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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 27, issue 5
Ann. Geophys., 27, 2087–2099, 2009
https://doi.org/10.5194/angeo-27-2087-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 27, 2087–2099, 2009
https://doi.org/10.5194/angeo-27-2087-2009
© Author(s) 2009. This work is distributed under
the Creative Commons Attribution 3.0 License.

  06 May 2009

06 May 2009

Solar and magnetospheric forcing of the low latitude thermospheric mass density as observed by CHAMP

S. Müller1,*, H. Lühr2, and S. Rentz2 S. Müller et al.
  • 1Free University Berlin, Institute for Meteorology, Carl-Heinrich-Becker Weg 6–10, 12165 Berlin, Germany
  • 2Helmholtz-Centre Potsdam, GFZ – German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
  • *now at: Norwegian Meteorological Institute, P.O. Box 6314, 9293 Tromsø, Norway

Abstract. We have studied the dependence of the thermospheric mass density at equatorial latitudes on the influence of various drivers. This statistical study is based on CHAMP accelerometer measurements. Our aim is to delineate the influences of the different contributions. For the isolation of the effects we make use of a dedicated data selection procedure and/or removal of disturbing effects. In a first step all readings are normalised to an altitude of 400 km. For the investigation of the solar influences only magnetically quiet days (Ap≤15) are considered. The dependence on solar flux can well be described by a linear relation within the flux range F10.7=80–240. The slope is twice as steep on the day side as on the night side. The air density exhibits clear annual and semi-annual variations with maxima at the equinoxes and a pronounced minimum around June solstice. The thermosphere maintains during quiet days a day to night mass density ratio very close to 2, which is independent of solar flux level or season. The magnetospheric input causing thermospheric density enhancement can well be parameterised by the am activity index. The low latitude density responds with a delay to changes of the index by about 3 h on the dayside and 4–5 h on the night side. The magnetospheric forcing causes an additive contribution to the quiet-time density, which is linearly correlated with the am index. The slopes of density increases are the same on the day and night sides. We present quantitative expressions for all the dependences. Our results suggest that all the studied forcing terms can be treated as linear combinations of the respective contribution.

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