The influence of IMF BY on the high‐latitude thermospheric circulation during northward IMF

Abstract

Thermospheric neutral wind data, obtained by the Fabry‐Perot Interferometer (FPI) and Wind and Temperature Spectrometer (WATS) on the Dynamics Explorer 2 satellite during periods of northward interplanetary magnetic field (IMF Bz positive), show a dependence of the circulation on the magnitude and sign of the east–west component (By) of the IMF. Averaged thermospheric neutral wind vector fields, for the northern hemisphere, and data from several sets of satellite orbits, that illustrate the variation in the neutral wind structure as a function of time and IMF orientation, are presented and discussed. The results show that near December solstice, at solar maximum: (1) sunward neutral winds develop inside the geomagnetic polar cap during periods of persistent positive Bz and relatively low By; (2) typical time delays in the establishment of sunward winds following a northward turning of the IMF are approximately 4 hours; (3) sunward winds are not observed when the magnitude of the Bz component of the IMF is less than +4 nT; (4) the maximum‐velocity antisunward winds, in the central geomagnetic polar cap region, are on the duskside (dawnside) in the northern hemisphere during periods of northward IMF and By negative (positive). A similar asymmetry exists in the sunward flow in the center of the polar cap; (5) if the absolute magnitude of the By component of the IMF exceeds that of Bz (i.e., if the By/Bz ratio is greater than 1.0) then sunward winds inside the geomagnetic polar cap are rarely observed; instead, one sees a pattern of neutral winds similar to that expected for the given sign of By under southward IMF conditions; (6) the summer hemisphere has neutral wind signatures that are similar to those seen in the winter hemisphere for Bz northward, i.e., sunward winds occur in the polar cap for persistently northward Bz, provided the magnitude of By does not exceed that of Bz.