Sunspot Cycle and Its Influence
Propagation Page
The Phenomenon of Solar Flares and Shortwave Propagation
Solar flares, intense bursts of radiation from the Sun can significantly impact shortwave radio broadcasts on Earth. These flares occur when magnetic energy built up in the solar atmosphere is suddenly released. The energy from a solar flare can disrupt the Earth’s ionosphere, a layer of the atmosphere crucial for shortwave propagation. When solar flares happen, they can cause sudden ionospheric disturbances (SID), leading to degraded or completely blocked shortwave radio signals, a phenomenon often referred to as “solar flare and radio disturbances.”
Solar Flares and Radio Disturbances
The relationship between solar flares and radio disturbances is complex. Shortwave radio waves travel long distances by reflecting off the ionosphere. During a solar flare, the ionosphere’s density and composition change rapidly, causing shortwave signals to be absorbed rather than reflected. This can lead to shortwave radio blackouts, significantly weakening or losing transmission. Such occurrences are often termed “solar flares and radio blackouts.”
Solar activity, particularly solar flares, can significantly impact shortwave radio propagation, likely contributing to the issues you’re experiencing with broadcast reception. The National Oceanic and Atmospheric Administration’s (NOAA) Space Weather Prediction Center provides detailed and current information on space weather conditions that affect radio communications. The NOAA website provides various resources, including forecasts, reports, and models that track and predict solar activity and its impact on different aspects of space weather, including HF radio communications.
Solar flares emit X-rays that can penetrate the Earth’s ionosphere, particularly the D-layer, causing it to become more ionized. This increased ionization can reflect or absorb radio waves at different frequencies, leading to HF (High Frequency) radio communications disruptions. This is particularly problematic for frequencies in the 1 to 30 MHz range, commonly used for shortwave broadcasting. The impact of these solar flares is most intense on the Earth’s dayside, where the sun is directly overhead, and can cause radio blackouts.
Moreover, other space weather phenomena like Radiation Storms caused by solar protons can also disrupt HF radio communication. These protons, guided by Earth’s magnetic field, collide with the upper atmosphere near the poles, enhancing the D-Layer and blocking HF radio communication at high latitudes.
Sunspot Cycle and Its Influence
The sunspot cycle, approximately 11 years, significantly influences shortwave radio propagation. Sunspots, dark spots on the Sun’s surface, are indicators of solar magnetic activity, which can lead to solar flares. During periods of high sunspot numbers (SSN), the Sun is more active, increasing the likelihood of solar flares. High SSN usually means better shortwave propagation conditions due to a more reflective ionosphere, except during solar flares. Understanding the “sunspot cycle” is essential for predicting shortwave radio propagation conditions.
SSN (Sun Spot Number) and Shortwave Propagation
SSN, or Sun Spot Number, is a simple count of the number of sunspots and groups of sunspots visible on the Sun’s surface. A higher SSN indicates a more active Sun, which can enhance or disrupt shortwave propagation. Increased solar radiation can boost the ionosphere’s reflectivity during a high SSN period, improving shortwave signal reach. However, the increased solar activity also raises the risk of solar flares, which can cause shortwave radio blackouts. Therefore, the relationship between SSN and shortwave propagation is a delicate balance.
Source: https://www.nexus.org/solar-flares-impact-on-shortwave-radio-broadcasts/
Propagation
You and the ionosphere . . . a reader participation post By Jock Elliott, KB2GOM
Here’s a shocker for you: we live at the bottom of the sky. Above us there are multiple layers of the atmosphere, pressing down on us at 14.7 pounds per square inch.
Of particular relevance to us as shortwave listeners and hams, there is a special layer of the atmosphere, not shown on the chart above called the ionosphere. The ionosphere starts around 30 miles above us and extends up to about 600 miles and includes parts of the layers above.
The Sun’s upper atmosphere, the corona, is very hot and produces a constant stream of Ultra-Violet and X-rays, some of which reach our atmosphere. When the high energy UV and X-rays strike the atmosphere, electrons are knocked loose from their parent atoms and molecules, creating a layer of electrons.
