Relating the Phases of Magnetic Reconnection Growth to Energy Transport Mechanisms in the Exhaust

AbstractThe efficiency of energy conversion during magnetic reconnection is related to the reconnection rate. While the stable reconnection rate has been studied extensively, its growth between the time of reconnection onset and its peak has not been thoroughly discussed. We use a 2D particle‐in‐cell simulation to examine how the reconnection rate evolves during the growth process and how it relates to changes near the x‐line. We identify three phases of growth: (a) slow quasi‐linear growth, (b) rapid exponential growth, and (c) tapered growth followed by negative growth after the reconnection rate peaks. We associate phase 1 with the breaking of x‐line uniformity by a localized density depletion that changes the in‐plane electric field structure near the neutral line, followed by the expansion of the inflow region and the enhancement of inflow Poynting flux Sz associated with the out‐of‐plane electric field Ey in phase 2. We show how the Hall fields facilitate rapid growth in phase 2 by opening up the exhaust and relieving the electron‐scale bottleneck to allow rapid energy transport across the separatrices. We find that in phase 3, the inflow of electromagnetic energy accumulates until the downstream electromagnetic energy density saturates toward the initial upstream asymptotic value. Finally, we examine how the electron outflow and the downstream ion populations interact in phase 3 and how each species exchanges energy with the local field structures in the exhaust.