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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 25, issue 9
Ann. Geophys., 25, 1949–1958, 2007
https://doi.org/10.5194/angeo-25-1949-2007
© Author(s) 2007. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 25, 1949–1958, 2007
https://doi.org/10.5194/angeo-25-1949-2007
© Author(s) 2007. This work is distributed under
the Creative Commons Attribution 3.0 License.

  02 Oct 2007

02 Oct 2007

A new method for studying the thermospheric density variability derived from CHAMP/STAR accelerometer data for magnetically active conditions

M. Menvielle1,2, C. Lathuillère3, S. Bruinsma4, and R. Viereck5 M. Menvielle et al.
  • 1CNRS/IPSL, Université Versailles St-Quentin, Centre d'études des Environnements Terrestre et Planétaires, UMR 8639, 94107 Saint Maur Cedex, France
  • 2Univ. Paris Sud, 91405 Orsay Cedex, France
  • 3Laboratoire de Planétologie de Grenoble, UJF, CNRS, BP 53, 38041 Grenoble Cedex 9, France
  • 4CNES, dept. of Terrestrial and Planetary Geodesy, 31401 Toulouse Cedex 4, France
  • 5NOAA Space Environment Centre, 325 Broadway, Boulder, CO 80305-3328, USA

Abstract. Thermospheric densities deduced from STAR accelerometer measurements onboard the CHAMP satellite are used to characterize the thermosphere and its response to space weather events. The STAR thermospheric density estimates are analysed using a Singular Value Decomposition (SVD) approach allowing one to decouple large scale spatial and temporal variations from fast and local transients. Because SVD achieves such decomposition by using the reproducibility of orbital variations, it provides more meaningful results than any method based upon data smoothing or filtering.

SVD analysis enables us to propose a new thermosphere proxy, based on the projection coefficient of the CHAMP densities on the first singular vector. The large scale spatial variations in the density, mostly related to altitude/latitude variations are captured by the first singular vector; time variations are captured by the associated projection coefficient.

The study presented here is focused on time dependent global scale variations in the thermospheric density between 50 N and 50 S geographic latitudes. We show that the time variations in the projection coefficient do in fact represent those in the global density that are associated with magnetic activity as well as with solar EUV radiations. We also show that the NRLMSISE-00 empirical model better accounts for the density forcing by Solar radiations when tuned using Mg II indices. Using the so modified model with an additional geomagnetic parameterization corresponding to quiet geomagnetic situation enables one to define time reference values which are then used to evaluate the impact of geomagnetic activity. The ratio of CHAMP density projection coefficient to the quiet model projection coefficient is a global quantity, independent of altitude and latitude, which quantifies the thermospheric density response to auroral energy deposition. It will serve as a proxy of the response of thermospheric density to geomagnetic activity forcing.

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