We used reanalyzed Jicamarca radar measurements to study the response of equatorial ionospheric electrodynamics and spread F during the main phase of the large September 2017 geomagnetic storm. Our observations near dusk on 7 September show very large upward drifts followed by a large short‐lived downward drift perturbation that completely suppressed the lower F region plasma irregularities and severely decreased the backscattered power from the higher altitude spread F. We suggest that this large short‐lived westward electric field perturbation is most likely of magnetospheric origin and is due to a sudden and very strong magnetic field reconfiguration. Later in the early night period, data indicate large, mostly upward, drift perturbations generally consistent with standard undershielding and overshielding electric field effects, but with amplitudes significantly larger than expected. Our analysis suggests that occurrence of storm‐time substorms is one of the major factors causing the large nighttime westward and eastward electric field perturbations observed at Jicamarca near the storm main phase. Our analysis also suggests that magnetospheric substorms play far more important roles on the electrodynamics of the equatorial nighttime ionosphere than has generally been thought.

We present examples of unusual “stair‐step” differential flux spectra observed by the Apollo 14 Suprathermal Ion Detector Experiment on the lunar dayside surface in Earth's magnetotail. These spectra exhibit a relatively constant differential flux below some cutoff energy and then drop off precipitously, by about an order of magnitude or more, at higher energies. We propose that these spectra result from photoions accelerated on the lunar dayside by nonmonotonic potentials (i.e., potentials that do not decay to zero monotonically) and present a model for the expected differential flux. The energy of the cutoff and the magnitude of the differential flux are related to the properties of the local space environment and are consistent with the observed flux spectra. If this interpretation is correct, these surface‐based ion observations provide a unique perspective that both complements and enhances the conclusions obtained by remote‐sensing orbiter observations on the Moon's exospheric and electrostatic properties.

The equatorial ionization anomaly (EIA) is a prominent global-scale structuring of the terrestrial low-latitude ionosphere in the form of a trough in the F-region electron density centered on the geomagnetic (dip) equator with two peaks (crests) in the electron density on each side offset in latitude by ±15°. Originally discovered from analysis of ionosonde data (Appleton, 1946;Bailey, 1948), the EIA has been observed extensively with various ground-based and spacecraft instrumentation over many decades. It is now well-understood that EIA is formed by the combined effects of upward (vertical at the equator) E × B plasma drift and diffusion along magnetic field lines. These two forces work together to form the so-called equatorial fountain effect (e.g., Anderson, 1973;Balan et al., 2018;Hanson & Moffett, 1966;Kelley, 2009). The low-latitude E × B drift velocity is dominated by the neutral wind dynamo during geomagnetically quiet times but during active times, includes highly variable transient contributions from prompt penetration effects and longer lived effects from the disturbance dynamo (e.g., Fejer & Maute, 2021). The sensitivity of the EIA to the low-latitude E × B drift velocity (e.g., Anderson, 1973) makes it an excellent diagnostic for studying equatorial electrodynamics (e.g., Balan et al., 2009).The observed north-south asymmetry of the ionization anomaly crests with respect to the geomagnetic equator is caused by the season-dependent asymmetry in the meridional neutral winds (Anderson & Roble, 1981). Effects from the neutral atmosphere can also cause a longitudinal variation in the anomaly's occurrence and size, presumably due to the longitudinal variation of the quiet-time dynamo electric field (e.g., Fejer et al., 2008). In addition, the magnitude of the anomaly crests exhibit a 4-peak (sometimes also identified as a 3-wave) pattern in longitude for a given local time (LT), as observed by remote Far Ultraviolet (FUV) nightglow data in the evening LT sector (

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