HF Radio Propagation and Sunspots
- an overview of the basics of the effects that sunspots have on HF radio
propagation and radio communications in general including two way radio
communications, maritime mobile radio communications, mobile radio, radio
broadcasting and amateur radio communications.
HF radio communications of various forms including two way
radio communications, maritime mobile radio communications, radio broadcasting,
amateur radio communications, and in fact any form of radio communications that
uses the HF bands and ionospheric radio propagation is very dependent upon the
state of the ionosphere. The higher the levels of radiation received from the
Sun, the greater the levels of ionisation in the ionosphere and in general this
brings better propagation conditions for HF radio communications.
It is found that the number of sunspots on the Sun has a
considerable effect on the levels of radiation emitted and hence impacting on
the ionosphere. In turn this has a marked effect on radio communications of all
forms. Sunspots are therefore of great interest to anyone involved in HF radio
communications, as it affects the radio propagation conditions so significantly.
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
Summary
The sunspot activity is of great importance to anyone
involved in HF radio communications. Whether two way radio communications,
maritime mobile communications, general mobile communications, point to point
radio links, amateur radio communications, radio broadcasting or whatever form
of radio communications. The level of sunspot activity has an enormous effect on
the ionosphere and hence on HF radio propagation conditions. Accordingly even a
superficial understanding is advantageous.
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