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What is radio?
The term "radio" comes from "radiation," which refers to electromagnetic (EM) energy moving as waves from one place to another.
Radio waves may propagate like light waves without a line of sight. For example, reflection, scattering, and diffraction influence radio waves, as shown in the following figure:
Multipath propagation characteristics
Radio waves are a part of the broader electromagnetic (EM) spectrum:
The entire electromagnetic spectrum consists of various types of waves classified by their frequency and wavelength. The layout is from high to low frequency (short to long wavelength), consists of different regions, each with distinct features and applications.
Below see the radio spectrum bands from low to high frequency (long to short wavelength):
Here we focus on HF (shortwave) bands, which range from 3 to 30 MHz, propagating as skywaves over long distances between Earth's surface and the ionosphere.
160 m (1.800–2.000 MHz): Officially part of the MF range but 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 (Region 2, the Americas).
60 m (5.3305–5.4069 MHz): Five 2.8 kHz USB channels; availability varies by country.
40 m (7.0–7.2 MHz) (Regions 1 & 3); up to 7.3 MHz (Region 2, the Americas).
30 m (10.100–10.150 MHz): WARC 1979; only CW and digital transmissions.
20 m (14.000–14.350 MHz): The most popular band.
17 m (18.068-18.168 MHz): WARC 1979.
15 m (21.000–21.450 MHz)
12 m (24.890–24.990 MHz): WARC 1979.
10 m (28.000–29.700 MHz) is the widest HF ham band.
The decline and comeback of skywave HF radio:
High Frequency (HF) radio experienced a decline due to the constantly changing nature of the ionosphere, which led to unresolved issues such as interference, fading, and limited bandwidth. The advent of satellites in the 1960s provided a more reliable, high-quality solution for long-distance communication, leading to HF radio's reduced usage. However, after 55 years of decline (1965–2020), HF communications via skywaves are making a comeback. The high costs and vulnerabilities of satellite-based global communications have renewed interest in HF radio. Technological advancements such as digital voice, automatic link establishment (ALE), and spread spectrum techniques have significantly improved the reliability and cost-effectiveness of HF communications. As a result, there is a growing interest in utilizing skywaves for long-distance communication.
The Advantages of skywave HF Radio over sattelites:
Skywaves travel longer distances and can reach places not covered by satellites.
No infrastructure required.
Low power wireless transmitters are sufficient over very long distances.
Key Points:
Essential for various sectors: Used in aviation, emergency services, maritime communication, and military operations for long-distance communication.
Crucial during emergencies: Vital for establishing communication and coordinating rescue efforts when traditional networks fail.
The term “HF band conditions” refers to the quality and reliability of HF radio signals transmitted within a specific band between two points on Earth via skywaves, influenced by the ionosphere’s dynamics.
Explore HF propagation key terms explained on this page:
Can we forecast changes in HF conditions over the next hour?
Yes, we can forecast short-term changes in HF propagation based on real-time data.
What is the difference between forecasting and predicting HF conditions?
The main distinction between forecasting and prediction lies in the time frame. Forecasting refers to short-term estimations derived from real-time measurements, while prediction often involves longer-term projections based on historical patterns and statistical models.
HF propagation prediction is a method used to evaluate the quality of radio transmissions between specific locations on Earth via the ionosphere. Some general guidelines can help determine the best times and directions for band openings:
Pre-dawn: 40 and 80 meters for dedicated hams.
Sunrise: 20 meters opens for distances of 3,000–6,000 km.
Late Morning: 10 and 15 meters may open for over 10,000 km.
Afternoon: 20 meters may open to trans-equatorial routes.
Evening: 40 meters offers opportunities for DX communication.
Late Evening: 80 meters can remain open for extended communication.
Night: 160 meters is ideal for those with large antennas.
Why do we need HF propagation forecasts?
HF propagation forecasts are essential due to the constantly changing nature of the ionosphere, which affects radio communications. By predicting key factors like the MUF, we can optimize frequency selection and timing for communication with specific locations. These forecasts are crucial for anticipating and mitigating disruptions caused by space weather events, such as solar flares and coronal mass ejections (CMEs), which can trigger geomagnetic storms.
In summary, HF propagation forecasts are necessary for:
Selecting optimal communication frequencies and timing.
Planning antenna systems effectively.
Enhancing communication link reliability and coverage.
How has the forecasting of HF propagation conditions evolved over the last 30 years?
Remarkable advancements in space technology, software-defined radio (SDR), and the internet have revolutionized our understanding of radio wave propagation. Before the 1990s, propagation charts and reports were often published in amateur radio magazines. Today, real-time solar indices and computer programs provide more accurate, up-to-the-minute propagation data via online tools.
Using multiple methods will provide a better understanding of the propagation conditions, as no single method can offer all the necessary information. Please find below examples and explanations of these methodes.
3.1 Use FT8 signals with PSKReporter to monitor and visualize real-time HF propagation.
In the past, scanning HF bands with analog receivers was time-consuming. Today, digital modes and waterfall displays allow quick analysis of band activity. FT8 facilitates real-time monitoring of active stations, while PSKReporter provides insights into band openings.
Below is a PSKReporter map from April 2, 2023:
Signals received were analyzed by WSJT-X v2.6.1 software (running on a PC) reporting online to PSKReporter.
Receiving station: Malahite DSP v1.3 RX conected to K-180WLA 70cm diameter Magnetic Loop antenna (MLA)
3.2 Track Global 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.
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.
3.3 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.
3.4.1 DXView—Real-time ham activity map designed by Jon Harder, NG0E.
Click on the map above to see the global radio ham activity for the previous 15 minutes on 11 ham bands (1.8–54 MHz). Data is gathered from online sources: WSPRnet,
Reverse Beacon Network
(CW, FT4, FT8), and DX Cluster. JavaScript is required to view the graphics.
3.4.5 PSK Reporter is an amateur radio signal-reporting and spotting network.
3.4.6 WSPR is a weak-signal radio communication protocol used for sending and receiving low-power transmissions to test propagation paths on the ham bands. Useful Links include: WSPRnet, WSPR Rocks, WSPR Live.
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.
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. All these topics are covered below.
Using multiple methods and tools is crucial for a better understanding of propagation conditions and accurate assessment. Find on this page real-time charts, space weather reports, solar-terrestrial data, apps, and tools that may help you plan your activity.
Skywave propagation basics
Chapter 4. HF Propagation Modes
This chapter reviews the primary modes of high frequency (HF) radio propagation.
Skip Distance - Dskip: A zone of silence (dead zone) between ground wave and sky wave with no reception.
Where h is the height, fMUF is maximum usable frequency and fc denotes the critical frequency
Special cases:
NVIS: Near Vertical Incidence Skywave operates at 2–8 MHz, using low horizontal antennas to address dead zones.
In late spring or early fall, low VHF (30 to 150 MHz) signals can be unpredictably "reflected" back to Earth via Sporadic E
Currently this page does not cover the following propagation modes:
Aurora propagation: an Aurora event may create opportunities for QSOs on 50 MHz and 145 MHz (6 and 2 meters), while lower HF bands are disturbed;
Backscatter propagation;
Meteor Scatter propagation;
Moon Bounce (EME) propagation;
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 significantly influences the quality of skywave communications. The sun's activity affects the ionosphere, which is essential for reflecting radio waves back to Earth and enabling long-distance communication. Solar EUV radiation ionizes the atmosphere, creating the ionosphere that bounces HF radio waves, allowing them to propagate beyond the horizon.
