Neutral winds and [formula omitted] emission rates in the lower thermosphere as measured with WINDII/UARS during the April 4–5th 1993 and February 1994 geomagnetic storms

Abstract

WINDII data provide neutral winds and O( 1 S) volume emission rates in the lower thermosphere between 90 and 200 km on a global scale. Data during two major geomagnetic events in April 1993 and February 1994 were selected for studying geomagnetic effects on this region. According to previous work, under quiet conditions, the dynamics in the altitude range between 90 and 130 km is mainly controlled by tidal forcing, winds have local time-dependent phases and latitude-dependent amplitude; above 130 km winds are relatively stable and weak with speeds mostly <100 m s −1 . During these two major geomagnetic storms, neutral winds at altitudes above 130 km in both polar regions are severely disturbed; the wind velocities follow the two-cell pattern of ion convection with maximum wind speeds increasing up to 600– 700 m s −1 . Below 130 km the tidal structure is retained at low- and mid-geomagnetic latitudes; but at high-geomagnetic latitudes (50° and above) wind profiles display complex disturbances, as a result of the interaction of irrotational tidal forcing and rotational geomagnetic forcing. During the February 1994 storm, neutral winds at mid-geographic latitudes show a much stronger disturbance in the southern hemisphere than in the northern hemisphere. Auroral enhancements in the green emission reach geographic latitudes 45°N/S during the two major storms. The emission rate at high geographic latitudes in the E-region increases to 2–5 times its usual value, but in the lower F-region the response of the emission rate is less dramatic, most of the time it is enhanced at high geographic latitudes but depleted at mid-geographic latitudes.