes in Ionospheric Climate in Terms of foF2 ^{Jan Lastovicka (2022)}
        There is not only space weather; there is also space climate. Space climate includes the ionospheric climate, which is affected by long-term trends in the ionosphere.
    35. Ionospheric Monitoring and Modeling Applicable to Coastal and Marine Environments ^{Ljiljana R. Cander and Bruno Zolesi (2019)}
    36. Statistically analyzing the ionospheric irregularity effect on radio occultation ^{M. Li and X. Yue, Atmos. Meas. Tech., 14, 3003–3013, 2021}
    37. Analysis of Ionospheric Disturbance Response to the Heavy Rain Event ^{Jian Kong, Lulu Shan, Xiao Yan, Youkun Wang - Remote Sens. 2022, 14(3), 510}
    38. A simplified HF radio channel forecasting model ^{Advances in Space Research, Volume 69, Issue 6, 15 March 2022, Pages 2477-2488, Moskaleva, Zaalov
        Estimating the maximum electron density in the ionosphere in relation to the critical frequency of the ionospheric F2 layer (foF2). The suggested forecasting model estimates the electron density profile one day ahead. Comparisons of observed and predicted vertical ionograms indicate the technique's utility.}
    39. Ionospheric current ^{Upper Atmospheric Science Division of the British Antarctic Survey}
    40. Radio Propagation Prediction for HF Communications (2018) ^{Dept. of Appl/ Physics & Tel., Midlands State Univ., Gweru, Zimbabwe}
    41. A Preliminary Systematic Study of HF Radio Propagation from a Source in the Subarctic
        Using HAARP and the Ham WSPR Network (2018) ^{Citizen Space Science, Fallen, C. T.}
    42. The influence of high latitude off-great circle propagation effects on HF communication systems and radiolocation ^{M. Warrington, A.J. Stocker, N. Zaalov (2002)}
    43. Analyzing the current ionospheric conditions ^{Google search}

    Recent Theories, Methods and Models

    44. Develop ionosphere computer models to enhance HF radio propagation ^{Military Aerospace 2022}
        Develop new ways to model the ionosphere in real time to help predict the propagation of high-frequency (HF) radio waves for improved communications and sensing.
    45. Recommendation: Ionospheric Characteristics And Methods Of Basic MUF, Operational MUF AND Ray-Path Prediction ^{ITU 1995}
    46. Recommendation: Propagation Factors Affecting Frequency Sharing In HF Terrestrial Systems ^{ITU 1994}
    47. Recommendation: HF propagation prediction method ^{ITU 2001}
    48. 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.
    49. Investigation of Two Prediction Models of Maximum Usable Frequency for HF Communication
        Based on Oblique- and Vertical-Incidence Sounding Data (2022) ^{atmosphere MDPI}
        MOF were compared to predicted MUF. The INGV model outperformed for MUF prediction over Beijing
        and its adjacent mid-latitude regions, according to the root-mean-square error comparison.
    50. Investigation of Two Prediction Models of Maximum Usable Frequency for HF Communication
        Based on Oblique- and Vertical-Incidence Sounding Data (2022) ^{atmosphere MDPI}
        MOF were compared to predicted MUF. The INGV model outperformed for MUF prediction over Beijing
        and its adjacent mid-latitude regions, according to the root-mean-square error comparison.
    51. Use of electron density profiles in HF propagation assessment: part 1- Requirements, prediction and forecasting (1991) ^{Advances in Space Research Journal}

    ITM Processes

    52. Terrestrial Atmosphere ITM (Ionosphere, Thermosphere, Mesosphere) Processes ^{NASA Visualization (2018)}
    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 Three-Dimensional Simulations of the Gradient Drift and Secondary Kelvin–Helmholtz Instabilities in Ionospheric Plasma Clouds ^{Almarhabi, Lujain & Skolar, Chirag & Scales, Wayne & Srinivasan, Bhuvana (2023)}
    56. Articles about "Ionospheric Plasma Clouds" ^{Google search}

    Vertical Coupling (Atmosphere - Ionosphere)

    57. Sprite (lightning) ^Wikipedia
    58. Upper-atmospheric lightning ^Wikipedia
    59. Transient Luminous Events: Lightning above our atmosphere ^AccuWeather
    60. NASA ScienceCasts: Observing Lightning from the International Space Station ^NASA
    61. Severe Weather 101: Lightning Types ^NOAA
    62. Transient Luminous Events (TLEs) ^SKYbrary
    63. Investigations of the Transient Luminous Events with the small satellites, balloons and ground-based instruments ^{Safura Mirzayeva 2022 Master Thesis}
    64. Stunning jellyfish sprite seen in the night sky during Texas storm Aug 2020 ^{The Weather Network}
    65. Solar cycle changes to planetary wave propagation and their influence on the middle atmosphere circulation (1997) ^{Arnold & Robinson, Annales Geophysicae, vol. 16, no. 1, pp. 69–76, 1998}
    66. Electrodynamical Coupling of Earth's Atmosphere and Ionosphere: An Overview (2011) ^{International Journal of Geophysics}
    67. A review of vertical coupling in the Atmosphere-Ionosphere system:
        Effects of waves, sudden stratospheric warmings, space weather, and of solar activity (2015) ^{Journal of Atmospheric and Solar-Terrestrial Physics}
    68. Electrodynamical Coupling of Earth's Atmosphere and Ionosphere: An Overview (2020) ^{University of Lucknowת India}
    69. A Review of Low Frequency Electromagnetic Wave Phenomena Related to Tropospheric-Ionospheric Coupling Mechanisms (2012) ^NASA
    70. Influence of lightning on electron density variation in the ionosphere (2015) ^{University of Cape Town, Dsc Thesis}
    71. TEC variations detected over southern Africa due to lightning storms published by South African National Space Agency
    72. Statistical Study of Global Lightning Activity and Thunderstorm-Induced Gravity Waves in the Ionosphere (2023) ^{Swati Chowdhury, New Zeeland}
    73. Eyes on the Earth Earth's Weather / Tropospheric Weather ^NASA/JPL

Index of Terms

1. A glossary of basic terms
2. Index of terms referred to this website

    Last but not least:

The world is changing as the radio amateur spectrum is being sold off to commercial users since few amateurs operate SHF and above.

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