New Aspects of the Coupling Between Thermosphere and Ionosphere, with Special regards to CHAMP Mission Results
The recent years have brought enormous progress in our understanding of the characteristics and the variability of the thermosphere. This is to a good part based on the high-resolution measurements of the accelerometer on board the CHAMP satellite. It sampled the air drag from the solar maximum in 2001 to the minimum in 2009. The thermosphere reacts quite differently to solar and magnetic forcing. This is in particular evident when comparing the mass density on the day and night sides at low latitudes. We were able to delineate the variations due to solar flux, season, and magnetic activity. Another important finding is the close relation of thermospheric features to the geomagnetic field geometry. Examples of this are the equatorial mass density anomaly, the cusp density anomaly, and the high-speed wind channel along the dip equator. None of these phenomena is so far considered in any of the atmospheric models. Thermospheric wind is a prime driver for ionospheric dynamics. As an example we present the F-region dynamo current system and its variation with local time. This wind-driven dynamo over the equator is assumed to cause special ionospheric effects after sunset. Typical post-sunset features at low latitude are the equatorial plasma bubbles. Here it is shown that some of these bubbles are accompanied by depletions in neutral density. Neutral depletions can form only when the ion-neutral friction is high enough. In the pressure balance equation, the neutral depletions make up a significant part. The comprehensive set of CHAMP neutral and plasma measurements enabled us to interpret the ionosphere–thermosphere as a coupled system.
KeywordsZonal Wind Solar Flux Plasma Bubble Night Side Equatorial Ionisation Anomaly
We thank Claudia Stolle and Stefan Maus for fruitful discussions about the ionosphere–thermosphere results presented here. We also thank W. Köhler for processing the CHAMP accelerometer data. The CHAMP mission is sponsored by the Space Agency of the German Aerospace Center (DLR) through funds of the Federal Ministry of Economics and Technology, following a decision of the German Federal Parliament (grant code 50EE0944). The data retrieval and operation of the CHAMP satellite by the German Space Operations Center (GSOC) is acknowledged.
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