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Electron density measurements from the Floating Potential Measurement Unit (FPMU) onboard the International Space Station allow us to observe the structure of the equatorial ionosphere. During two geomagnetically quiet time periods, we examined the equatorial F‐region structure at night using FPMU electron density measurements along with Swarm spacecraft electron density measurements and Total Electron Content from ground‐based Global Navigation Satellite System receivers for comparison. During these time periods, the equatorial ionization anomaly (EIA) extended to local times late as post‐midnight in some cases. The EIA occurrences at night showed a longitudinal dependence. The mean density of the EIA peaks exhibited a 3‐wave pattern in longitude likely due to lower atmospheric planetary wave activity, similar to the longitudinal dependence previously observed in the EIA.

Most of the low‐latitude ionospheric radar observations in South America come from the Jicamarca Radio Observatory, located in the western longitude sector (∼75°W). The deployment of the 30 MHz FAPESP‐Clemson‐INPE (FCI) coherent backscatter radar in the magnetic equatorial site of São Luis, Brazil, in 2001 allowed observations to be made in the eastern sector (∼45°W). However, despite being operational for several years (2001–2012), FCI only made observations during daytime and pre‐midnight hours, with a few exceptions. Here, we describe an upgraded system that replaced the FCI radar and present results of full‐nightF‐region observations. This radar is referred to as Measurements of Equatorial and Low‐latitude Ionospheric irregularities over São Luís, South America (MELISSA), and made observations between March 2014 and December 2018. We present results of our analyses of pre‐ and post‐midnightF‐region echoes with focus on the spectral features of post‐midnight echoes and how they compare to spectra of echoes observed in the post‐sunset sector. The radar observations indicate that post‐midnightF‐region irregularities were generated locally and were not a result of “fossil” structures generated much earlier in time (in other longitude sectors) and that drifted into the radar field‐of‐view. This also includes cases where the echoes are weak and that would be associated with decaying equatorial spreadF(ESF) structures. Collocated digisonde observations show modest but noticeableF‐region apparent uplifts prior to post‐midnight ESF events. We associate the equatorial uplifts with disturbed dynamo effects and with destabilizingF‐region conditions leading to ESF development.