Off-diagonal Ion Pressure Term in Collisionless Magnetic Reconnection

Magnetic reconnection converts magnetic energy into particle energy, with ion outflows serving as a prominent manifestation, particularly in geospace. However, ion processes and signatures related to energy conversion remain incompletely understood in collisionless magnetic reconnection. In this study, we analyze in-situ data and simulations to identify a distinct signature in the off-diagonal component of the ion pressure tensor. This signature displays a bipolar reversal that correlates with ion outflows across the reconnection X-line. The bipolar signal reflects distorted velocity distributions during ion acceleration. The primary cause of this distortion is partial cyclotron motion around the reconnected magnetic field, combined with the acceleration by reconnection electric fields. Hall electric fields further enhance this cyclotron motion by inducing dawnward ion motion as ions enter the current layer from the inflow region. The bipolar reversal in the off-diagonal ion pressure term is a candidate for supporting ion-scale reconnection electric fields.