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  • Communications Effects of Space Weather
  • HF Communications and Space Weather
  • Ionospheric Disturbances and HF communications

Ionospheric Disturbances and HF communications

  • Electric currents caused by the arrival of charged particles alter the properties of the ionosphere, particularly the F-layer critical frequency (foF2) which determines the maximum usable frequency (MUF) that can be used on HF circuits. The response of F-layer critical frequency is complicated as it depends on the time of the day, the season, the latitude, and the nature of the disturbance itself. In many cases, the critical frequency is enhanced early and then depressed later in the storm. These variations in ionospheric properties, particularly the depressions of critical frequencies, need to be anticipated by HF communicators. It is often assumed that ionospheric disturbances occur whenever magnetic disturbances do. This is substantially correct but the relationship between them is complex and there is certainly no one-to-one relationship between a magnetic disturbance index, local or world-wide, and the level of ionospheric disturbance as measured by critical frequency depression, communications disruption or any other parameter. Quite severe magnetic disturbances can occur with little apparent ionospheric effect and vice-versa. Periods of severe disturbance will affect more than the MUFs. Irregularities in the ionosphere result in signals travelling by more than one path and this can produce interference and consequent difficulties in communications. Ionospheric disturbances have a similar variation during the solar cycle as do geomagnetic disturbances. In general, disturbances are more frequent at the high parts of the solar cycle. In some cycles, a second and sometimes larger disturbance peak occurs during the declining phase of the cycle.

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