Relating the Phases of Magnetic Reconnection Growth to the Temporal Evolution of X-line Structures in a Collisionless Plasma

The 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 the peak reconnection rate has not been thoroughly discussed. We use a 2D particle-in-cell (PIC) simulation to examine how the non-ideal reconnection electric field evolves during the growth process and how it relates to changes near the x-line. We identify three phases of growth: 1) slow quasi-linear growth, 2) rapid exponential growth, and 3) tapered growth followed by negative growth after the reconnection rate peaks. Through analysis of the structural changes of the EDR, we associate the early phases with the breaking of x-line symmetry through the erosion of the pre-onset bipolar Ez and the emergence of a diverging Ex pattern at the neutral line in phase 1 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, relieving the electron-scale bottleneck and allowing large Poynting flux across the separatrices. We find that the rapid inflow of electromagnetic energy accumulates until the downstream electromagnetic energy density in phase 3 approaches 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.