Significance of solar activity for amateur radio

Stars are great. Beautiful to look at and fascinating, yet with still many secrets. One of these stars is especially great - our sun. Without it, nothing would function on our planet; it powers our lives. Almost all the energy we use comes from the sun. Whether we use stored solar energy from fossil fuels or use solar energy directly, the suns energy is what powers us.

And for us radio amateurs, the sun has another, very special meaning. Because it is also the main cause for what can be achieved on shortwave and other bands, i.e. how well we can reach distant places by radio worldwide. We are talking about the propagation conditions...

How does
a star work - very

A star functions as a constant battle of two forces in balance: gravity (from the outside in) and radiant energy (from the inside out).

It is gravity that compresses the matter of a star (mostly hydrogen and some helium). This enormous pressure causes high temperatures in the core, the matter of the star becomes a so-called plasma and a fusion process starts. In this process, atomic nuclei of lighter elements fuse to form heavier elements. The energy emitted in this process is minuscule, but because it happens so often, the sum of the radiation energy produced is extremely high. High enough to build up a radiation pressure that can withstand the equally strong force of gravity. This creates an equilibrium that can be maintained as long as the sun has enough 'fuel'.

After some conversion processes, this radiation from inside the sun presents itself to us as heat and light. Beyond that, however, there is still a very high portion of non-visible radiation. These are radio waves up to the X-ray range, i.e. strong ionizing radiation.

Earth's protective shield - source of joy for radio amateurs

So the light and heat arriving here is the primary source of energy for all life on Earth. And to prevent too much of this radiation from reaching us (which can also be harmful), the Earth has a protective shell that absorbs (filters) and redirects some of the radiation. This protective shell consists of several layers. The lowest layer we know well, it contains the oxygen we breathe. Above it come other layers with different functions.

An area further up we call the 'ionosphere'. Here the air is already extremely thin. Because of this low density, the radiation of the sun can penetrate and excite the molecules of the air energetically. In the process, the electrons are released from the atomic shell; this is referred to as ionization of the gas. And it is precisely this ionization that ensures that radio waves are diffracted at these layers and deflected back to earth. Suddenly our radio signal reaches 'around the corner' and if everything fits even around the whole world.

How well the radio signal is deflected at the ionospheric layers depends on several parameters. Among other things, the average density of the molecules is important, this can change over time. And, of course, the strength of the solar radiation is of great importance. And this is also subject to various fluctuations.

Important cycles for radio amateurs.


One change we most clearly: the day and night change. At night, without irradiation, the ionization disappears after a few minutes to hours. This has different effects depending on the frequency. On long and medium wave, attenuating layers disappear and suddenly you hear stations from all over Europe at night. On shortwave the diffraction disappears, where before we could work Asia or the USA now nothing can be heard.

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We can observe another rhythm annually. Decisive is the angle under which the sunbeams arrive here. In winter the angle is very low on the northern hemisphere of the earth and the duration of the day is also only short, the energy input into the ionosphere is lower. In summer, the angle is very steep and the sun can act much longer due to the longer daytime hours. Thus the energy input is much higher, damping layers damp longer, diffractive layers act longer. As a result, HF propagation works better on the lower bands (160m, 80m) in winter, for example, because there is less noise and we can hear better. In summer there are effects on HF and VHF, which are not to be observed otherwise in the year. An example is the so-called "Sporadic-E" propagation on the upper bands (6m and higher).

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11 years

And then there is a very important cycle, and that is the every 11 years repeating change of the magnetization of the sun. The sun (and presumably every other star) is a giant dynamo, that is, a rotating something that has an electric and magnetic field. We observe that the magnetic field of the sun tilts, i.e. reverses polarity, about every 11 years. After two such passes the magnetic north pole is again at the starting point, one should talk therefore actually of a 22-year cycle. But since each tilting process north to south, south to north looks the same to us, we talk about an 11-year cycle.

The physical processes in the sun, which lead to this cycle, are not yet completely understood. Intensive research is being done here by many scientists around the world. Important for us radio amateurs are the changes in the sun's radiation intensity and the associated change in ionization in our earth's atmosphere.

Every 11 years there is a maximum in the activity of the sun. At this maximum the solar radiation is much higher and also more turbulent than at the minimum. Such a maximum of activity lasts several months. The increase of the activity from the minimum to the maximum is steeper, the change from the maximum to the minimum is flatter. A visible sign of increasing solar activity are the sunspots, darker areas on the surface, which can be observed with special optics (Attention, never look directly into the sun with the unprotected eye!). The next solar activity maximum is expected for the year 2025.

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Strong at Maximum –
Solar Eruptions

The maximum and the time before it are accompanied by frequent eruptions on the solar surface, so called 'flares'. These flares often have an immediate effect on the propagation of radio waves on shortwave, since they are almost always associated with strong radiation bursts. This radiation arrives here on Earth after a short time (usually after a few minutes) and can cause considerable interference. This is because too much radiation is also not good for the ionization of the upper atmospheric layers - which is actually useful for us.

If the radiation is too strong, the diffraction property often breaks down completely and very quickly, resulting in a total "radio blackout" on the side of the Earth facing the sun. This blackout can last up to one or two hours.

