A Case Study of Ionospheric Storm-Time Altitudinal Differences at Low Latitudes During the May 2021 Geomagnetic Storm

Previous studies paid little attention to the ionospheric storm-time altitudinal differences due to insufficiency of ionospheric measurements. In this work, multiple instrumental observations were used to investigate the ionospheric storm-time response at low latitudes in the American and Asian-Australian sectors during the May 2021 geomagnetic storm. The ground-based Global Navigation Satellite Systems (GNSS) total electron content (TEC) and Low Earth Orbit (LEO) satellite topside TEC presented opposite (positive/negative) variations in the low-latitude and equatorial region of both sectors during this storm. The electron density profiles from the Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) and the Sanya Incoherent Scatter Radar showed a good agreement and well explained the opposite variations between GNSS TEC and LEO satellite topside TEC. The F2-layer peak height (hmF2) and peak density (NmF2) displayed inverse variations, and the feature was present mostly in the regions between equatorial ionization anomaly crests. The combined modulation effects of the storm-time zonal electric fields and the field-aligned transports possibly resulted in the contrary variations of hmF2 and NmF2 in the low-latitude and equatorial region, leading to the storm-time altitudinal differences during this storm. Relatively, the storm-time thermospheric composition disturbances might be a minor factor responsible for these differences. The ionospheric storm-time responses have been the research hotspot for nearly 80 years. When geomagnetic storms occur, the temperature, electron density, and electric field of the ionosphere are violently disturbed due to the enhanced energy coupled into the ionosphere, which are called ionospheric storms. The increases (decreases) of ionospheric electron density and total electron content are called positive (negative) storm during ionospheric storm. The ionospheric storm-time responses present complex variations such as sectoral differences, latitudinal differences, and altitudinal differences. The altitudinal differences are mainly reflected in that the ionospheric storm-time responses are inconsistent at different altitudes. This study focuses on the ionospheric storm-time altitudinal differences at low latitudes during the May 2021 geomagnetic storm. In the low-latitude and equatorial region, the ionospheric storm-time electron densities at different altitudes show inconsistent variations. The storm-time zonal electric fields and field-aligned transports may play significant roles in the ionospheric storm-time altitudinal differences in the low-latitude and equatorial region during this storm. The ionospheric storm-time altitudinal differences were studied by comparing Global Navigation Satellite Systems (GNSS) total electron content (TEC), Swarm, COSMIC-2, Sanya Incoherent Scatter Radar, Ionospheric Connection Explorer, and Global Scale Observations of the Limb and Disk The ground-based GNSS TEC and topside Low Earth Orbit TEC revealed opposite variations in both the Asian-Australian and American sectors Storm-time zonal electric fields and field-aligned transports possibly played key roles