Variation of the topside ionosphere during the last solar minimum period studied with multisatellite measurements of electron density and temperature

Abstract

Using the ionospheric measurements of CHAMP, DEMETER, and DMSP F15, the seasonal and spatial variations of the topside ionosphere during the last solar minimum period were investigated and compared with ionospheric models. In all the satellite measurements, equatorial ionization anomaly (EIA) shows clearly longitudinal asymmetry with wave number -3 or -4 patterns. Anomalous increases of N-e in the nighttime surpassing daytime N-e, known as the Weddell Sea anomaly (WSA) or midlatitude summer nighttime anomaly (MSNA), were also observed in the global N-e distribution with differences in detailed geometry of the geomagnetic field according to the altitude. In the nighttime ionosphere, the reduced T-e in the equatorial region at the DMSP altitude, identified as the equatorial plasma temperature anomaly (EPTA), was ascribed to the leftover of the prereversal enhancement of the upward plasma drift. Though the EIA, WSA, MSNA, and EPTA are all associated with the upward plasma movement, the difference in the thermal evolution is ascribable to the geometry of drift in which the plasma moves across the geomagnetic field line for the EIA and the EPTA, while along the field line for the WSA and the MSNA