The ionosphere is always changing, courtesy of NASA Goddard
The dance of radio waves within a vibrant airglow. Solar storms intensify the ionosphere's beauty, while Earth's weather below adds to the unique destination.
HF radio waves from Earth to the ionosphere cause free electrons to oscillate and re-radiate, resulting in wave refraction with a refractive index similar to geometrical optics.
Refraction of radio waves in the ionosphere The left two paths of signals demonstrate higher-frequency radio waves lost in space.
The refraction of radio waves in the ionosphere is characterized by their critical frequency. This is the highest frequency at which radio waves reflect back to Earth. Higher frequencies escape into space.
The D, E, and F "layers" are commonly used to depict the structure of the ionosphere, but this is not entirely accurate. Ionization exists throughout the ionosphere, with its level varying with altitude and fluctuating in time and geographical regions as a result of solar radiation.
There is also a C region below them, which has low ionization levels that don't affect HF radio propagation.
The term "region" is more appropriate.
It’s common to present the order of ionosphere regions affecting HF skywaves from the highest region downwards.
The ionospheric regions
The F-region, located between 180 and 600 km above the Earth, enables long-distance HF communication in the 3.5 to 30 MHz range.
This region consists H+, He+ (ionizedHydrogen and Helium ) has the highest free-electron density up to 1012 electrons per m3 excited by solar radiation 10-100 nano-meter EUV. It splits during the day into sub-regions F1 and F2. It slowly dissipates after sunset.
The E-region (90-150 km) dissipates a couple of hours after sunset.
This region consists O2+ (ionizedOxygen) up to ~1011 electrons per m3 excited by solar radiation 1–10 nano-meter EUV.
During intense Sporadic E(Es) events (particularly near the equator) it sprodically reflects frequencies in the 50-144 MHz bands.
The D-region is the lowest at 50-90 km above ground. It is effective only during daytime, blocking radio frequencies below the LUF from reaching the higher E and F regions. This absorbing region dissipates at sunset.
Why does the density of free electrons increase sharply with height between 50 km and 250 km?
The density of free electrons is affected by the balance of two opposing processes: ionization and recombination (the capture of a free electron by a positive ion).
The F-region gets most of the UV radiation compared to the lower E and D regions, while the rate of electron-ion recombination is much faster in the lowest D region (due to the higher gas density). As a result, the free-electron density of the high-set F-region (at noon) is significantly higher than that of the E and D regions.
At most, only one thousandth (1/1000) of the neutral atmosphere is ionized.
7.2 Multi-refractions
The ionosphere bounces skywaves in multiple modes
Complex modes: F Skip / 1F1E, E-F Ducted, F Chordal, E-F ocasional and sporadic E. A modified version. The original figure is courtesy of the f.
The ionospheric physical conditions are: temperature distribution, free electron density, pressure, density, gas compositions, chemical reactions, and transport phenomena (horizontal and vertical winds), as illustrated below.
Temperatures distribution due to low or high solar flux
free electron density
ionic compositions.
Not shown on the right figure:
Gas pressure and density
gas compositions
chemical reactions of atomic ions with molecules
horizontal and vertical winds
Ionospheric physical conditions
The dynamics in the D-region are attributed to the reactions of the ions O+, N+, and NO+ with N2, O2, and NO.
The next sub-chapter explains the critical frequencies that affect skywave propagation.
7.5 The critical frequencies for HF radio propagation
The significant frequencies relevant for skywave propagation are foF2, MUF, OWF, and LUF.
7.5.1 The critical frequency (foF2) is the highest frequency below which a radio wave is reflected by the F2-region at vertical incidence, independent of transmitter power.
Vertical reflection from F2-region (layer)
The critical frequency is depndent on the collision frequency of the free-electrons and their density: Where fc is the critical frequency and Nmax 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.
Statistically, between 2005 and 2007, the global average critical frequency (foF2) varied from 1.8 MHz to 11 MHz, with an average of 7.5 MHz. The daily critical frequencies range from 6.8 MHz to 12 MHz.
7.5.2 The Maximum Usable Frequency (MUF) is a fascinating concept in skywave propagation. It is the highest radio frequency reflected by the ionosphere at a given incident angle, independent of transmitter power.
MUF illustration
7.5.2.1 The MUF determines the conditions for radio wave propagation between specific locations.
7.5.2.3 As a rule of thumb, the MUF is approximately 3-4 times the critical frequency.
7.5.2.4 Ionosondes determine the critical frequency, which varies significantly based on location and time. Some general trends can be observed:
7.5.2.5 Day vs. Night and Geographical Locations: The MUF varies with latitude and the day due to increased ionization from solar radiation. At night, the MUF decreases.
MUF vs. latitude, courtesy of Australian Space Weather Service.
This graph shows how the maximum usable frequency (MUF) for radio communications varies with geomagnetic latitude during the day and night.
Day Hemisphere: The red curve (F2 layer) peaks around 18 degrees, forming an "equatorial anomaly." The blue curve (E layer) remains relatively flat.
Night Hemisphere: The red curve shows a "mid-latitude trough" around 60 degrees latitude. Gradually growing towards the equator.
These variations are crucial for understanding radio wave propagation and planning telecommunications.
7.5.2.6 Seasonal Variations: The MUF is higher in summer due to the sun being directly overhead and lower in winter.
7.5.2.7 Solar Activity: High solar activity can increase the MUF by enhancing ionospheric ionization.
7.5.3 The Optimum Working Frequency (OWF) is usually 85% of the MUF.
7.5.4 Lowest Usable Frequency (LUF) is the frequency where signal strength rapidly decreases due to D-region absorption.
The LUF (also referred to as "absorption-limited frequency" (ALF) or FL) is a soft frequency limit, unlike the ionospheric skip MUF. Transmitter power can overcome some absorption effects, which affect the LUF. A ray at a frequency lower than the LUF is absorbed in the D layer.
Why does the LUF depend mainly on the D region? Below the D region, the free electron density is negligible for absorption. Above the D region, there are fewer neutral atoms, so the E and F regions absorb far less than the D layer.
Why is there no global average for the LUF? The Lowest Usable Frequency (LUF) varies significantly due to several factors, making it challenging to establish a global average. Here are the key reasons:
7.5.4.1 Day vs. Night: During the day, the LUF is higher because of increased D-layer absorption caused by solar radiation. At night, the ionosphere becomes less ionized, leading to a lower LUF.
7.5.4.3 Seasonal Variations: The LUF is higher in the summer due to increased solar activity and lower in the winter.
7.5.4.4 Solar Activity: Periods of high solar activity, such as solar flares, can increase the LUF by enhancing ionospheric absorption. Unexpected solar flares and solar particle events can also cause blackouts if the LUF exceeds the Maximum Usable Frequency (MUF).
Understanding these variations is crucial for effective HF radio communication, as it helps in selecting the optimal frequency for transmission.
NVIS provides the solution for the dead zone (between groundwave and skip). It is the only solution for communication coverage in hilly and/or jungle areas over short distances of a few hundred kilometers.
An illustration: How NVIS provides communications within a hilly area.
Typical operating frequencies are 2-4 MHz at night and 4-8 MHz during day.