Now, here’s the cool part: this layer – the ionosphere – is important because radio waves bounce off of it.
The sun, however, is not constant in its action on the ionosphere. The amount of UV and x-ray energy (photon flux) produced by the sun varies at by nearly a factor of ten as the sun goes through an 11 year cycle. The density of the ionosphere changes accordingly, and so does the ability of the ionosphere to bounce radio waves. When the sun is at peak activity, and the ionosphere is “hot,” SWLs and hams are likely to experience excellent long-range propagation. When the sun is quieter, long-range propagation diminishes.
Every 11-year solar cycle is unique, but early indications are that we may on the verge a cycle that favors long-range propagation: https://swling.com/blog/2022/03/termination-event-may-indicate-solar-cycle-strength/
The results can be spectacular. Decades ago, during a particularly hot solar cycle, I once spoke from my station near Albany, NY, to a station in the state of Georgia on a mere 4 watts. On another occasion, I conversed with a ham in Christchurch, New Zealand – a distance of over 9,000 miles – with 100 watts single sideband transmit power. During that same period, I would routinely listen to shortwave stations halfway around the world.
6 Meters!! The magic band. Should be some real magic happening at the end of 2024.
Ain't this solar maximum great?
Solar Cycle 25 is now much, much stronger than anyone anticipated,
and it's slowly growing stronger through at least this weekend.
Today's estimated international sunspot number is 281.
It's increasingly likely that we'll have widespread coast-to-coast and
worldwide 6 meter F2 propagation during about half of the days
between late October and at least early February. Widespread F2
openings are likely to bring 6 meter CW and SSB to life like we haven't
experienced in more than 20 years.
The first sign of enhanced 6 meter F2 will be increasingly frequent TEP
from Europe and North America to South America and the South Atlantic islands. TEP may begin very sporadically by late August and become increasingly frequent later in September and especially during October.
Coast-to-coast F2 propagation and propagation crossing the Atlantic
to Europe and Africa may begin sporadically during September and
October and become frequent and long lasting by early November.
Effective 6 meter antennas can be very small. 3 element Yagis are small, lightweight and very effective. 20 foot antenna height is adequate but sloping terrain or higher antennas perform much better. Heights higher than 50 feet are unnecessary and in many cases perform poorly.
Are you ready for this once in a lifetime experience?
73
Frank
W3LPL
Ionospheric Propagation of Radio Waves Gives Ham Radio Operators "Seven League Boots"!
Thanks to ionospheric propagation of radio waves, ham radio operators can rely on HF ionospheric radio signal propagation to communicate with fellow hams located way beyond the horizon.
The ionized layers of the ionosphere make HF radio wave propagation possible much beyond line of sight distances. These layers can be viewed as our "Seven League Boots" which, by leaps and rebounds, give our ham ra
I'll explain, in a moment, how the 'F' layer is the most useful ionized layer for DX.
Best of all, solar sunspot cycles improve HF propagation because they revitalize our ionosphere. The good news is, solar cycle 25 has begun! Ham radio operators, all over the world, are looking forward to its increasing activity.
The simplified drawing above illustrates how radio wave 'C' is refracted, by the ionized layer 'F', back toward the earth's surface, rebounds off the earth's surface a great distance away from its origin, goes upwards again as 'C1' to be refracted again by the 'F' layer and bounce off the earth further on as 'C2' and so on.
The radio signals 'A' and 'B', arriving at the ionized 'F' layer at too
Improve Your Communication with Eyebank Net’s Reliable information
The HF signals will gradually lose energy after each refraction by the 'F' layer and after each rebound off the earth's surface... until it is no longer discernible. But, by that time, it will have traveled thousands of miles and been heard by countless radio amateurs and shortwave listeners!
That's the magic of HF ionospheric radio signal propagation.
How Do Ionized Layers Form to Enable Ionospheric Propagation
Ionization of the upper reaches of earth's atmosphere occurs when ultraviolet radiation from the sun collides with hydrogen and helium molecules that are few and far between up there. These collisions detach electrons from the gaseous molecules.