But that's only the effect of the 'fast' radiation of a flare. How about slower effects? Often with such an eruption a mass ejection on the sun can be observed. Due to the sudden breaking of the magnetic field of the sun, ionized mass is ejected from the gravitational field of the sun and brought into space. The speed of these matter clouds is different, but in any case much slower than the radiation of the causing eruption. We observe their arrival only a few minutes after the event. The mass ejections usually arrive on Earth after one to three days - if at all.

Eruptions with mass ejection

Such eruptions associated with an ejection of solar mass are called "CME", coronal mass ejections. They are characterized by the direction (towards or past Earth), the velocity, and the strength and direction of the associated magnetic field.

If such a CME is directed toward Earth, it can have far-reaching consequences for us. From a purely statistical point of view, most of such eruptions with mass ejections do not take place in the direction of the Earth. This is good for us, because the effects can be quite drastic up to catastrophic with very strong events. The most beautiful effect is the aurora, the polar light at the north and south pole. Here the matter of the ejecta arrives on earth after hours or days and is - because the molecules are electrically charged - deflected towards the poles by the magnetic field of the earth. This can have an influence on radio propagation too. Concrete consequences for radio amateurs are for example reflection appearances on VHF at the aurora. But also strongly disturbed propagation conditions on shortwave...

Less nice consequences of such earth directed mass ejections are influences on satellites and technical equipment on earth. Satellites can be damaged or even destroyed in certain functions by the strong electric field. If that would affect the worldwide GPS system of navigation satellites, this could have very far-reaching effects on all of us. Another effect is the induction of strong currents in electrical transmission lines of our global power supply.

There have been events in thepast that have led to the temporary shutdown of substations - meaning a power outage for many homes, hospitals, businesses (refrigeration!) and industrial plants. These are consequences that can affect us all, radio amateur or not.

Observing the sun as a radio amateur

So what are the possibilities for radio amateurs to observe the activity of the sun? And then to understand its effects on propagation conditions on shortwave and VHF?

R, F, A, k - The four most important measurements

The activity of the sun is measured with many values. Four of them are very important for us - the Sunspot Relative Number 'R', a ratio that indicates the amount of sunspots currently present; and - the solar flux 'F', the intensity of radiation in a particular part of the radio spectrum (at 2695 MHz, the line of activity of excited hydrogen).

- The geomagnetic index 'A' describes the influence of the solar mass input (particle radiation) into the ionosphere

- The geomagnetic index 'k' describes the state of the Earth's magnetic field, whether it is quiet or disturbed by solar events

Sunspot relative index
Values of the sunspot relative index 'R' range from about 10 in a minimum to 150 in a typical maximum. Simple rule: the higher the 'R' value, the better the chances of 'good conditions', especially on the higher bands. However, the chance of a solar storm also increases, which can cause severe interference and even complete loss of communications on shortwave.
Solar Flux
Solar Flux shows values of 50-70 at minimum, and up to 200 at maximum activity. So rough rule of thumb is the same as above - higher values please the radio amateur, but also bring the danger of interference.
Mass input
At values around 10 there was and is little mass input into the ionosphere, the situation is calm, little interference is expected. The value can increase abruptly when an Earth-directed CME arrives, the maximum being around 30. Such a high value indicates a strong particle stream from the Sun to the Earth. As a consequence, an aurora may result, but also erratically better propagation conditions on the lower bands, which may then last for several hours.
Earth magnetic field state
"A value from 0 to 9 is given here, summarized by other observational data. Small value = quiet magnetosphere = good for us; high value = turbulent magnetics = bad for us radio amateurs. ."

Where can radio amateurs find up-to-date data?

We asked a real expert about this: Dr. Hartmut Büttig, DL1VDL. Hartmut has for years created and advised the propagation forecast of the DARC, he gives us in a short interview exciting insights into how he came to this and where you can get these measurement data:

with Hartmut Büttig DL1VDL

May 2022

To the interview

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Let's look forward to the next solar maximum in 2025!

Solar activity is a very complex process that is not yet fully understood by us and needs to be investigated further. So for us radio amateurs, this not only has direct implications for propagation conditions, but also represents a field of research in which we as laymen can certainly participate. In any case, it is fascinating to observe, and helps us to reach our goals (many DXCC countries!) faster and better. Let's look forward to the next solar maximum in 2025!


What are sunspots?

Sunspots are temporary phenomena on the solar surface that appear darker than the surrounding areas. They are zones of intense magnetic activity and can persist for a few days to several months.

How do sunspots affect radio communications?

Sunspots can significantly affect radio communications. They are the source of solar emissions that can affect the Earth's ionosphere. These emissions can disrupt or enhance the signal strength of high-frequency radio communications.

How often do sunspots occur?

The frequency of sunspots varies in a cycle of about 11 years. This cycle, known as the solar cycle, has periods of high activity, called solar maximum, and periods of low activity, called solar minimum.

What is the SSN index?

The SSN (Sunspot Number) Index is a measure of the number of sunspots and is used to determine solar activity. A higher SSN index indicates increased solar activity and therefore a potentially greater impact on radio communications.

What is the significance of the solar cycle forecast for radio amateurs?

For radio amateurs, the solar cycle forecast is important to estimate the quality of radio communications. Concrete consequences for radio amateurs are for example reflection appearances on VHF at the aurora. But also strongly disturbed propagation conditions on shortwave. Phases of low activity often allow more stable, but less far-reaching radio links.