컨텐츠 바로가기
  • Communications Effects of Space Weather
  • HF Communications and Space Weather
  • HF communication frequencies and the solar cycle

HF communication frequencies and the solar cycle

  • The ionosphere extends from a height of about 50 km up to over 500 km above the surface of the earth. It is formed from the ionisation of atoms of air (i.e. electrons from removed from the atom) by incoming solar radiation. The chemistry of the atmosphere then determines the structure of the ionosphere which is generally divided into layers labelled D, E, F1 and F2 (in order of increasing height). The F layer is of most importance for HF communications as it is present during day and night, it is located at the greatest height, and it reflects the highest frequencies in the HF band. It is the Extreme Ultraviolet (EUV) radiation from the sun which is responsible for forming and maintaining the ionosphere. This arises from the bright and hot regions which overlie sunspots. The number and size of sunspots varies with the solar cycle and so the properties of the ionosphere in turn exhibit a variation with the cycle. At the low point of the solar cycle, EUV radiation from the sun is weak and the density of charged particles in the F layer of the ionosphere is least. This means that only the lower frequency HF signals can be reflected. At the peak of the cycle, the EUV and the ionospheric density are both large and higher frequencies in the HF band can be reflected. While the solar cycle is very important in determining HF frequencies, there are many other important factors. These include: the season; the time of day; the latitude; and the geometry of the circuit. Prediction of the best frequencies for HF circuits is performed by computer programs such as the IPS ASAPS program (for more information see contacts at the end of this paper).

- IPS -