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 (F‐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.