As a result, positive hydrogen and helium ions are generated and negatively charged free electrons are liberated from their nucleus. These regroup into ionized layers above the earth.
However, ionized layers only form when the sun is "active", which it is for about 9-10 years, every eleven years or so. It's commonly called the 11-year sunspot cycle.
We can see the progression of the last few sunspot cycles in the graph shown earlier. You can obtain more information on the 11-year cycle of sunspots here.
The Ionized Layers and Their Respective Role in HF Radio Wave Propagation
Ionized Layer 'D'
During the day, the ionized layer 'D' mostly hinders ionospheric propagation of radio waves.
It is the ionized layer closest to the earth's surface. It is located between 60 km and 100 km (37-62 miles) above the earth.
In the daytime, it forms under the sun's intense UV radiation and constitutes a barrier preventing amateur radio signals in the 40-meter, 80-meter and 160-meter bands from getting far and from being heard in the intense atmospheric noise.
Meanwhile, signals 10 MHz and above can get through to reach the ionized layers above and make their way beyond the horizon.
The 'D' layer dissipates at sunset.
Signals in the 160-meter to 40-meter bands then become free to reach the 'F' layer and reach DX amateur radio stations like the other higher-frequency signals.
Ionized Layer 'E'
The 'E' layer lies between 90 km and 150 km (56-93 miles) above the earth but its most useful portion is located between 95 km and 120 km (59-75 miles) of altitude.
During daytime hours, in theory, layer 'E' could refract 5-20 MHz signals and help them along their way.
However, in reality, the 'D' layer (below) absorbs much of the energy of signals at these frequencies. Only signals in the 7-14 MHz range - transmitted near vertically - will be able to punch through the 'D' layer with enough remaining energy to reach the 'E' layer and be refracted along to reach as far as 1200 km (750 miles) at times.
That's where NVIS antennas come in handy.
The periods just before dawn and right after dusk are best to make use of the 'E' layer. At night, the 'E' layer disappears almost completely, while still remaining somewhat useful to the propagation of signals in the 160-meter band.
The "Sporadic E" Layer
Sometimes, dense ionized clouds will form suddenly in the 'E' layer and disappear just as suddenly, minutes, rarely hours later.
Sporadic 'E' propagation (Es) is useful at frequencies above 28 MHz, in the VHF range, rarely below. We cover their usefulness in extending the reach of VHF signals beyond the horizon on another page of this website.
Both 'E' and 'Es' propagation contribute to 50 MHz activity.
Ionized Layer 'F1'
During daytime hours, in summer, this layer will often be useful to the propagation of HF radio signals of the 30-meter and 20-meter bands. Its role in the propagation of HF signals is rather negligible.
Ionized Layer 'F2'
The 'F2' layer forms during daytime hours between 200 km and 400 km (125-250 miles) above the earth. It is higher in altitude in the summer than it is in the winter.
It is usually around all year round.
At night, layers 'F1' and 'F2' merge into one 'F' layer, a little lower than the daytime 'F2' was located.
The 'F2' ionized layer is present during the major part of a solar cycle.
However, it will sometimes disappear completely for days on end during a deep solar cycle minimum!
The 'F2' layer will reach its highest density at the peak of a solar sunspot cycle.
It will then refract toward earth radio signals ranging from 7 MHz to 30 MHz and enable them to reach distances as far as 4000 km from their origin, rebound off the earth to rise again to the 'F2' layer... and repeatedly do so… sometimes to travel right around the earth and come back from behind their point of origin!
During the better nine years or so of a solar cycle, QRP operators (5 watts of radiated power or less), using simple dipoles, can make DX contacts as far and as often as the QRO operators (using up to 200 to 300 times more power) using a multi-element directional antenna!
During such wonderful periods, every ham radio operator has an equal chance under the sun to make DX contacts.
Ionospheric Propagation of Radio Waves is a Complex Topic
The information I have presented to you in this article is a very brief summary of what could be said about HF ionospheric radio signal propagation. I have really only scratched the surface!
Countless scientific publications have covered many aspects of the subject since the discovery of the ionosphere's existence and, later, its role in the propagation of HF radio signals.
