Magnetopause Boundary Layers in Low Mach CME‐Driven Storm Environment

Abstract Magnetopause boundary layers (BLs) play an important role mediating plasma and energy exchange between the solar wind/magnetosheath and Earth's magnetosphere. Energy exchange across the magnetopause is enhanced during storms, yet little work has been done investigating BLs during storms. In this letter, using MMS and THEMIS observations, we investigate the structure and formation of magnetopause BLs during a large coronal mass ejection (CME) driven storm containing 20 hours of low Alfvén Mach number solar wind ( M A ). Separated by Earth radii (), MMS and THEMIS observe a low latitude boundary layer (LLBL), magnetosheath boundary layer (MSBL), and formation of a plasma depletion layer after a northward interplanetary magnetic field turning. MMS observations indicate lobe reconnection drives the MSBL and LLBL formation. Observations of CME ions on closed field lines demonstrate dual reconnection can trap solar wind plasma under sub‐Alfvenic solar wind conditions and constrain the trapped populations' dwell time.