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The low latitude ionosphere and thermosphere are strongly disturbed during and shortly after geomagnetic storms. We use novel Jicamarca radar measurements, ACE satellite solar wind, and SuperMAG geomagnetic field observations to study the electrodynamic response of the equatorial ionosphere to the 23, 24 April 2023 geomagnetic storm. We also compare our data with results from previous experimental and modeling studies of equatorial storm-time electrodynamics. We show, for the first time, unusually large equatorial vertical and zonal plasma drift (zonal and meridional electric field) perturbations driven simultaneously by multi storm-time electric field mechanisms during both the storm main and recovery phases. These include daytime undershielding and overshielding prompt penetration electric fields driven by solar wind electric fields and dynamic pressure changes, substorms, as well as disturbance dynamo electric fields, which are not well reproduced by current empirical models. Our nighttime measurements, over an extended period of large and slowly decreasing southward IMF Bz, show very large, substorm-driven, vertical and zonal drift fluctuations superposed on large undershield driven upward and westward drifts up to about 01 LT, and the occurrence of equatorial spread F irregularities with very strong spatial and temporal structuring. These nighttime observations cannot be explained by present models of equatorial storm-time electrodynamics. [ABSTRACT FROM AUTHOR]

We performed, for the first time, a season‐dependent geomagnetically quiet‐time climatology of mid‐ and low‐latitude ion densities during low and moderate solar flux conditions using Floating Potential Measurement Unit (FPMU) observations aboard the International Space Station (ISS) from 2008 to 2019. Our daytime observations indicate that the main characteristics of the equatorial ionization anomaly (EIA) at ∼400 km are consistent with those from other in situ and remote sensing probes. The FPMU daytime densities are also generally in good agreement with the corresponding results from the International Reference Ionosphere (IRI). However, the IRI does not reproduce the mid‐solar flux evening low‐latitude measured densities. In this period, the EIA exhibits strong longitude‐dependent crest‐to‐trough ratios and asymmetries due to the pre‐reversal enhancement (PRE) of the zonal electric field and thermospheric neutral winds. Our data also show strong structuring of the daytime and nighttime plasma densities. This includes a bulge, discussed for the first time here, in the EIA southern crest in the South Atlantic sector during the December solstice and equinox data, which we suggest being generated by the transport of plasma from the Weddell Sea anomaly (WSA). We also highlight and show the evolution of a midlatitude summer nighttime anomaly (MSNA) during the June solstice data in the North Atlantic sector. Our results give new insights into these two anomalies, where we show that they are stronger with increasing solar flux levels and that they last until the early morning. These latter results are not consistent with those from previous studies. Key Points: The FPMU daytime densities are generally in good agreement with the results from the International Reference Ionosphere (IRI)A bulge in the EIA southern crest possibly associated with the Weddell Sea anomaly (WSA) was observed during the daytimeTwo midlatitude summer nighttime anomalies (MSNAs) were observed until the early morning and were stronger in a higher solar flux period [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: We present measurements of the electron density in the ionized part of the terrestrial atmosphere using the Floating Potential Measurement Unit plasma instrument on board the International Space Station, at an altitude around 400 km. Near the geomagnetic equator, these measurements show that a well‐known daytime structure in the electron density extends all the way past midnight. This structure is the equatorial ionization anomaly, which consists of a trough at the equator and two peaks at approximately ±15° latitude. We also show that values of electron density in this structure are modulated in longitude, varying as a 3‐wave pattern, likely due to the underlying action of tidal planetary waves in the lower atmosphere. Key Points: During two geomagnetically quiet time periods, the equatorial ionization anomaly (EIA) was observed at local times as late as post‐midnightThe mean density of the EIA crests at night appears to exhibit a 3‐wave pattern in longitudeThis variation in longitude is consistent with previous observations from IMAGE [ABSTRACT FROM AUTHOR]

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‐night F‐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‐midnight F‐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‐midnight F‐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 spread F (ESF) structures. Collocated digisonde observations show modest but noticeable F‐region apparent uplifts prior to post‐midnight ESF events. We associate the equatorial uplifts with disturbed dynamo effects and with destabilizing F‐region conditions leading to ESF development. Key Points: The MELISSA 30 MHz radar system is described and examples of F‐region observations are introducedResults of the spectral analysis of pre‐ and post‐midnight F‐region echoes are presented and comparedResults indicate the local development of F‐region irregularities in the post‐midnight sector [ABSTRACT FROM AUTHOR]