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  • Communications Effects of Space Weather
  • HF Communications and Space Weather
  • Solar flares and HF communication fadeouts

solar flares and HF communication fadeouts

  • Solar flares produce copious amounts of electromagnetic radiation, the X-ray component of which increases the ionisation of the ionospheric D layer. HF communication generally depends on the reflection of signals from the higher F layer and such signals must travel through the D layer at least twice. Increased ionisation, combined with the higher density of neutral particles, results in the absorption of the signal in the D layer during a major solar flare. This effect is known as a sudden ionospheric disturbance (SID). The SID is observed as an increased attenuation of HF signals particularly at the lower frequencies. This is often referred to as a SWF (short-wave fadeout), SSWF (sudden short-wave fadeout) or a GSWF (gradual short-wave fadeout). The fadeout follows closely the pattern of the solar flare, being observed at the same time as the flare. Fadeouts mostly have a rapid onset of a few minutes and a slower decline lasting perhaps an hour (highly variable). A property of SWFs is that they affect the lower frequencies in the HF band more than higher frequencies which may not be affected at all. The high frequencies are the last to be affected (if at all) and the first to recover. An important feature of SWFs is that the ionospheric circuit is disturbed only when there is an ionospheric reflection point for the signal located in the sunlit hemisphere. No effect is observed if all the reflection points are located in the night hemisphere which is shadowed from the X-rays produced by the solar flare. The intensity of flares at X-ray wavelengths is a good indicator of the chance of a significant fadeout. Hence, we can predict fadeouts if we can predict the occurrence of X-ray flares. At present, exact predictions of the timing and strength of flares are not possible. However, by observing the structure of sunspot regions we can predict intervals of time (perhaps lasting several days) during which flares and fadeouts are likely. Being closely associated with solar flares, SWFs exhibit the same solar cycle distribution as do flares. SWFs are much more frequent near the peak of the cycle whilst they are relatively infrequent near solar minimum.

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