How can GOCE and TerraSAR-X contribute to the topside ionosphere and plasmasphere research?

In this study we analyze concurrent observations derived from two low-Earth orbit satellite missions - the GOCE (Gravity field and steady state Ocean Circulation Explorer) satellite with unprecedented low orbit of 250-km and the TerraSAR-X satellite with orbit altitude of 515-km. The science payloads of both satellites have no instruments for ionospheric research, but they include a dual-frequency GPS (Global Positioning System) receiver. GPS measurements from the precise orbit determination GPS antenna on board these satellites can be used to determine TEC (total electron content) at the topside ionosphere-plasmasphere system for different conditions. Results derived from these missions are presented for the first time. The comparison was done for June and December solstice conditions at low and moderate solar activity levels for quiet geomagnetic activity. We obtain the quantitative estimates of electron content and its percentage contribution to the ground-based GPS TEC for topside ionosphere/plasmasphere at altitudes 250-500-km and above 500-km. Similarities and differences in such electron content distribution on a global scale are discussed. Distinctive features of the Weddell Sea Anomaly and equatorial ionization anomaly are evident in the topside TEC derived from GOCE GPS data. We report the Weddell Sea Anomaly presence in TEC at early morning local time.