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.

中文翻译：

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DMSP卫星在太阳最大黄昏观测到的高度为840 km的电离层赤道电离异常的发生

尽管人们公认赤道电离异常（EIA）的电子密度双峰通常会随着高度的增加而合并成一个单峰，因此在较高的上层电离层中EIA的特征变得不明显，但仍不清楚在什么高度双峰结构可以扩展。在这项研究中，我们使用DMSP F12-F15卫星在840 km高处的测量值，通过检查离子密度在轨道上的纬度分布，检查了2000-2002年太阳最大年份黄昏区的EIA发生率（EOR）。仍然可以在DMSP高度处观察到EIA，并且在春分季节的傍晚特定经度处的EOR可以达到30％。EOR表现出取决于季节的波状经度结构。而且，不规则发生率与EOR相似，在纵向和季节上都有变化，并且它们的局部时间演化表明前者比后者早到达高峰。赤道垂直线的观测E  ×  B等离子体漂移表明，东侧电场的逆转增强（PRE）在日落后840 km高度处的赤道电离异常的形成中起重要作用，但在该高度发生赤道电离异常剂量不能精确地代表东向电场逆转增强的特征。