Localized Magnetopause Erosion at Geosynchronous Orbit by Reconnection.

This study presents observations of magnetopause reconnection and erosion at geosynchronous orbit, utilizing in situ satellite measurements and remote sensing ground‐based instruments. During the main phase of a geomagnetic storm, Geostationary Operational Environmental Satellites (GOES) 15 was on the dawnside of the dayside magnetopause (10.6 MLT) and observed significant magnetopause erosion, while GOES 13, observing duskside (14.6 MLT), remained within the magnetosphere. Combined observations from the THEMIS satellites and Super Dual Auroral Radar Network radars verified that magnetopause erosion was primarily caused by reconnection. While various factors may contribute to asymmetric erosion, the observations suggest that the weak reconnection rate on the duskside can play a role in the formation of asymmetric magnetopause shape. This discrepancy in reconnection rate is associated with the presence of cold dense plasma on the duskside of the magnetosphere, which limits the reconnection rate by mass loading, resulting in more efficient magnetopause erosion on the dawnside. Plain Language Summary: The boundary of Earth's magnetosphere, knows as the magnetopause, moves back and forth in response to the upstream solar wind conditions, mainly solar wind dynamic pressure and the north‐south component of the interplanetary magnetic field (IMF). It is well known that the magnetopause moves closer to the Earth during periods of southward IMF compared to northward IMF. This phenomenon occurs because reconnection between the IMF and Earth's magnetic field removes magnetic flux at the boundary layer, resulting in magnetopause erosion. Occasionally, the magnetopause approaches geosynchronous orbit when the magnetosphere is highly compressed and/or eroded, mainly during geomagnetic storms. Statistical studies have shown the magnetopause erosion often shows a dawn‐dusk asymmetry at geosynchronous orbit. This study presents concurrent observations of magnetopause erosion and related dayside reconnection at geosynchronous orbit by utilizing data from THEMIS, Geostationary Operational Environmental Satellites, and Super Dual Auroral Radar Network radar. The observations suggest that the presence of cold dense plasma in the duskside magnetosphere weakens reconnection rates and, consequently, it contributed to the dawn‐dusk asymmetry of magnetopause erosion. Key Points: We present observations of magnetopause erosion at geosynchronous orbit caused by reconnectionMagnetopause erosion showed dawn‐dusk asymmetryAsymmetric erosion can plausibly be attributed to a reduced reconnection rate at the duskside magnetopause due to cold dense plasma [ABSTRACT FROM AUTHOR]