a b s t r a c tSporadic-E is a problem in high latitude HF communications because of its irregular and as yet unpredictable behaviour. This paper characterises the change in E-layer critical frequency (f o E) that occurs in the four hours following a storm sudden commencement (SSC) event which is a precursor to magnetospheric storms which adversely affect ionospheric communications.Following checks for any seasonal or solar activity dependency in the data, further analysis determines the occurrence of full blanketing (screening) Sporadic-E layer formation at high latitudes as seen on vertical ionosondes in Northern Finland (671 Latitude). An appropriate threshold value of f o E is proposed that could be used following the commencement of ionospheric storms, indicating that there is a high probability that F-layer screening will occur. As far as the authors are aware this is the first statistical analysis of the onset of Sporadic-E following SSC.

a b s t r a c tIonospheric 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 B z 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.

1] Following particle precipitation at high latitudes that occurs as a result of geomagnetic disturbances, the electron density of the E region of the ionosphere undergoes substantial variation. These variations modify the E layer propagation characteristics which adversely affect HF propagation paths both unexpectedly and in many cases for long periods of time. This paper presents two case studies of how the enhancement of E region electron density immediately following storm sudden commencement affects high-latitude HF communications links in and through the auroral zone. Using data gathered from ionosondes, incoherent scatter radar and an oblique sounder, the Advanced Composition Explorer Satellite, and riometers and magnetometers, the physical effects of particle precipitation on the ionospheric E layer and the resulting effect on HF links is revealed. Both case studies show that besides the expected occurrence of severe short-term absorption and the reduction in the critical frequency of the F layer caused by a redistribution of the electron density in the upper layers of the ionosphere, there is the likelihood a sporadic E layer will form and/or be enhanced because of significant and sustained particle precipitation. When the critical frequency of the sporadic E layer increases above the norm and in cases exceeds the critical frequency of the F layer, there are severe implications for many HF radio communication systems used at high latitudes unless corrective action is taken.

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Many services rely on the output of propagation planning tools to predict the future state of the ionosphere and the availability of suitable communications channels. While this approach is adequate for services operating at high-latitudes during quiet ionospheric conditions it is insufficient to deal with a disturbed ionosphere. This paper presents the results of empirical modelling of the changes that occur in the D-and E-regions of the ionosphere following the onset of disturbed conditions as defined by the occurrence of a storm sudden commencement.