Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufmsa31a..08b&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #SA31A-08
Physics
0355 Thermosphere: Composition And Chemistry, 1241 Satellite Orbits, 1294 Instruments And Techniques
Scientific paper
The German satellite CHAMP has been launched in a circular, nearly-polar orbit at 460 km altitude in July 2000. The two main mission objectives are the mapping of the magnetic and gravity fields of the Earth. The French accelerometer STAR, positioned at the center of mass of CHAMP, measures the nongravitational accelerations acting on it. Therefore, a secondary objective concerns the monitoring of atmospheric density, through analysis of drag observations. The CHAMP mission profile is particularly interesting for upper atmosphere studies, since it provides a nearly complete latitudinal coverage, while complete solar local time sampling is achieved every four months approximately. During its 5-year mission, the solar activity will be covered from maximum (2000-2001) to minimum (2005) conditions with the same instrument. This last point is important in view of the heterogeneous density database available nowadays. The total atmospheric density can be derived from the STAR data. The accuracy of this derivation depends on the uncertainty of the STAR calibration parameters (bias and scale factor), the data preprocessing, the CHAMP satellite macromodel, as well as the magnitude of the thermospheric winds. These winds increase with geomagnetic activity, attaining speeds of up to 1000 m/s at CHAMP's altitude. This causes an ever larger uncertainty of the derived neutral densities, which increases approximately 5% per 200 m/s. The differences between modeled- and observed drag accelerations have been analyzed qualitatively under low- and high geomagnetic activity conditions. The STAR data are also used to compare two proxy-indicators for solar activity: F10.7 (radio flux), which is used in most upper atmosphere models, and the chromospheric Mg II index. It has been shown recently that the latter index is more representative of the EUV/UV emissions, which heat the upper atmosphere. The difference between the two indices on time scales of days to several solar rotations can be large during high solar activity. In this presentation, thermosphere model predictions, based either on F10.7 or Mg II, are compared to STAR density observations, demonstrating the importance of the employed proxy.
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