Storm-Time Plasma Density Peaks at Middle Latitudes: Observations, Statistics, and Mechanisms During Recovery Phases

We investigate storm-time ionospheric plasma density peak structures at middle latitudes using multi-instrument observations and a statistical analysis. Case studies of the April 2023 geomagnetic storm and the May 2024 superstorm reveal distinct types of plasma enhancements over the Asian–Australian sector during the recovery phase. The April 2023 event exhibits a narrow mid-latitude peak with clear equatorward motion and slight westward drift, characterized by strong O⁺ dominance. Its formation is closely associated with equatorward thermospheric winds and F-layer uplift, while the sharp boundaries of the structure are linked to storm-time O/N₂ depletion and subauroral polarization stream (SAPS) flows.

In contrast, the May 2024 superstorm produces plasma density peaks along ±40° MLAT accompanied by low-latitude enhancements. These structures display pronounced westward evolution and are primarily formed through the stretching and transport of high-density plasma remnants from the storm-enhanced density (SED) base region within a SAPS channel. Plasma composition and ion drift observations confirm their ionospheric origin. The subsequent reshaping of the overall structure and the asymmetric behavior of low-latitude enhancements highlight the role of polarization electric fields associated with equatorial plasma bubbles.

To assess the generality of these phenomena, we perform a statistical analysis of 92 geomagnetic storms (Dst < −50 nT) from 2020 to 2024. Fifty-five events exhibit similar mid-latitude peak structures over East Asia, with most occurring during the recovery phase. These results demonstrate that storm-time mid-latitude plasma density peaks are common but can arise from different physical pathways. We propose that their formation and evolution are governed by varying combinations of thermospheric winds, composition changes, SAPS-driven transport, and electrodynamic processes during geomagnetic storm recovery.