In situ electron density measurements by the CHAllenging Minisatellite Payload and the Defense Meteorological Satellites Program F17 satellites show that the midlatitude ionization at altitudes of ∼350 and 850 km is enhanced in the late evening. The enhancements increase to maximum around midnight and are clearly observed till early morning as the equatorial ionization decays to minimal level. They appear in the winter hemisphere during June and December solstices and in both hemispheres during equinox. The enhancements are well confined between ±30° and ±50° magnetic latitude, with the magnetic flux tubes of L = 1.3 − 2.4 connecting to the plasmasphere. Furthermore, coincident longitudinal variations exist in both the ionospheric enhancements and the plasmaspheric total electron content, especially during the solstice months. The coincidence may suggest essential plasma transport between the ionosphere and the plasmasphere. These facts support the idea that the plasmasphere provides extra plasma to the midlatitude ionosphere through downward plasma influx along the magnetic field lines to form the nighttime ionization enhancements when the sunlight is absent.

The diurnal variation of the ionosphere is mainly controlled by solar radiation via photoionization, which expects a midday electron density (N e ) maximum and a night minimum. Meanwhile, there are also deviations from this main cycle, for example, the long-known Weddell Sea Anomaly (WSA; Bellchambers & Piggott, 1958) characterized by higher nighttime ionization. This phenomenon is later globally observed, which tends to occur at midlatitude in the summer hemisphere with prolonged sunlight at high altitudes (e.g., Lin et al., 2010;Thampi et al., 2011;Yin et al., 2022).Another nighttime anomaly exists in the winter hemisphere, with N e turning to increase when the photoionization fully stops after sunset. Here we use the winter nighttime enhancement (WNE) to refer to this kind of anomaly that has been investigated at first by incoherent scatter radars (ISR), ionosondes and total electron content (TEC) measurements (e.g.,

Ionospheric plasma density data from the Planar Langmuir Probe onboard the CHAllenging Minisatellite Payload (CHAMP) are used to investigate the latitudinal profile of the ionospheric plasma density. Along with the well‐known profiles with features of the equatorial ionization anomaly and equatorial plasma bubbles, a third type is observed as smooth profiles with large‐scale deep equatorial trough (DET) where the plasma density is extremely low. A global survey focusing on magnetically quiet conditions reveals that the DET profiles are generally a post‐sunset phenomenon, and their appearance depends on longitude and season. During equinoxes in active solar years, the strong pre‐reversal enhancement electric field is believed to uplift the ionosphere and produce the DET profiles. While around June solstice, the DET profiles are clustered at normal45°W–normal0° E longitude, where the zonal electric field is weak but the magnetic meridional component of the neutral wind is most significant. The SAMI2 model is then employed to simulate the ionospheric plasma distribution in the magnetic meridional plane with enhanced neutral wind, which successfully reproduced the main features of the observed DET profile. The results indicate that the neutral wind can obviously affect the equatorial ionosphere by transporting plasma in the magnetic flux tube through neutral‐ion interaction. The neutral wind is previously known to work with the inclined magnetic field at mid‐latitude, while this study emphasizes its effect around the magnetic equator especially when the electric field forcing (boldE×boldB) is weak or absent.

Interplanetary shocks are abrupt changes in plasma properties that propagate through the heliosphere. The impingement of an IP shock onto the Earth's magnetosphere can have profound space weather consequences (

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