Dependence of the high‐latitude thermospheric densities on the interplanetary magnetic field

Abstract

The systematic analysis of the interplanetary magnetic field (IMF) By and Bz influences on observed thermospheric density is presented. For this purpose, the high‐latitude southern summer thermospheric total mass density near 400 km altitude, derived from the high‐accuracy accelerometer on board the Challenging Minisatellite Payload (CHAMP) spacecraft, is statistically analyzed in magnetic coordinates. The difference density distributions, which are obtained by subtracting values for zero IMF from those for nonzero IMF, vary strongly with respect to the direction of the IMF: Difference densities for negative By show significant enhancements in the early morning hours and hours around dawn but show reduced values in the dusk sector. For positive By the difference densities are opposite in sign to those for negative By. Density differences for negative Bz show significant increases in the cusp region and premidnight sector but a small decrease in the dawn sector. The difference densities at high latitudes tend to be weakest when Bz is positive. We suggest that the high‐latitude thermospheric density variations for different IMF conditions, especially in the dawn and dusk sectors, can be strongly determined by thermospheric winds, which are associated with the ionospheric convection and vary strongly with respect to the IMF direction. We also suggest that density variations, especially in auroral and cusp regions, are also influenced by the local heating associated with ionospheric currents, which vary with IMF conditions.