Variations of electron density and temperature in ionosphere based on the DEMETER ISL data

Observations of the Langmuir Probe Instrument (ISL, Instrument Sonde de Langmuir) onboard the DEMETER satellite during four years from 2006 to 2009 were used to analyze the tempo-spatial variations of electron density (N_e) and temperature (T_e) in the ionosphere. Twenty four research bins with each covering an area with 10° in longitude and 2° in latitude were selected to study the spatial distributions of N_e and T_e. The results indicate that both N_e and T_e have strong annual variations in the topside ionosphere at 660 km altitude. The semiannual anomaly and equinoctial asymmetry which are usually well known as the features of F-layer also exist in the topside ionosphere at low- and mid-latitudes. The yearly variation of N_e is opposite to the peak electron density of the F2-layer (NmF2) at higher latitudes in daytime and both are similar in nighttime. Also the yearly variations of T_e at low-latitude are contrary to that at 600 km in daytime and similar in nighttime. An interesting feature of nighttime T_e at low-latitude is an obvious annual variation in the northern hemisphere and semiannual variation in the southern hemisphere. The yearly variations of T_e in daytime have negative and positive correlation with N_e at mid- and high-latitudes, respectively. Both N_e and T_e in the neighborhood bins at the same latitude have a high correlation. In ionospheric events analyzing, this information may help to understand the characteristics of the variation and to distinguish the reliable abnormality from the normal background map.