Energization of Separatrix Electrons by Plasma Double Layers During Asymmetric Magnetic Reconnection

Abstract Particle acceleration in magnetic reconnection is a critical area that demands investigation for a better understanding of energization processes in space plasma. NASA's Magnetospheric Multiscale Mission detected multiple separatrix crossings during the asymmetric magnetic reconnection at Earth's subsolar magnetopause. This unique observation revealed a highly field‐aligned beam of electrons and the coexistence of strong parallel electric fields greater than −23 mV/m along with bipolar electric field structures. For the first time, we report the observation of multiple plasma double layers (DLs) in asymmetric magnetic reconnection. Based on observations, we have identified that DLs are generated through the current‐driven Buneman instability along the reconnection separatrix layer, a finding well supported by existing theoretical and simulation studies. Such local electric fields can accelerate thermal electrons up to the keV range, which can undergo further energization downstream of the reconnection.