SEASONAL EFFECTS IN THE IONOSPHERE-THERMOSPHERE RESPONSE TO THE PRECIPITATION AND FIELD-ALIGNED CURRENT VARIATIONS IN THE CUSP REGION

The seasonal effects in the thermosphere and ionosphere responses to t he precipitating electron flux and field-aligned current variations, o f the order of an hour in duration, in the summer and winter cusp regi ons have been investigated using the global numerical model of the Ear th's upper atmosphere. Two variants of the calculations have been perf ormed both for the IMF B-y < 0. In the first variant, the model input data for the summer and winter precipitating fluxes and field-aligned currents have been taken as geomagnetically symmetric and equal to tho se used earlier in the calculations for the equinoctial conditions. It has been found that both ionospheric and thermospheric disturbances a re more intensive in the winter cusp region due to the lower conductiv ity of the winter polar cap ionosphere and correspondingly larger elec tric field variations leading to the larger Joule heating effects in t he ion and neutral gas temperature, ion drag effects in the thermosphe ric winds and ion drift effects in the F2-region electron concentratio n. In the second variant, the calculations have been performed for the events of 28-29 January, 1992 when precipitations were weaker but the magnetospheric convection was stronger than in the first variant. Geo magnetically asymmetric input data for the summer and winter precipita ting fluxes and field-aligned currents have been taken from the patter ns derived by combining data obtained from the satellite, radar and gr ound magnetometer observations for these events. Calculated patterns o f the ionospheric convection and thermospheric circulation have been c ompared with observations and it has been established that calculated patterns of the ionospheric convection for both winter and summer hemi spheres are in a good agreement with the observations. Calculated patt erns of the thermospheric circulation are in a good agreement with the average circulation for the Southern (summer) Hemisphere obtained fro m DE-2 data for IMF B-y < 0 but for the Northern (winter) Hemisphere t here is a disagreement at high latitudes in the afternoon sector of th e cusp region. At the same time, the model results for this sector agr ee with other DE-2 data and with the ground-based FPI data. All ionosp heric and thermospheric disturbances in the second variant of the calc ulations are more intensive in the winter cusp region in comparison wi th the summer one and this seasonal difference is larger than in the f irst variant of the calculations, especially in the electron density a nd all temperature variations. The means that the seasonal effects in the cusp region are stronger in the thermospheric and ionospheric resp onses to the FAC variations than to the precipitation disturbances.