Localized Plasma Density Peak at Middle Latitudes During the April 2023 Geomagnetic Storm

This paper conducts a multi-instrument analysis of a latitudinal plasma density peak at the middle latitudes during the early recovery phase of the April 2023 geomagnetic storm. The total electron content (TEC), peak density of the F layer, and the in situ plasma density from Swarm and Defense Meteorological Satellite Program (DMSP) satellites all capture this peak feature. This narrow latitudinal peak structure appeared around 50 degrees N and extended from 40 degrees E to 150 degrees E in longitude with a prolonged duration of about 8 hr from sunset to midnight. This mid-latitude peak reveals a noticeable equatorward motion and a slight westward shift. According to the plasma composition observations from DMSP satellites, this peak structure shows an O+ ions dominance, which means that this peak is more likely to be formed by an internal rather than an external source from the plasmasphere. Meanwhile, the middle latitude Fabry-Perot interferometer (FPI) observed strong equatorward thermospheric winds, and the peak height of the F layer presented a visible elevation, which suggests that the equatorward wind lifting caused the plasma density enhancement. Besides, the O/N2 ratio significantly decreased at lower and middle latitudes, and ion drift observations showed a distinct subauroral westward channel. Based on these simultaneous measurements, this structure's sharp equatorward and poleward boundaries might be related to the O/N2 ratio change and the subauroral polarization stream (SAPS) flow separately. The ionospheric response to geomagnetic storm disturbances exhibits diverse plasma density structures. During the early recovery phase of the April 2023 geomagnetic storm, a distinct latitudinal plasma density peak is observed at middle latitudes. The formation of the mid-latitude plasma density peak structure is not settled yet, even though some homologous structures have been reported. This study investigates the spatial and temporal features of this mid-latitude peak structure and the dominant ion composition. The investigation is achieved by utilizing the total electron content, F layer parameters, and in situ plasma observations. Furthermore, the observations of neutral wind, thermospheric composition, and ion drift observations show a good correlation with the spatial-temporal characteristics of this peak structure. These clues suggest that the formation of the mid-latitude peak structure during the April 2023 geomagnetic storm may be attributed to a combined effect involving equatorward neutral winds, thermospheric composition change, and subauroral polarization stream flow. A mid-latitude peak is present in total electron content, F layer peak density, and in situ plasma density during the storm recovery phase The mid-latitude peak covers 40 degrees-150 degrees E in longitude with a noticeable equatorward motion and slight westward shift The peak structure may be associated with the thermospheric equatorward wind, composition change, and subauroral polarization stream