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Abstract A coronal mass ejection erupted from the Sun on 21 April 2023 and created a G4 geomagnetic storm on 23 April. NASA's global‐scale observations of the limb and disk (GOLD) imager observed bright equatorial ionization anomaly (EIA) crests at ∼25° Mlat, ∼11° poleward from their average locations, computed by averaging the EIA crests during the previous geomagnetic quiet days (18–22 April) between ∼15°W and 5°W Glon. ReversedC‐shape equatorial plasma bubbles (EPBs) were observed reaching ∼±36° Mlat (∼40°N and ∼30°S Glat) with apex altitudes ∼4,000 km and large westward tilts of ∼52°. Using GOLD's observations EPBs zonal motions are derived. It is observed that the EPBs zonal velocities are eastward near the equator and westward at mid‐latitudes. Model‐predicted prompt penetration electric fields indicate that they may have affected the postsunset pre‐reversal enhancement at equatorial latitudes. Zonal ion drifts from a defense meteorological satellite program satellite suggest that westward neutral winds and perturbed westward ion drifts over mid‐latitudes contributed to the observed latitudinal shear in zonal drifts. 

Abstract We study the variations of the topside ionospheric ion density measured by Defense Meteorological Satellite Program satellites during the intense magnetic storm on 7–10 November 2004. It is found for the first time that quasi‐periodic enhancements in the ion density with a period of ∼6 hr occur nearly simultaneously at 0630, 0830, and 0930 local time in the dawn sector during the storm main phase with southward interplanetary magnetic field (IMF). The quasi‐periodic density enhancements extend from the southern subauroral latitudes to the northern subauroral latitudes. In the dusk sector, the topside ion density during the storm main phase is increased at middle latitudes for ∼12 hr but shows decrease or relatively small increase over the magnetic equator, indicating that penetration electric fields dominate the ion density redistribution. Similar quasi‐periodic enhancements in the topside ion density are also observed in the dawn sector during other intense magnetic storms. The solar wind and IMF do not have quasi‐periodic variations in this storm case. Periodic processes in geospace, such as periodic substorms in the magnetosphere, waves and tides in the atmosphere, and traveling ionospheric disturbances, cannot explain the observed periodic enhancements of the ionospheric ion density. We suggest that the magnetosphere‐ionospheric‐thermospheric system may have an intrinsic period of ∼6 hr and that oscillations of the magnetosphere‐ionospheric‐thermospheric system with this period can be excited during intense magnetic storms, although the mechanisms for the generation of the long‐periodic oscillations are not understood. 

Abstract We have used measurements of the Defense Meteorological Satellite Program (DMSP) satellites to study variations of electron temperature in the subauroral ionosphere during the magnetic storm on 17–25 March 2015. This magnetic storm had a long recovery phase of 7 days, and the ionospheric behavior over the entire storm time was seldom investigated. In this study, we find that the electron temperature at subauroral latitudes was continuously enhanced for 8 days, from the storm onset to the end of the recovery phase. The maximum electron temperature during the storm times was 1000–4000 K higher than the maximum electron temperature during quiet times. This long‐lasting enhancement of subauroral electron temperature was attributed to energy transfer among the solar wind, magnetosphere, ring current, plasmasphere, and ionosphere driven by high‐speed solar wind streams and fluctuating interplanetary magnetic field during the entire 8‐day period of the storm. The electron temperature enhancements were quite symmetric in the post‐midnight sector but became strongly asymmetric near dawn between the southern and northern hemispheres. The asymmetric enhancements of electron temperature near dawn may be related to the magnetic declination and the daytime midlatitude trough in the southern hemisphere. Large daily variations of maximum electron temperature in the post‐midnight sector were observed and may be related to the offset between geomagnetic and geographic latitudes. These DMSP observations provide new insight on ionospheric response to intense magnetic storms. 

Abstract We identified a few new storm‐time ionospheric phenomena by analyzing disturbances in topside ion density, electron temperature, and ion temperature at ∼840 km altitude measured by theDefense Meteorological Satellite Programsatellites during the 20 November 2003 magnetic storm. The storm‐time ion density enhancements showed different features at different local times. Longitudinal structures in the enhanced ion density occurred in the morning sector and extended from equatorial regions to middle latitudes. Ion density increase due to enhanced fountain effect was observed in the evening sector and lasted for ∼18 hr. A positive ionospheric storm occurred during the late recovery phase of the storm and was associated with increased atomic oxygen to molecular nitrogen column density ratio. Electron temperature at subauroral latitudes reached 8000 K during the storm, ∼4000 K higher than the quiet‐time temperature. The subauroral temperature enhancement lasted for 2–3 days. Simultaneous enhancements in the ion density, electron temperature, and ion temperature from subauroral to equatorial latitudes occurred in the night‐time ionosphere and lasted for ∼18 hr. A negative correlation between ion density and electron/ion temperature variations occurred in the dusk sector for ∼12 hr. An enhanced ion temperature crest in the winter hemisphere during the magnetic storm lasted for 2 days. A decrease in the ion temperature crest was also observed with an increase of the ion density. These new features in the ionospheric density and temperature, together with the results from previous studies, provide a more comprehensive scenario of the ionospheric response to the superstorm.