Abstract
Abstract. The seasonal effects in the thermosphere and ionosphere responses to the precipitating electron flux and field-aligned current variations, of the order of an hour in duration, in the summer and winter cusp regions have been investigated using the global numerical model of the Earth's upper atmosphere. Two variants of the calculations have been performed both for the IMF By < 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 those used earlier in the calculations for the equinoctial conditions. It has been found that both ionospheric and thermospheric disturbances are more intensive in the winter cusp region due to the lower conductivity of the winter polar cap ionosphere and correspondingly larger electric field variations leading to the larger Joule heating effects in the ion and neutral gas temperature, ion drag effects in the thermospheric winds and ion drift effects in the F2-region electron concentration. 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. Geomagnetically asymmetric input data for the summer and winter precipitating fluxes and field-aligned currents have been taken from the patterns derived by combining data obtained from the satellite, radar and ground magnetometer observations for these events. Calculated patterns of the ionospheric convection and thermospheric circulation have been compared with observations and it has been established that calculated patterns of the ionospheric convection for both winter and summer hemispheres are in a good agreement with the observations. Calculated patterns of the thermospheric circulation are in a good agreement with the average circulation for the Southern (summer) Hemisphere obtained from DE-2 data for IMF By < 0 but for the Northern (winter) Hemisphere there is a disagreement at high latitudes in the afternoon sector of the cusp region. At the same time, the model results for this sector agree with other DE-2 data and with the ground-based FPI data. All ionospheric and thermospheric disturbances in the second variant of the calculations are more intensive in the winter cusp region in comparison with the summer one and this seasonal difference is larger than in the first variant of the calculations, especially in the electron density and all temperature variations. The means that the seasonal effects in the cusp region are stronger in the thermospheric and ionospheric responses to the FAC variations than to the precipitation disturbances.Key words. Ionosphere (ionosphere · atmosphere interactions; ionosphere · magnetosphere interactions; ionospheric disturbances).
Highlights
IntroductionThere have been many reported ground-based and satellite investigations of the thermospheric and ionospheric responses to the soft electron precipitation and ®eld-aligned current variations in the cusp region (e.g., Shepherd, 1979; Kelly and Vickrey, 1984; Kofman and Wickwar, 1984; McCormac and Smith, 1984; Oliver et al, 1984; Robinson et al, 1984; Smith, 1984; Vennerstrom et al, 1984; Wickwar, 1984; Thayer et al, 1987; Sandholt et al, 1994; Wu et al, 1996)
In the present investigation we have studied mainly the seasonal eects in the thermospheric and ionospheric responses to the soft electron precipitation and ®eld-aligned current variations, of the order of an hour in duration, in the summer and winter cusp regions simultaneously using the same model as in Namgaladze et al (1996b)
This dierence is due to the dierent background conductivities in the summer and winter polar caps not connected with each other by the geomagnetic ®eld lines because they are open there. It is the main cause of the seasonal eects in the ionospheric and thermospheric responses to the precipitation and ®eld-aligned current variations in the summer and winter cusp regions
Summary
There have been many reported ground-based and satellite investigations of the thermospheric and ionospheric responses to the soft electron precipitation and ®eld-aligned current variations in the cusp region (e.g., Shepherd, 1979; Kelly and Vickrey, 1984; Kofman and Wickwar, 1984; McCormac and Smith, 1984; Oliver et al, 1984; Robinson et al, 1984; Smith, 1984; Vennerstrom et al, 1984; Wickwar, 1984; Thayer et al, 1987; Sandholt et al, 1994; Wu et al, 1996). Namgaladze et al.: Seasonal eects in the ionosphere-thermosphere response cusp have been investigated for the equinoctial conditions of 22 March, 1987 (low solar activity) by Namgaladze et al (1996b) using the global numerical model of the Earth's upper atmosphere (Namgaladze et al, 1988, 1996a). This three-dimensional time-dependent model describes the ionosphere, thermosphere and protonosphere of the Earth as a single system and includes the calculations of the electric ®elds both of magnetospheric and thermospheric (dynamo) origin. The responses of the electron concentration, ion, electron and neutral temperature, wind velocity and electric ®eld potential to the variations of a precipitating 0.23 keV electronux intensity with and without variations in the ®eld-aligned current density in the cusp have been calculated by solving the corresponding continuity, momentum and heat balance equations
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