NVIS requires suitable antennas (like a low dipole at hight of 0.1-0.25 wavelengths) to improve vertical radiation and reduce lower-angle radiation, contrary to what is customary for long-range communication.
NVIS offers enhanced resistance to fading (constant signal level), and minimal attenuation, making it suitable for low transmit power levels and omnidirectional coverage, allowing flexibility in setup and placement.
To avoid skip zones on 40 m band use NVIS when f0F2 is higher than 8.5 MHz. Switch to 80 m if the day is on the downward slope. Optimize antenna radiation pattern for the desired takeoff angle. Optimum NVIS height for horizontal dipoles: 0.18–0.22λ for TX and 0.16λ for RX. courtesy of Chris, N6CTA.
The 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 region is gone, but the reflecting F-region remains. This is because the F-region receives sunlight while the D-region does not.
Note: Because of the denser air at lower altitudes, the D-region dissolves before sunset, and ion recombination is faster.
The height of the F and D regions 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.
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 observed values of foF2, MUF, and LUF between two locations.
The ionosonde (invented in 1925) is an "HF radar" that emits short pulses of radio radiation into the ionosphere in order to determine optimal frequencies for HF communication.
How does it work?
Typical ionodonde modes are vertical and oblique:
The ionosonde, also known as a chirpsounder, sweeps the HF range from 2 to 30 MHz. The transmitted (Tx) frequency increases at a rate of about 100 kHz per second and is digitally modulated in 25 kHz increments. Matching receivers (Rx) detect and analyze echo signals.
The ionosonde measures the time it takes for reflected pulses to arrive after being transmitted. The height (calculated from the time delay) is plotted versus frequencies. This recording, known as an ionogram, shows the plasma density distribution in ionospheric regions at various altitudes (48–600 km).
Every 15 minutes, real-time data from ionosonde stations throughout the world is reported via the internet.
Some stations aren't always active, and significant regions of the globe are uncovered yet with ionosonde stations, as shown on the above map.
An ionogram is a visual representation of the height of the ionospheric reflection of a specific HF radio frequency.
A typical ionogram E, F1 and F2 indicate ionospheric regions
Ionograms usually contain a dual representation:
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;
A curve in vertical direction representing the density of free-electrons per cubic centimeter, as a function of height.
8.3 Day-night Cycle - Diurnal changes
The diurnal cycle on Earth occurs every 24 hours, with the sun affecting ionosphere characteristics. The E and F layers have larger electron densities during daylight, while the D layer disappears at night. The MUF and LUF rise with the sun and diminish after sunset.
Typical diurnal changes
MUF - F region maximum usable frequency OWF - optimum working frequency EMUF - E region maximum usable frequency LUF - lowest usable frequency
References:
MUF, OWF, and LUF explained
Real-time MUF conditions
8.4 Seasonal phenomena
Enhanced solar EUV radiation increases free-electron densities more in summer than winter, and more near the equator than the poles.
The dynamics of ionospheric regions near the equator and mid-latitudes
As a result, HF propagation conditions on the bands above 10 MHz are better in the summer and closer to the equator, whereas propagation conditions on the bands below 10 MHz are better in the winter and at high latitudes.
8.5 Near real-time propagation charts
See below for eight online charts showing HF propagation conditions, all based on recent ionosonde measurements:
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.
NVIS Map shows wolrdwide distribution of foF2provided by KC2G updated every 15 minutes
The next 3 NVIS maps are provided by the Australian Space Waether Forecast Center (ASWFC) updated every 15 minutes
Greyline (Grayline) map showing near real-time propagation conditions; MUF values from selected regions and a few solar and geomagnetic indices—updated every 3 hours (by Paul L Herrman, N0NBH).
Real-time MUF 3000 Km Propagation Mapupdated every 15 minutes
This map was developed between 2018 and 2021 to assist radio amateurs in finding the best times and frequencies for contacts by displaying HF propagation conditionsat a glance.
This map shows the estimated MUF, calculated from ionograms.
A radio path of 3,000 Km is being considered for unification.
The colored regions of this map, which are rebounded by iso-frequency contours, illustrate the Maximum Usable Frequency that is expected to bounce off of the ionosphere on a 3000 Km path. The 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 colored discs that show their location. A number inside a disc indicates the calculated 3000km MUF from the critical ionospheric frequency, foF2. The information from 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, foF2 (as shown in the next map).
Additional Notes:
The accuracy is insufficient for professional radio services because:
Inaccuracy results from the limited coverage of radio stations. Therefore, the map is based on data interpolation. The algorithm aims to find the MUF (or FoF2) at scattered points globally, but achieving accurate extrapolation from few data points is challenging. The guessing process is better in areas close to measurement stations, but uncertainty increases in distant areas.
Stations may provide inaccurate or disagreeing data, leading to peculiar results in an attempt to align measurements. Stations may go off-line or re-appear, causing unexpected changes in the model's global picture due to the limited number of data points initially used.
Real-time ionosonde data sharing has been discontinued by countries like Russia, China, Japan, the US Space Force, and NOAA in recent years. Some ionosondes are only available through NOAA, and if GIRO outages occur, maps on this site may stop updating.
There is uncertainty associated with predicting the ionosphere's state using vertical sounding data.
Geomagnetic storms and blackouts, resulting from elevated X-ray flares and proton ejections, can significantly impact the accuracy of MUF estimation derived from vertical sounding data. This map does not refer to D-region absorption or to ionospheric noise. The propagation model is too simple.
The effects of geomagnetic storms and blackouts are implicitly included in the results of ionograms. But it is impossible to predict band conditions for the next few hours. Geospace dynamic models are still being developed.
The "MUF(3000km)" project is the result of research and development by Andrew D Rodland - KC2G, which is based on an earlier work by Matt Smith - AF7TI. WWROF financing and data from ionosonde operators all over the world, provided by GIRO and NOAA made it feasible.
The colored regions of this map, which are rebounded by iso-frequency contours, illustrate the critical frequency that is expected to bounce off of the ionosphere at near vertical angle. The ham bands (160, 80, 60, 40, 30 ,20m) are designated by iso-frequency contours: 1.8, 3.5, 5.3, 7, 10.1, and 14 Mhz.
foF2, as measured by ionosondes, is the raw data that powers the site. Colored discs indicate the location of stations. A number inside each disc represents the critical frequency, foF2.
Another NVIS map provided by the Australian Space Weather Service is updated every 15 minutes. It displays contours of the critical ionospheric frequency - foF2. There are a few differences between this map and the KC2G map, mainly due to the choice of frequencies for the contours. The KC2G map highlights ham bands. The following map, however, is designed for commercial use.
Near real-time critical ionospheric frequency map Click on the map to view the source page. There is further information.
T Index Map - foF2 is provided by Australian Government Space Wheather Services
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.
Near real-time foF2 anomaly map Click on the map to view the source page.
The plot above shows a near real-time foF2 anomaly map. The anomalies are calculated by subtracting the median foF2 for the last days from the currently observed foF2. The current foF2 and dataset are used to calculate the median foF2 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.
Recent foF2 measurements at various locations of Australia, New Zealand and East Antarctica
foF2 Plots courtesy of Australian Space Weather Forecasting Centre Click on the map to view the source page.
The recent LUF (ALF) chart provided by Australian Space Weather Alert System
The map below shows the LUF for 1500 km HF circuits. If the frequency you wish to use is lower than the LUF, then communication is unlikely; if it is higher than the LUF, then communication is possible.
