DIURNAL AND SEASONAL OCCURRENCE OF POLAR PATCHES

Analysis of the diurnal and seasonal variation of polar patches, as id entified in two years of HF-radar data from Halley, Antarctica during a period near sunspot maximum, shows that there is a broad maximum in occurrence centred about magnetic noon, not local noon. There are mini ma in occurrence near midsummer and midwinter, with maxima in occurren ce between equinox and winter. There are no significant correlations b etween the occurrence of polar patches and the corresponding hourly av erages of the solar wind and IMF parameters, except that patches usual ly occur when the interplanetary magnetic field has a southward compon ent. The results can be understood in terms of UT and seasonal differe nces in the plasma concentration being convected from the dayside iono sphere into the polar cap. In summer and winter the electron concentra tions in the polar cap are high and low, respectively, but relatively unstructured. About equinox, a tongue of enhanced ionisation is convec ted into the polar cap; this tongue is then structured by the effects of the interplanetary magnetic field, but these Halley data cannot be used to separate the various competing mechanisms for patch formation. The observed diurnal and seasonal variation in the occurrence of pola r patches are largely consistent with predictions of Sojka et al. (199 4) when their results are translated into the southern hemisphere. How ever, the ionospheric effects of flux transfer events are still consid ered essential in their formation, a feature not yet included in the S ojka et al. model.