LAUSR

Although it is accepted that the electron density double peaks of the Equatorial Ionization Anomaly (EIA) generally merge into a single‐peak with increasing altitude and thus the signature of EIA becomes inconspicuous in the higher topside ionosphere, it is still unclear that to what altitudes the double‐peak structure can extend. In this study, we used measurements of the DMSP F12‐F15 satellites at ∼840 km altitude to examine EIA occurrence rate (EOR) in the dusk sector during solar maximum years 2000–2002 by checking latitudinal profiles of the ion density orbit by orbit. The EIA can still be observed at DMSP heights, and the EOR can reach 30% at specific longitudes at dusk side in equinox seasons. The EOR behaves wave‐like longitude structure that depends on seasons. Moreover, the irregularity occurrence rate shows similar longitudinal and seasonal variations with EOR, and the local time evolutions of them show that the former peaks earlier than the latter. The observations of equatorial vertical E × B plasma drifts indicated that the prereversal enhancement (PRE) of eastward electric field plays a significant role in the formation of the equatorial ionization anomaly at an altitude of 840 km just after sunset, but the occurrence of the equatorial ionization anomaly at this altitude dose not precisely represent the characteristics of the prereversal enhancement of eastward electric field.