For short circuits compared with 1500 km, the LUF values are likely to be too high, but communications will still be possible at slightly lower frequencies. For much longer circuits, slightly higher frequencies than the suggested LUF can still be affected by the fadeout.
The following real-time chart is event-driven and updates only when a flare of magnitude M1 or greater is observed.
LUF plot courtesy of Australian Space Weather Forecasting Centre Click on the map to view the source page.
Chapter 9. Ionospheric Dynamics
The upper atmosphere and ionosphere interact in a two-way nonlinear manner, known as the atmosphere-ionosphere system. The meteorological processes below influence the system, as do the solar and geomagnetic processes above. Gravity waves and planetary waves play significant roles in the energy and momentum budget of the thermosphere, while geomagnetic storms cause significant dynamical and thermal changes. Understanding this system requires a whole-hearted approach. This chapter discusses the effects of this complicated system on sky waves.
Sporadic E (Es) indicates occasional reflections from highly ionized "plasma clouds" in the lower E region.
Reflection from Sporadic E plasma cloud
Operators may use Es for making mid-range contacts on the VHF amateur bands: 50 MHz (6 m), 70 MHz (4 m), and 144 MHz (2 m).
Sporadic E Propagation in 2 minutes courtesy of Andrew McColm, VK3FS
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 the summer solstice at VHF bands, which under normal conditions can only propagate by line-of-sight.
9.2 Ionospheric Clouds
All the ionospheric regions consist of "plasma clouds" as illustrated below:
The dynamic ionosphere causes signal fading (QSB) over time. Small-scale irregularities in the ionosphere are observed at all levels, with periodic motions attributed to neutral atmospheric waves interacting with ionized components in the upper atmosphere. While understanding is limited, the research promises short-term changes.
Additionally the ionosphereic regions are disrupted by (1) The chaotic solar activity and (2) The tropospheric weather from far below.
What effect does tropospheric weather have on the ionosphere?
Storms, hurricanes, and strong wind patterns can all temporarily alter the TEC caused by EUV solar radiation.
In other words, the ionosphere and troposphere are coupled by a variety of mechanisms.
For instance, a lightning storm can cause electrodynamic interaction, as shown in the following figure.
The following figure illustrates electrodynamical coupling of the Troposphere with the Ionosphere:
Ionospheric clouds due to Troposphere-Ionosphere coupling
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.
The Digisonde Directogram was developed to identify ionospheric plasma irregularities.
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.
9.3 Ionospheric Disturbances
The ionospheric disturbances, SID, TID, GRB, are explained below.
9.3.1 "Sudden Ionospheric Disturbances" (SID) are any one of several ionospheric perturbations resulting from abnormally high ionization or plasma density in the D region of the ionosphere and caused by solar flares and/or solar particle events (SPE).
The SID affects HF sky wave signal strengths, with lower frequencies being more heavily absorbed and resulting in a larger decrease in signal strength (see figure below).
Fadeout signal strength vs. time, courtesy of Australian Space Weather Service
During a strong SID, the LUF will increase to a frequency higher than the MUF, thus closing the usable frequency window, an event called a fadeout or blackout.
The current fadeout (SWF: short-wave fadeout) event, if any, is shown below. Real-time chart provided by the Australian Space Weather Alert System.
9.3.2 "Traveling Ionospheric Disturbance" (TID) is a wave-like structure passing through the ionosphere that alters the altitude and angle of refraction of sky waves. TIDs travel horizontally at 5–10 km/minute, with varying phases, amplitudes, and angles of arrival. Some originate in auroral zones (high latitudes).
Probing traveling F region ionospheric disturbances
The Super Dual Auroral Radar Network (SuperDARN) is an international network of 35 HF radars (8 MHz–22 MHz) located in the northern and southern hemispheres.
SuperDARN site in Holmwood SDA, Saskatoon
The SuperDARN are designed to study F region Ionospheric dynamics, instability, disturbances and storms. The research covers geospace phenomena, including field-aligned currents, magnetic reconnection, and mesospheric winds. It tests theories of polar cap expansion and contraction under changing IMF conditions, observing large-scale responses to substorms. The collaboration includes various institutions.
9.3.3 "Cosmic Gamma-ray Bursts" (GRB) may also cause communications disturbances. Measurable effects are rarely observed.
On October 9, 2022, there was a cosmic gamma-ray burst that affected all ionospheric and stratospheric regions. These are intense explosions observed in distant galaxies, the brightest and most extreme events in the universe. NASA describes them as the most powerful class of explosions since the Big Bang. Afterglows are longer-lived and typically emitted at longer wavelengths.
Studies are being done on this phenomenon.
Chapter 10. Total Electron Content (TEC)
What is TEC? TEC is the total number of free electrons present along a path between two points.
Why is TEC important for HF propagation conditions?
TEC correlates with the critical frequency, foF2, and is therefore implemented in a variety of ionosphere models. Moreover, the total electron content can provide additional information about the structure and dynamics of the ionosphere. It can detect and monitor ionospheric disturbances, such as those caused by solar flares or geomagnetic storms.
Units: 1 TEC Unit (TECU) is the number of free electrons per square meter (x1016) for a shell height of 400 km directly above a certain point. Values in Earth’s atmosphere can range from a few to several hundred TEC units.
How is TEC measured? Data is gathered from GPS receivers worldwide, observing carrier phase delays in radio signals from satellites above the ionosphere, often using GPS satellites.
The effect of Tropospheric weather:
The troposphere and ionosphere are separate atmospheric regions with distinct functions. However, they do interact through various processes.
Tropospheric lightning may induce changes in total electron content and consequently affect HF propagation conditions.
Thunderstorms can also worsen the signal-to-noise ratio, in particular in the lower HF bands; i.e., tropospheric weather may affect these conditions, especially in tropical regions.
Thus, monitoring and modeling TEC patterns and variations allows us to better understand and prepare for the constantly changing atmospheric conditions.
The Ionosphere Monitoring and Prediction Center operated by the German Aerospace Center (DLR) provides near real-time TEC map:
Near real-time TEC map
An animated map courtesy of HB9VQQ demonstrates past TEC variations:
Banners and widgets are visual aids used to display global propagation conditions. They help radio operators quickly understand current global conditions and make informed decisions about their operations. The following banners and widgets were created by Paul L. Herrman (N0NBH).
Extreme Ultra Violet (EUV) radiation creates the ionosphere, especially the F2-region. Since EUV is fully absorbed by the ionosphere, it doesn't reach the ground, making direct measurement impossible for ground-based devices. Before the space age, scientists relied on two indirect markers to gauge the ionization levels of the F2-region.
SSN - Sunspot Number is a count of the number of dark spots seen on the sun.
Higher SSN values correlate with improved conditions on 14 MHz band and above:.
SFI - Solar flux index refers to the intensity of solar radio emissions at 10.7 cm (2,800 MHz).
Higher flux correlates with increased ionization levels of the E and F regions, enhancing HF radio propagation conditions.
Understanding the correlation between 10.7 cm solar flux and sunspots:
Radio telescopes in Ottawa (February 14, 1947-May 31, 1991) and Penticton, British Columbia, have routinely reported solar flux density at 2,800 megahertz since June 1, 1991. Levels are determined every day at local noon (1700 GMT in Ottawa and 2000 GMT in Penticton) and then corrected to within a few percent for factors such as antenna gain, air absorption, bursts in progress, and background sky temperature.
