Observed characteristics of sudden commencement absorption

Ionospheric absorption at high latitudes that occurs coincidently with storm sudden commencements (SSC) is categorised as sudden commencement absorption (SCA). Having discounted any solar activity effects on the measurement of SCA this paper describes, for the first time, the basic characteristics of SCA measured over half a solar cycle.It is revealed that the measurement of SCA is not affected by Solar activity, the local time of occurrence of the SSC or the direction of Bz immediately before or after the SSC event. We also show that the median value of expected SCA is best predicted by the step change in IMF magnetic field coincident with the SSC event. This strong correlation between the median value of SCA and the IMF magnitude that occurred during the shock has been formulated to aid propagation prediction.