Sunspot Cycle and Its Influence

 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.” 

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.

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, 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/

 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. 

 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. 

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.

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.

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.

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.

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.

 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