The solar flux is quoted in terms of SFU (Solar Flux Units) = 10-22 Watts per meter2 per Hz.
The amount of solar radiation varies around the world. Even with correction factors added, it is difficult to obtain a consistent series of results. To overcome this, the reading from the Penticton Radio Observatory in British Columbia, Canada, is used as the benchmark. As a result, these numbers are extremely useful for predicting ionospheric radio propagation.
The undisturbed solar surface, developing active regions, and short-lived enhancements above the daily level all contribute to 10.7 cm radio flux. Levels are determined and corrected to within a few percent.
Sunspot Number records have been traced back to the 17th century. However, these records are open to subjective observation and interpretation.
The 10.7 cm wavelength (2,800 MHz) also coincides with the daily number of sunspots. Both databases are interchangeable. The 10.7 cm Solar Flux data is more stable and reliable. See a comparison table between SSN and SFI.
11.3 - Geomagnetic indices explained
Geomagnetic indices measure disturbances in Earth's magnetic field, which can disrupt HF propagation by increasing atmospheric noise and weakening radio signals. These indices are crucial for understanding the potential impacts on all communication systems, satellite operations, and even power grids.
K and A are local indices
K-index: This index represents short-term (3-hour) geomagnetic activity at a specific geomagnetic station. It quantifies disturbances in Earth’s horizontal magnetic field by comparing geomagnetic fluctuations, measured by a magnetometer, to a quiet day. The K-scale is logarithmic, allowing for a more manageable representation of the wide range of geomagnetic activity magnitudes.
A-index: This index averages K values to provide a linearized view of geomagnetic activity. It is important for predicting and understanding the effects of geomagnetic storms on HF communications.
Kp and Ap are global planetary indices
K and A indices measure local geomagnetic activity at a single observatory. A global average of these indices is calculated from 13 mid-latitude geomagnetic observatories, marked as Kp and Ap:
Kp: Average of K-indices from 13 observatories, indicating broad geomagnetic activity.
Ap: Daily global geomagnetic activity, derived from the Kp index.
The HPo (GFZ) indices are less commonly referenced. This higher time resolution can be crucial for predicting and mitigating the impacts of geomagnetic storms on various technologies.
The half-hourly Hp30 and hourly Hp60, developed at GFZ (German Research Center for Geosciences), offer improved time resolutions compared to the three-hourly Kp. Together with the linear versions Ap30 and Ap60, they are collectively known as the HPo index, providing near-real-time data from about 13 geomagnetic observatories.
11.4 - A review of the propagation indices
HF propagation indices are essential tools for amateur radio operators to evaluate and predict radio wave propagation conditions. The key indicators include the Maximum Usable Frequency (MUF), Lowest Usable Frequency (LUF), and ionospheric noise levels. These indicators are correlated with solar indices such as the Sunspot Number (SSN), Solar Flux Index (SFI), and X-ray flux, as well as geomagnetic indices like the A and Kp indices. Understanding all these parameters is crucial for accurately estimating HF propagation conditions.
The observed indices of propagation conditions over the last 30 days, courtesy of QRZCQ Please be aware of the various acronyms in the above chart SF:=Flux index; SN:=Spot number; AI:=A index; KI:=Kp index; and XR:=X-Ray index.
Solar events are classified into two types: quiet and active. Both affect HF skywave propagation conditions.
12.1 - Quiet sun
The sun emits Electromagnetic Radiation across a wide spectrum from Gama-rays to ELF (long radio waves).
The spectrum of solar electromagnetic radiation
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 chromospherecourtesy of 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-regionionization.
Lyman series-alpha Hydrogen-spectral-line at a wavelength of 121.6 nm ionizes Nitric Oxide (NO) at the D-region causing mostly
absorption of HF bands below 10 MHz.
12.2 - Active Sun
Solar activity is driven by the eleven-year periodic reversal of the sun's magnetic field. There is a helical dynamo in the sun's core and a chaotic dynamo near the surface.
The main solar phenomena associated with HF radio propagation on Earth are:
Sunspots (last from a few days to a few months); the number of spots varies in 11-year solar cycle (a deterministic chaos)
Sunspots are dark, cooler regions of the Sun's surface created by local magnetic activity.
These local magnetic fields 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.
Two images of the Sun, that were taken at the same time (February 3, 2002)
by Solar and Heliospheric Observatory (SOHO) satellite courtesy of European Space Agency and NASA.
on the left: Sunspots in visible light on the right Extreme Ultra Violet (EUV 30.4 nm)
Q. What is the reason for analyzing sunspots in both visible and ultraviolet light?
A. "Visible light" is what we see with our bare eyes. Magnetic disturbances are only detectable in EUV light.
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 following video chronicles solar activity from Aug. 12 to Dec. 22, 2022, as captured by NASA’s Solar Dynamics Observatory (SDO). From its orbit in space around Earth, SDO has steadily imaged the Sun in 4K x 4K resolution for nearly 13 years. This information has enabled countless new discoveries about the workings of our closest star and how it influences the solar system.
133 Days on the Sun - courtesy of NASA Goddard
Historical SFI.
12.4 Solar storms
For centuries, people have been observing sunspots without knowing what they are.
Coronal Mass Ejections (CMEs) often appear as twisted ropes. They can occur alongside solar flares or spontaneously, disrupting the solar wind and damaging systems both near-Earth and on its surface. The magnetic fields of CMEs merge with the interplanetary magnetic field.
(A) The "Solar Flares" are bursts of energeric radiation (X-ray and EUV, 1–10 Å) from the Sun.
A Solar Flare courtesy of NOAA, May 2023
Solar flares enhance the ionization of the ionosphere, specifically the D-region at 50-90 km altitude.
A coronal mass ejection (CME) is a significant ejection of plasma mass from the sun's corona into the heliosphere, following solar flares.
Image of coronal mass ejection (CME) captured by NASA and ESA's SOlar and Heliospheric Observatory (SOHO) Credit: NASA / GSFC / SOHO / ESA
CMEs release large amounts of matter into the solar wind and interplanetary space, primarily consisting of electrons and protons. Pre-eruption structures require magnetic energy, while post-eruption structures form magnetic flux ropes and prominences.
The types of CMEs:
Halo CMEs appear as a halo around the sun; often directed towards or away from Earth.
Partial Halo CMEs cover part of the sun; less impactful than full halos.
Narrow CMEs are confined to a narrow width; less likely to impact Earth directly.
Fast CMEs travel faster than 500 km/s. They can cause significant geomagnetic storms.
Slow CMEs travel slower than 500 km/s. Generally, they have a lesser impact.
Each type can affect Earth's magnetosphere differently, potentially causing geomagnetic storms and disruptions.
Solar Proton Event (SPE) refers to protons heading Earth by solar wind or a CME.
Solar Energetic Particles (SEP) include electrons, protons, and alpha particles that are ejected from the Sun at high speeds.
Reaching Earth, they interact with the magnetosphere.
Guided by the Earth's magnetic field, the charged particles are attracted by the north and south magnetic poles causing auroras.
Fadeouts and blackouts of radio communication occur when highly charged particles reach Earth.
Energetic protons penetrate all the way down to the D-region, boosting ionization levels close to the poles.
