Seasonal variations of high-latitude field-aligned currents inferred from Orsted and Magsat observations

[1] In this paper we report on field-aligned currents inferred from high-precision three-component geomagnetic field observations made on board the Danish satellite Orsted over polar regions. Because of a slow drift in local time of the satellite orbit through the noon-midnight'' sector, we were able to study the seasonal dependence of the dynamic properties of the dayside and nightside field-aligned current systems over the Northern and Southern Hemispheres. We find an average over-the-pole distance between dayside and nightside currents of 32 during summer but 37 during winter and 36 during equinox. The decrease in the size of the summer polar cap is caused by a shift of both daytime and nighttime current systems to higher magnetic latitudes. For comparison, the dawn-dusk cross-polar distance of the Region 1/Region 2 field-aligned currents has been determined from high-precision data observed by Magsat, a satellite flown in 1979-1980 in a dawn-dusk'' orbit. The latter results show that the dawn-dusk distance between R1/R2 currents exhibits little seasonal dependence and amounts to 34 for all seasons in both polar caps. The seasonal dependence is confirmed for the high-latitude field-aligned intensities; they are larger by a factor of 1.5-1.8 in the sunlit (summer) polar cap in comparison with the winter hemisphere. Our results suggest that the R1/R2 and dayside field-aligned currents are well balanced between the pairs of downward/upward currents for all seasons as well as between hemispheres during equinox. We were not able to confirm results reported in earlier studies that the net currents tend to increase with an enhancement of ionospheric conductivity caused by the solar illumination or substorm activity.