Effects of sudden stratospheric warming (SSW) on the lunitidal modulation of the F-region dynamo

Abstract

The E-region dynamo in the equatorial ionosphere changes significantly during Sudden Stratospheric Warming (SSW) events. However, little is known about the modulation of the F-region dynamo by SSW events. During the SSW event of December 2001 the vertical current density in the daytime equatorial F-region, as observed by the CHAMP satellite, exhibited oscillations with a 13-day period. This period reflects the lunar tide in the CHAMP satellite data. Two independent drivers are known to contribute to the F-region vertical current: thermospheric zonal wind (F-region dynamo) and vertical electric field (E-region dynamo). The thermospheric zonal wind as observed by CHAMP was modulated in a similar way as the F-region vertical current. Vertical electric field in the equatorial F-region can be estimated from the magnetic and plasma/neutral observations of the CHAMP satellite. The obtained values also show quasi 13-day modulations. The thermospheric wind and vertical electric field drove the F-region vertical currents in opposite directions, and the polarity of the net current generally followed the former (-region dynamo). From the observed phase delays of the 13-day oscillations we conclude that the F-region vertical current during the SSW event was not only modulated by the variation of the thermospheric wind but also by the variations of the E-region electric field and the F-region plasma density distribution.