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) of transpolar transmissions. These blackout events often last between 24 and 48 hours.
Sunspots, unlike flares and CMEs, are statistically predicted. Sub-topic 12.5 discusses the Solar Cycle. Sub-topic 12.6 presents long term prediction for Radio Flux at 10.7 cm.
12.5 Solar Cycle
Sunspots change in eleven year cycles. There are many sunspots during solar maximum and few during solar minimum.
Solar Cycle Sunspot Number Progression Source: The International Space Environment Service (ISES)
An animated overview of the Solar Cycle; published by NASA in May 2013
Solar magnetic flips are associated with solar maximum, when the number of sunspots is near its maximum, but it is often a gradual process that can take up to 18 months. The reversal will most likely take three to four months to complete.
The sunspot cycle begins when a sunspot appears on the sun's surface at roughly 30 degrees latitude. The formation zone then travels toward the equator. At its peak intensity, the sun's global magnetic field reverses its polar regions, as if the positive and negative ends of a magnet were flipped at each of the sun's poles.
There have been 24 (11-years) solar cycles since 1749. The magnetic field of the sun totally flipped every 11 years or so. In other words, the sun's north and south poles switched places. After two reversals (22 years), the solar magnetic field returns to its former orientation. This is known as "Hale cycle".
Understanding the complex interactions between solar magnetic fields, sunspots, and the solar cycle is crucial for comprehending the Sun's dynamic behavior and its impact on Earth, specifically HF propgation conditions.
The Current 25th Cycle began in 2020. The number of sunspots observed far exceeds predictions.
July 2024 marked the peak of Solar Cycle 25, with a monthly average sunspot number of 196.5, a new high. The last time this occurred was in December 2001. Despite predictions of a similar cycle size to previous cycles, Solar Cycle 25 exceeded these expectations.
Sunspot Number Progression during Solar Cycles 24 and 25
The recent Estimated International Sunspot Number (EISN)
Solar flux like sunspot number can be also used to show the observed and predicted Solar Cycle.
Solar Cycle Notable Events
In the 19th century, more than 150 years ago, extreme events had been observed.
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.
Comparison of the recent Solar Cycles by Jan Alvestad:
The current solar cycle (25) is stronger than the previous cycle (24) but weaker than the three previous cycles (21-23)
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
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
Solar flares are also characterized by radio wave radiation at different frequencies
Different sunspot cycles can have different radio burstdistributions at 245 MHz.
That is to say that the sunspot cycles can vary and that they may not be considered identical.
The NOAA Space Weather Prediction Center predicts monthly sunspot number and 10.7 cm radio flux. The Sunspot Number indicates the number of sunspots visible on the solar surface, whereas the 10.7 cm Radio Flux quantifies the solar radio emission at a 2,800 MHz. A combination of observational data, analytical methods, and AI techniques contributes to predicting solar flux.
There are two recommended sources:
Predicted sunspot number and Radio Flux at 10.7 cm
are provided by NOAA / NWS Space Weather Prediction Center
This is a multi-year (2022-2040) forecast of the monthly Sunspot Number and the monthly F10.7 cm radio flux.
The Predicted values are based on the consensus of the Solar Cycle 24 Prediction Panel.
27-Day Outlook of 10.7 cm Sun Radio Flux and the Earth Geomagnetic Indices prepared by the US Dept. of Commerce, NOAA, Space Weather Prediction Center.
The 27-day Space Weather Outlook Table , issued Mondays by 1500 UTC, is a numerical forecast of three key solar-geophysical indices; the 10.7 cm solar radio flux, the planetary A index, and the largest daily K values. A complete summary of weekly activity and 27-day forecasts since 1997, plus an extensive descriptive, are online as the weekly.
3 Day Geomagnetic and Aurora ForecastSolarHam. Kevin, VE3EN SolarHam website relays data and images from various sources.
Solar Flares
are sudden, intense bursts of radiation from the sun's surface. They can disrupt communications and navigation systems on Earth and pose risks to astronauts.
Solar Wind
is a continuous flow of charged particles, mainly electrons and protons, from the sun. It affects the entire solar system and can influence space weather around Earth.
CMEs - Coronal Mass Ejections are massive bursts of solar wind and magnetic fields rising above the solar corona or being released into space. CMEs can cause significant geomagnetic storms when they collide with Earth's magnetosphere.
The Magnetosphere acts as a shield against the solar wind. It deflects most of the charged particles, protecting our planet from harmful solar radiation.
Geomagnetic storms
are disturbances in Earth's magnetosphere caused by enhanced solar wind. They can lead to beautiful auroras but also disrupt power grids, satellites, and communication systems.
An illustration of the Space Weather environment The Lagrange Mission was designed to monitor hazardous CMECoronal Mass Ejections headed toward Earth.
On the right side (of the above picture), you may see an illustration of the Magnetosphere, which protects Earth from Solar Wind. The magnetosphere is a part of a dynamic, interconnected system that responds to solar, planetary, and interstellar conditions. It is disturbed when solar wind interacts with the space environment surrounding Earth.
The Lagrange point L1 allows a sattelite to maintain a constant line with Earth as it orbits the Sun.
Animation of a satellite trapped at the L1 point of the Sun-Earth-Moon gravitational system. Published by Space Weather Live
Australian Space Weather Service: "Events beyond the Earth's atmosphere impact our technology and the near-Earth space environment."
Wikipedia: "A branch of space physics and aeronomy, or heliophysics, is concerned with the time-varying conditions within the Solar System, emphasizing the space surrounding the Earth."
Definition of Space Weather by INPE, Brazil: "There is no full agreement among all the members of the international communities on the definition of space weather. Therefore, they list the definitions of space weather according to some organizations."
The solar wind is a stream of charged particles emitted by the sun's corona into outer space. These particles interact with Earth’s magnetosphere and magnetic field, significantly affecting skywave propagation and triggering auroras around the Earth’s poles.
The illustration above depicts the solar wind reaching Earth’s magnetosphere. It shows how the solar wind rearranges the magnetosphere, compressing the magnetic field on the side facing the sun while elongating it on the opposite side.
The illustration above shows the solar wind reaching Earth’s magnetosphere. It demonstrates how the solar wind compresses the magnetic field on the side facing the sun while elongating it on the opposite side.
The solar wind can vary greatly in speed, density, temperature, composition, and the interplanetary magnetic field (IMF). These variations are influenced by solar activity, such as coronal mass ejections (CMEs) or coronal holes. Although predicting exact changes in the solar wind is challenging, there is some correlation with sunspots and solar flares.
The Interplanetary Magnetic Field (IMF) is the magnetic field carried by the solar wind. It interacts with Earth’s magnetosphere, and when aligned with Earth’s magnetic field, it can cause magnetic reconnection events, leading to geomagnetic storms.
The solar wind can reach Earth between 20 to 30 minutes after a solar storm begins and up to four days later.
Earth’s magnetosphere is the region around the planet dominated by its magnetic field. It acts as a shield, protecting us from the harmful effects of solar wind and cosmic radiation.
The magnetosphere is a "magnetic bubble" that surrounds Earth. Its shape depends on the solar wind and the orientation of the Earth’s magnetic field.
Earth's Magnetic field—the geomagnetic field.
The orientation of the Earth’s magnetic field is composed of two variables: (1) the Earth's axis is tilted 23.5 degrees to the ecliptic plane, and (2) the Earth's magnetic field is tilted 11 degrees relative to the Earth's axis.
