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.
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).
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.
Strong at Maximum –
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
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
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!
Interview with Hartmut Büttig, DL1VDL
Thank you for being available for a short interview!
How did you get involved with monitoring solar activity and making propagation forecasts for radio amateurs?
My start was an "emergency situation".
In March 1994, the then radio weather editor Alfred Müller, DL1FL, with whom I already corresponded as HF referee, asked me to find him a successor immediately for health reasons. Since there was only one week until the next newsletter, I dealt with the subject of radio weather myself. On 8 April 1994, I wrote the first radio weather report based on DL1FL's model. I received spontaneous support from Ulrich Müller, DK4VW, who gave me the daily radio weather data for DK0WCY. Prof. Walter Eichenauer, DJ2RE, also supported me with technical tips. The new task was interesting for me and an additional challenge. I noted the propagation conditions in the calendar daily in the morning before work and in the evening. In addition, there was Grayline data, which I used anyway for DX QSOs on the lower bands. Later, the newsletter editors asked me to explain radio weather terms from time to time. Thus, a small radio weather lexicon was created .
How do you see the current solar activity cycle? Normal? Above or below average?
When writing about sunspot cycle developments, one must always be aware that very complicated solar, atmospheric and ionospheric processes have to be packaged into simple statements. The "error bar" is thereby large. Interrelationships, for example the coupling between the lower atmospheric layers and the ionosphere, are not yet scientifically understood. Model calculations now substantiate the assumption that the eleven-year cycle is timed by the solar system itself .
After the last sunspot minimum in December 2019, hardly anything happened until early 2021. Currently, solar activity is increasing faster than predicted . This is a good sign, although we can only guess whether a first peak from the maximum is now emerging, for example. The past cycles also had two maxima .
What tips do you have for newcomers for the next few years until the maximum? How can they best profit from it?
Our hobby should inspire us and those who want to start successfully will find material for self-study in the link collection.
It was always important for me to discuss my questions with friends.
You can do interesting tests yourself. If you know CW, you can start a CQ call and check the Reverse Beacon Network to see where your callsign is heard . This immediately gives a picture of the currently open propagation paths. Listening to the beacons of the IARU IBP also gives an overview of which areas can be reached with omnidirectional antennas and less than 100 W transmitting power . All beacons of the IBP are equipped in the same way. The DX clusters are also mirrors of activity, although one should not be disappointed if DX stations are not audible in our area when they are reported from other geographical regions .
The joy of amateur radio only comes from activity on the bands!
What sources of information do you recommend to an active radio amateur to inform him- or herself about the current solar activity and the effects on radio weather?
"Old hands" actually have their favourite sources of information.
I use the following collection of links for radio weather reports:
First, the links cited in the text:
Further links (examples, without evaluation)
 : www.solarham.net - (Current solar activity, more links)
 : www.solen.info/solar - (Tabular solar data, Coronal holes, solar wind)
 : https://www.solarham.net/cmetracking.htm - (solar wind tracker)
 : https://www.darc.de/funkbetrieb/hf-prognose/ - (current forecast)
 : https://www.voacap.com/hf/ - (Propagation planning)
 : https://www.ionosonde.iap-kborn.de/actuellz.htm - (Ionograms, MUF)
 : Usage of the Digisonde at IAP Juliusruh (darc.de) Explanation of Ionograms
 : https://tropo.f5len.org/forecasts-for-europe/ - (Tropo DX Forecast)
 : https://www.tvcomm.co.uk/g7izu/ - (Data about Sporadic-E layer)
: https://ara35.fr/wp-content/uploads/2020/09/IndicesSolairesAngl1.pdf (Solar Weather Data - Understanding Main Polar Parameters – PDF document to download)
: https://qrznow.com/160-meter-band-enigma-shrouded-mystery/ - (160M basic information)