Research is ongoing, involving and scientists and ham radio operators alike.
For more on our sun's behaviour, visit the Solar and Heliospheric Observatory
by VE2DPE
https://www.hamradiosecrets.com/ionospheric-propagation-of-radio-waves.html
The ARRL Solar update
10-17-25
Region AR4246 underwent significant evolution, growing in overall
size while gaining multiple new spots. Subsequently, AR4246 was the
main provider of activity which included an M1.2 flare on October
13. Several coronal mass ejections (CMEs) with potential
Earth-directed components are in the mix at this time.
Additional modeling efforts are underway to perhaps confirm those
suspicions. The largest flare was a long-duration M4.8/2n on October
15. Modeling efforts showed the ejecta to be on a northward
trajectory and not on an Earthward course.
Other notable activity included a prominence eruption off the ENE
limb-first visible in LASCO C2 imagery on October 14. Given the
location of the event, initial analysis suggests this ejecta to be
well into foul-ball territory and not Earth-directed. Additional
modeling efforts are underway to perhaps confirm this suspicion.
Slight decay was observed in the intermediate portion of Region
AR4248 as it grew in length. Region AR4247 was in decay as well. The
remaining regions were unremarkable in comparison. No new regions
were assigned numbers this period. Minor to moderate (R1-R2) radio
blackouts are likely for the next 3 days, with a slight chance for
an isolated R3 (strong) event, due to the current and potential
flare activity of Regions AR4246 and AR4248.
Solar wind parameters continued to reflect the influence of a
negative polarity coronal hole high speed stream (CH HSS). Solar
wind speeds finally dipped below 600 km/s and phi was predominantly
in the negative solar sector.
Solar wind parameters are expected to continue at enhanced levels,
although gradually waning, and continuing through October 17, due to
CME arrivals from the October 11 to 13 timeframe originating from
AR4246. Solar activity reached moderate levels due to M-class flare
activity.
Weekly Commentary on the Sun, the Magnetosphere, and the Earth's
Ionosphere, October 16, 2025, by F. K. Janda, OK1HH:
"In line with forecasts, solar activity continued to gradually
increase, including several moderately strong flares. These were
mostly observed in the active region NOAA 4246, located in the
northwest of the solar disk, while approaching its limb.
"The second of the two larger active regions is NOAA 4248, also
located in the northwest, but closer to the central meridian and the
solar equator, while relatively close to the extensive coronal hole
No. 87. In the coming days, AR NOAA 4248 will move into the active
longitudes. Therefore, its eruptive activity will increase. Between
it and the aforementioned coronal hole, a source of intensified
solar wind will form, which will hit Earth during the coming week.
"Even earlier, on October 16, when this text is being written, we
expect a G2 geomagnetic disturbance, which will first cause a brief
improvement and then a deterioration in ionospheric propagation of
short waves, which have been rather average so far. The exception
was a shorter increase in MUF on October 15 between 1000 and 1200
UTC, probably caused by an intensification of the solar wind.
"Solar activity should remain at current levels for the rest of the
month. Fluctuations in propagation conditions, mostly around average
with occasional deterioration, will depend on the irregular
occurrence of geomagnetic disturbances."
The latest video report from Dr. Tamitha Skov, WX6SWW, can be found
on YouTube at: https://www.youtube.com/watch?v=pAZtua4DZUs .
For more information concerning shortwave radio propagation, see
http://www.arrl.org/propagation and the ARRL Technical Information
Service web page at, http://arrl.org/propagation-of-rf-signals. For
an explanation of numbers used in this bulletin, see
http://arrl.org/the-sun-the-earth-the-ionosphere . Information and
tutorials on propagation can be found at, http://k9la.us/ .
Also, check this:
"Understanding Solar Indices" from September 2002 QST.
The Predicted Planetary A Index for October 18 to 24 is 5, 5, 15,
10, 8, 5, and 5, with a mean of 7.6. Predicted Planetary K Index is
2, 2, 5, 3, 3, 2, and 2, with a mean of 2.7. 10.7-centimeter flux
is 150, 150, 150, 150, 145, 140, and 145, with a mean of 147.1.
