Combined CHAMP-EISCAT studies on local thermospheric mass density enhancements in the CUSP

Rentz, S.1, Lühr, H.1 and Rietveld, M.2

1 GFZ Potsdam, Potsdam, Germany
2 EISCAT Scientific Association, Tromsų, Norway

Using onboard-measured accelerometer data the neutral atmospheric density and wind distribution at orbit altitudes of the CHAMP satellite (ca. 400km) can be examined both on global scale and on local scale. A prominent investigated local phenomenon is the thermospheric mass density anomaly in the cusp region. CHAMP frequently records air drag peaks during cusp overflights. In case studies, the arising density variations were discussed. The results initiated a multi-instrumental satellite-EISCAT-campaign called SIRCUS, where incoming magnetosheath plasma was supposed to be a possible driving mechanism. It can penetrate down to lower altitudes in the cusp region and cause ionospheric currents which may fuel Joule heating. In the meantime, several years of CHAMP data are available. Hence, the investigation of the local phenomenon was extended to a statistical analysis, using three years of accelerometer measurements. The study reveals dependences on solar activity, season and solar zenith angle. The amplitude of the density enhancement strongly depends on the level of solar EUV radiation. It decreases by a factor of 3 between 2002 (average yearly F10.7 = 179) and 2004 (average yearly F10.7 = 107). In the northern hemisphere, the density distribution displays a clear seasonal variation with weakest amplitudes in summer and largest amplitudes during autumn and winter (mean ratio autumn/summer = 1.1). Whilst the density enhancement is confined to the noon sector during high solar zenith angles, it is shifted by about 1.5 hours towards the afternoon during low solar zenith angles. The statistical analysis was complemented by a combined CHAMP-EISCAT campaign in October 2006. While the CHAMP spacecraft maps the thermospheric responses, the ESR32, ESR42, and the Tromsų VHF radar simultaneously measure the ionospheric parameters, which are used to calculate conductivities, electric field, and Joule heating rates. The measurements will help to identify the altitude range of the heated area and the driving processes. Therefore the combined studies contribute to a better understanding of the coupling processes between ionospheric plasma and thermospheric neutral gas.