Sunspots and Radio Propagation

 


 

It is widely known in there is a close link between the number of sunspots on the sun and the propagation conditions experienced on the short wave bands. In periods when the number of sunspots is high, radio conditions, especially on the bands at the top of the short wave spectrum are very good. At other times few signals may be heard at the top of the short wave spectrum and lower frequencies must be used instead.

What are sunspots?

If the sun is viewed by projecting its image onto a screen, dark areas can be seen from time to time. These can last anything from a few hours right up to several weeks. These spots are cool areas (relatively speaking) on the surface of the sun. The temperature is around only 3000°C against a sizzling 6000°C for the rest of the surface. It is much hotter under the surface reaching temperatures in excess of a million degrees Celsius.

Note: Under no circumstances should the sun be viewed directly, even though dark glasses. In the past many people have had their sight damaged by doing this.

These sunspots are areas where there is intense magnetic activity. The fields in these areas are enormous and as a result the surface of the sun is disrupted. In these areas the surface cools dramatically causing a darker region to be perceived.

Around the sunspot there is an area called a plage. This is slightly brighter than the surrounding area and it is a large radiator of cosmic rays, ultra-violet light and x-rays. In fact it results in the overall level of radiation coming from the sun to increase. In turn this increased radiation level from around the sunspots causes the ionosphere to become ionised to a greater extent. This means that higher frequencies can be reflected from the ionosphere.

As sunspots appear in groups, especially the larger ones a sunspot number was devised. This is not the number of sunspots that are observed but a number indicating the level of sunspot activity. The number is very closely related to the actual amount radiation received from the sun. In this way it is a good measure of solar activity. The daily readings are smoothed mathematically to take out the erratic variations to give the Smoothed Sunspot Number. Sometimes the abbreviation SSN is seen, and it is this smoothed sunspot number that it refers to.

Eleven year cycle

The number of sunspots on the surface of the sun varies with time. At times very few or even none may be visible, whereas at other times the number is very much greater. Although the number varies greatly over short periods of time as the sun rotates, careful analysis using the SSN reveals a longer term trend. It is found that over a period of approximately eleven years over which the sunspots vary. At the peak of this cycle conditions on the bands at the top of the short wave spectrum are very good. Low power stations can be heard over remarkably long distances. At the bottom of the cycle bands around 30 MHz will not usually support normal propagation via the ionosphere.

Sunspots have been observed by the Chinese since before the birth of Christ. However it was not until the mid-eighteenth century that astronomers started to keep records of sunspot numbers. By looking at these over the years it is possible to see the trend since then, and the cycles which have occurred since then. Cycle number 22 officially started in September 1986. It started with a sunspot number of 12 and rose rapidly over the following 33 months to reach a peak of 158. From its peak the sunspot number fell slightly and rose again to give a second, smaller peak before falling to bring the cycle to an end in 1996

© Ian Poole, last updated 6th January 1999