[P-EM15] Dynamics of Magnetosphere and Ionosphere

[PEM15-04] Magnetopause reconnection under sub-Alfvenic solar wind with northward magnetic field

*Hiroshi Hasegawa¹, Richard E. Denton², Li-Jen Chen³, Qiang Hu⁴, Masaki N Nishino¹, Kyoung-Joo Hwang⁵ (1.Institute of Space and Astronautical Science, JAXA, 2.Dartmouth College, 3.NASA Goddard Space Flight Center, 4.University of Alabama Huntsville, 5.Southwest Research Institute)

Keywords:magnetic reconnection, solar wind, magnetopause, Earth's magnetosphere

Solar wind is a supersonic plasma flow emanating from the Sun, but on rare occasions the solar wind at 1 AU can be slower than the local Alfvén speed. Such a sub-Alfvénic solar wind was observed on 24 April 2023 when a magnetic cloud, a substructure of coronal mass ejections, impinged on Earth’s magnetosphere. We present observations on this day by the Magnetospheric Multiscale (MMS) spacecraft around the dayside magnetopause during an interval of sub-Alfvénic solar wind with northward interplanetary magnetic field, the conditions for which the solar wind-magnetosphere interaction is poorly understood. Our analysis of MMS data shows that magnetic reconnection at the observed magnetopause was more efficient than under typical high Alfvén Mach number solar wind conditions. This more efficient reconnection can cause complex interaction of reconnected magnetic field lines, possibly leading to more vigorous transport of solar wind plasmas into the magnetosphere, and can energize ions and electrons more efficiently. The results could provide significant insights into coupling between the solar wind and other planets, such as at Mercury, or between planetary magnetospheric plasmas and moons under low Alfvén Mach number conditions.