Global Response of Low and Mid-Latitude Ionosphere to the Severe Geomagnetic Storm of April 2023

The response of topside ionosphere over low and midlatitudes to the severe geomagnetic storm of 23–24 April 2023 are studied using in-situ plasma measurements made by the Swarm satellite constellation at altitudes of 470 and 510 km. The local time coverage during the period was on dawn and dusk sectors. The geomagnetic storm is a two-step storm reaching a minimum SYM-H index magnitude of −233 nT. Observations reveal positive ionospheric storm with plasma density enhancements during sunlit periods of the main phase of the geomagnetic storm. This enhancement is significant over the Southern hemisphere. A clear increase in the equatorial electric field due to prompt penetration field is inferred through the increased separation of EIA crests and plasma density enhancements. This event showcased a longlasting prompt penetration electric field covering both the main phases. Interestingly, during the temporary recovery phase between the first and second main phases, the ionospheric observations are similar to those in the main phases with no indication of an overshielding electric field. During predawn hours of temporary recovery phase, a negative ionospheric storm is observed only over low latitudes. During the recovery phase, disturbed dynamo effects are also observed. However, for the first five hours in the early recovery phase, the effects were still similar to that of main phase. While the ring current recovery indicated by SYM-H indices took more than 36 hours, the disturbance dynamo effects were seen in the ionosphere only for about 11 hours, and the ionosphere was behaving similar to the quiet periods after 16 hours of start of the recovery phase. The observations are explained by means of undershielding prompt penetration electric fields during the main phases and development of disturbed dynamo effect well into the recovery phase.