Observations of thermospheric neutral winds and temperatures at Cachoeira Paulista (23°S, 45°W) during a geomagnetic storm

Abstract

Observations of nighttime thermospheric neutral wind velocities and temperatures from measurements of Doppler shifts and widths, respectively, of the OI 630.0 nm airglow emission line, using a Fabry-Perot interferometer, have been carried out at Cachoeira Paulista (23°S, 45°W), Brazil, since 1988. In this paper we present observations of thermospheric neutral winds and temperatures during the period July 6–10, 1991, which included the strong magnetic disturbance of July 8–9, 1991 when the Kp value reached 8. During this storm the meridional and zonal neutral wind velocity patterns were strongly modified. On July 9–10, the observed meridional wind during the storm shows a behaviour opposite to that observed during winter and the observed zonal wind at the beginning of the night shows a behaviour similar to its quiet time behaviour (to the east), but at 01:15 LT the zonal wind changes its direction rapidly to the west and at 03:45 LT it reverses to east again. Also, from the observed thermospheric temperatures, inferred from the thermal broadening of the 630.0 nm line emission, we find that during this event there is thermospheric cooling over Cachoeira Paulista. This result is surprising, because we would normally expect thermospheric heating during geomagnetic storms, since a large amount of energy is deposited in the thermosphere at high latitudes.