The geomagnetic conditions, influenced by solar activity, significantly impact the HF propagation conditions due to the Earth's magnetic field and the state of the surrounding magnetosphere.
13.4 Geomagnetic activity
Geomagnetic activity refers to the disturbances in Earth’s magnetic field caused by solar wind and other solar phenomena. These disturbances can range from minor fluctuations to major geomagnetic storms.
13.5 Impact of Geomagnetic Storms on Radio Communications
Geomagnetic storms are significant disturbances in Earth’s magnetosphere caused by solar wind shock waves or coronal mass ejection (CME) from the Sun that disrupt HF communication and trigger auroras:
Geomagnetic storms are more frequent during periods of high solar activity.
Geomagnetic storms occur one to four days after a CME.
These storms alter the normal conditions of the magnetosphere and ionosphere.
Radio waves of certain frequencies may experience high absorption levels, leading to rapid fading events and unusual radio propagation paths.
There are two main effects on radio communications: storms can reduce the MUF in equatorial regions, worsening conditions for HF bands, while increasing the MUF in polar regions, improving low VHF contacts.
HF absorption levels are higher, especially in the lower bands near the equator, which can cause a complete fadeout of HF signals.
Illustration of geomagnetic storms—"auroras" as seen from earth close to the polar regions:
Public-domain photographs show electrical discharge in ionospheric regions near Earth's polar regions.
The correlation between geomagnetic storm G-scale, Kp and Ap indices, and HF propagation conditions is quite intricate.
A geomagnetic storm has three phases: initial, main, and recovery. The initial phase involves an increase in the Disturbance Storm Time (Dst) index by 20 to 50 nano-Tesla (nT) in tens of minutes. The Dst index estimates the globally averaged change of the horizontal component of the Earth's magnetic field at the magnetic equator based on measurements from a few magnetometer stations. Dst is computed once per hour and reported in near-real-time.
Current The solar wind affects ionospheric conditions and HF propagation. The Rice Space Institute’s provides "real-time dials" showing the current solar wind (SW) and the interplanetary magnetic field as measured by ACE.
Speed x100 km per second
Density of charged energetic particles per unit volume
Pressure The force per unit area required to stop the solar wind flow; nP = nano pascals
Proton's Temperature
Background color indications: no disruptionspotential disruptionsdisruptions.
The Current Planetary Geomagnetic Activity — Kp and K-index distribution — low, middle and high latitude
Recent geomagnetic activity, measured by the Kp index, provided by the European Space Weather Service Network
Real-time K index near Australia provided by ASWFC Real-time Australia K index The current Australia K index map
Recent 8 days of UK K indices and the global Kp provided by British Geogolocal Survey
The global Kp
13.7 Space weather prediction
Space weather prediction rather forecasts are based on observations from space and gound:
Space observatories
Satellites play a crucial role in predicting space weather and its impact on HF radio propagation:
ACE (Advanced Composition Explorer): Provides real-time data on solar wind and geomagnetic storms, giving up to an hour's advance warning of space weather events that can impact Earth.
Located at the L1 Lagrange point, where Earth’s and Sun’s gravitational forces balance.
GOES (Geostationary Operational Environmental Satellites):
Tracks solar flares and other space weather phenomena, aiding in timely alerts and mitigating potential impacts on HF propagation and space technology.
DSCOVR (Deep Space Climate Observatory): Monitors real-time solar wind, providing early warnings for geomagnetic storms.
SDO (Solar Dynamics Observatory): Delivers detailed images of the sun divided into four spectral bands.
SOHO (Solar and Heliospheric Observatory): Monitors solar activity and space weather.
STEREO (Solar and Terrestrial Relations Observatory): Consists of STEREO-A (Ahead) and STEREO-B (Behind),
which orbit the Sun near the stable Lagrange Points L4 and L5 to provide a 3D view of solar phenomena from multiple perspectives.
The Parker Solar Probe significantly contributes to the prediction of space weather. By flying closer to the Sun than any previous spacecraft, it collects unprecedented data on the solar wind and the Sun’s corona.
Ground-based observatories
Ionosondes: Measure the ionosphere’s electron density profile by transmitting radio waves and analyzing the returned signals. They help determine the ionospheric layers’ height and density, crucial for predicting HF radio wave propagation.
Magnetometers: Measure geomagnetic fluctuations, providing data on the Earth’s magnetic field. Near the equator, they help monitor geomagnetic storms and disturbances that can affect HF propagation by altering the ionosphere’s structure.
Radio telescopes: Detect solar radio emissions, which can indicate solar flares and other disturbances. By monitoring these emissions, scientists can predict space weather events that might impact HF radio communication.
Ground-based observatories, combined with satellite data, provide a comprehensive picture of space weather conditions affecting HF propagation.
13.8 Low Accuracy of Geomagnetic Storm Predictions
Geomagnetic storm predictions have low accuracy because only about 12% of coronal mass ejections (CMEs) actually reach Earth. This means we receive warnings about potential geomagnetic storms that frequently (~88% of the time) do not occur.
Historical data shows that only a few solar storms have reached the intensity of the Carrington Event, such as the one in Quebec in 1989 and a series of storms in 2003. In 2012, a powerful CME sped toward Earth's orbit but narrowly missed.
The ionization level of the ionosphere and the operating frequency significantly influence skywave propagation, a key element in global HF communications.
Solar events can disrupt HF propagation, leading to unpredictable conditions.
Since the 1970s, satellites have largely replaced legacy HF communications, but recent technological advances have mitigated many of the traditional disadvantages.
Today, we can predict wave propagation conditions more accurately.
Practical Techniques:
Use weak signal digital modes (FT8, JT65, WSPR) for probing the ionosphere.
Utilize PSKReporter for real-time feedback and strategy adjustments.
Map for amateur radio uses (HamRadio) : In the shape of a terrestrial world globe, the MUF and Aurora Borealis layer. This gives the distances and directions of antennas between amateur radio stations, the position on the Maidenhead Locator grid.
The Ionosphere June 2024 Andrew McColm, VK3FS The ionosphere, generated by solar radiation, plays a crucial role in radio communication with the D, E, and F2 regions outlined in this video.
The Ionosphere Part 2 Aug 2024 Andrew McColm, VK3FS Ionized oxygen atoms in the E region are critical to good DX in the 50–144 MHz bands. EUV and activity from our sun affect the MUF and LUF in the 3–30 MHz bands.
Propagation of radio waves explainedJean-Paul Suijs, PA9X Radio waves; Earth’s atmosphere (from Troposphere to Ionosphere); Main Propagation modes; Ionospheric 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.
EVE OverviewSolar Phys. - The Solar Dynamics Observatory The EVE project (real-time high-resolution EUV measurements) was designed to improve understanding of the evolution of solar flares and extend the related mathematical models used to analyze solar flare events.
Dr. Tamitha Skov is a space weather physicist that reviews solar storms and anlyzes how they affect spacewetaher. She specializes in forecasting and analyzing space weather processes in the heliosphere and exosphere. Her work extends to both traditional media and social platforms. As a credentialed space weather forecaster, she helps the public understand the effects of space weather.
