Bipolar Reversal in the Off‐Diagonal Ion Pressure Term in Collisionless Magnetic Reconnection

Abstract Magnetic reconnection is a fundamental process that converts magnetic energy into particle energy, with wide applications in space plasmas. A key manifestation of this energy conversion is the acceleration of fast ion outflows. However, ion processes and their associated signatures in energy conversion remains only partially understood in collisionless magnetic reconnection. In this study, we utilize statistical analyses of in‐situ observations and particle‐in‐cell (PIC) 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 originates from distorted ion velocity distributions during acceleration. These signals are confirmed by statistical in‐situ evidence for the first time and captured by PIC simulations. PIC simulations further indicate the peak of the off‐diagonal ion pressure is near the magnetic pileup region associated with the ion outflow. Trajectories of ions in the distorted velocity distributions are traced within the PIC simulations. Ions in the distorted distributions undergo partial cyclotron motion around the reconnected magnetic field ( B z ) and acceleration by the reconnection electric fields. The observation of bipolar reversal in the off‐diagonal ion pressure term indicate its spatial gradient, which could contribute to supporting ion‐scale reconnection electric fields. These findings provide new insights into the fundamental features of energy conversion in collisionless magnetic reconnection.