Solar wind Magnetospheric Multiscale (MMS):
Four Magnetospheric Multiscale (MMS) spacecraft, flying in a tetrahedral formation, detect charged particles and magnetic fields in space, helping scientists understand how solar wind interacts with Earth’s magnetosphere. This mission, involving Rice University, studies magnetic reconnection, acceleration, and turbulence in space.
Mastering HF Communication: Decoding Space Weather Data August 2023 Chris, N6CTA The article explains how radio amateurs can use real-time space weather data to optimize HF communication. In particular it mentions the Hp30 index, MUF , f0F2, and eSFI. These metrics are crucial for selecting the right frequency bands and forecasting propagation conditions. It provides practical insights and essential links to help enthusiasts determine the best conditions for different types of HF operations, including high-band DX, low-band DX, and NVIS operations. Additionally, it includes tips on antenna optimization to enhance HF communication effectiveness.
Gamma-ray burstWikipedia Gamma-ray bursts are the most intense explosions in the universe, observed in distant galaxies, with longer-lived afterglows and longer wavelengths emitted.
Space weather effects on communications systems June 2020 Mark MacAleste, CISA–Cybersecurity and Infrastructure Security Agency Effects, Operational Impacts, Mitigations, and Research Gaps for Communications Systems
Online Activity and Band Monitoring
Gathering information of real-time activity on the ham bands
HF Propagation ToolsHamwaves - Serge Stroobandt, ON4AA Real-time online dashboard of solar activity influencing HF propagation on Earth.
Real‑time HF propagation space weatherHamwaves - Serge Stroobandt, ON4AA Real-time online dashboard of solar activity influencing HF propagation on Earth.
An Open-Source IRI-based Nowcasting Tool for Ionospheric Electron Density and HF Propagation Andrew D Rodland (2022 Harvard Abstracts)
An overview of the software and the models behind prop.kc2g.com, a website using the IRI-2016 model, conditioned on near-real-time ionosonde data, to provide global maps of MUF(3000) and foF2. While primarily designed for radio amateur use, this system is useful for nowcasting of F region ionospheric density and mesoscale low elevation HF propagation characteristics.
The Advanced Stand Alone Prediction System (ASAPS)ASWFC Australian Space Weather Forecasting Centre offer three software products to predict HF propagation:
1. GWPS - designed for HF operators working in defence and emergency services
2. ASAPS Kernel - The Advanced Stand Alone Prediction System designed for government, defence and emergency services
3. Consultancies - designed for industry, defence and emergency services
DR2W - Predict Propagation ConditionsDK9IP (Winfried), DH3WO (Wolfgang), DJ2BQ (Ewald), ZS1AO/DJ2HD (Mathew)
A Long-term forecasting cannot take into account unpredicted ionospheric and magnetic disturbances or anomalies
VOACAP Online Application for Ham RadioJari Perkiömäki, OH6BG / OG6G
VOACAP forecats monthly average of the expected reliability with diurnal and seasonal variations.
A Long-term forecasting cannot take into account unpredicted ionospheric and magnetic disturbances or anomalies.
IOCAP Application Introduction VideoSANSA The South African National Space Agency (SANSA) created i/o cap Primary Work Surface, an operational HF communication solution. It's a modern, user-friendly HF frequency prediction tool that's simple to use and accurate. In a software program, it blends space weather research and practical HF experience.
DX Toolbox - Shortwave / Ham Radio / HF Radio PropagationBlack Cat Systems This is a software application that provides a range of tools for HF radio operators, including propagation forecast based on the Solar Terrestrial Dispatch (STL) model.
It also includes a real-time solar data display and a grayline map.
Proplab-Pro v3: RevieweHamManualspacew.com
Proplab-Pro 3.2 (Build 45, March 2023) Three-dimensional ray-tracing ionosphere; can run as standalone; not free.
Mathematical models and numerical procedures simulate dynamic processes in HF radio propagation, considering interactions between the Sun's and Earth's surfaces, sun, space weather, ionosphere, and atmosphere.
Amateur propagation programs, accessible via the internet, provide graphical solutions and simulate ionospheric effects using near-real-time data or well-known functions, achieving high accuracy.
VOACAP, a US government-funded HF propagation prediction engine, has been continuously improved over the years, with its software technology firmly rooted in the 1980s.
Predicting and Monitoring PropagationDXLab * 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.
PropView forecasts LUF and MUF between two locations over a 24-hour period using VOACAP, ICEPAC, and IONCAP engines. It can specify locations via latitude/longitude entry or DXCC prefix entry. PropView can build schedules for the IARU/HF beacon network and monitor the NCDXF/IARU International Beacon Network. It interoperates with Commander and DXView for automatic monitoring and location display.
ITU-R DirectoryITU Software, Data and Validation examples for ionospheric and tropospheric radio wave propagation and radio noise
ITU-R P.533 model This is an ITU table links to Software, Data and Validation examples for ionospheric and tropospheric radio wave propagation and radio noise in a wide range of propagation conditions. The ITUR HF Prop experimental software, was written by G4FKH and HZ1JB, and is based on the ITU-R P.533 method.
It uses a probabilistic approach to estimate radio coverage with algorithms that are supposed to be more accurate than other similar programs.
VOACAP–Voice of America Coverage Analysis Program is a professional hf system performance prediction tool VOACAP predicts monthly average expected reliability, considering diurnal and seasonal variations, but does not account for unpredicted ionospheric and magnetic disturbances or anomalies.
ITUR HF Prop
Prediction of HF circuits based on Recommendation ITU-R P.533 model - an improved (2017) point-to-point propagation prediction tool, based on an ITU engine, developed by Gwyn Williams, G4FKH.
Amateur RadioWikipedia
The name of the hobby "Amateur Radio" or "Ham Radio" refers to a non-commercial communication, wireless experimentation, self-training, private recreation,
radiosport, contesting, and emergency communications activity that may use radio transmitters and receivers.
Amateur radio stationWikipedia Read about different types of stations used by amateur radio operators.
Radio AmateurWikipedia "Radio Amateur" or "Radio Ham" is the person usualy a licensed operator who communicates with other radio amateurs on amateur radio frequencies.
Automatic Link Establishment (ALE) is a feature that enables a radio station to select the best frequency to establish a connection with another HF radio station or network of stations. It replaces traditional prediction techniques and reliance on trained operators.
Short and long term prediction of ionospheric HF radio propagationJ. Mielich und J. Bremer (2010) A modified ionospheric activity index AI has been developed on the basis of ionospheric foF2 observations.
Such index can be helpful for an interested user to get information about the current state of the ionosphere. Using ionosonde data.
Long-Term Changes in Ionospheric Climate in Terms of foF2Jan 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.
A simplified HF radio channel forecasting modelAdvances 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.
Ionospheric currentUpper Atmospheric Science Division
of the British Antarctic Survey
Comparison of observed and predicted MUF(3000)F2 in the polar cap regionRadio Science AGU (2015) Comparison of ICEPAC, VOACAP, and REC533 models reveal diurnal and seasonal variations. Summer diurnal variation is not represented by the VOACAP or ICEPAC models. REC533 surpasses VOACAP during the winter and equinox months. ICEPAC performs poorly during periods of low solar activity.
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.
Understanding HF Skywave PropagationFor Radio Amateurs: Beginners and Advanced
Table of contents
References
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Peak (He II) EUV radiation at a wavelength of 30.4 nm is the most important solar emission contributing to half of the Ionospheric F-region ionization
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