The Effect of a Guide Field on Local Energy Conversion During Asymmetric Magnetic Reconnection: Particle‐in‐Cell Simulations

We use theory and simulations to study how the out‐of‐plane (guide) magnetic field strength modifies the location where the energy conversion rate between the electric field and the plasma is appreciable during asymmetric magnetic reconnection, motivated by observations (Genestreti et al., 2017). For weak guide fields, energy conversion is maximum on the magnetospheric side of the X line, midway between the X line and electron stagnation point. As the guide field increases, the electron stagnation point gets closer to the X line, and energy conversion occurs closer to the electron stagnation point. We motivate one possible nonrigorous approach to extend the theory of the stagnation point location to include a guide field. The predictions are compared to two‐dimensional particle‐in‐cell (PIC) simulations with vastly different guide fields. The simulations have upstream parameters corresponding to three events observed with Magnetospheric Multiscale (MMS). The predictions agree reasonably well with the simulation results, capturing trends with the guide field. The theory correctly predicts that the X line and stagnation points approach each other as the guide field increases. The results are compared to MMS observations, Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) observations of each event, and a global resistive‐magnetohydrodynamics simulation of the 16 October 2015 event. The PIC simulation results agree well with the global observations and simulation but differ in the strong electric fields and energy conversion rates found in MMS observations. The observational, theoretical, and numerical results suggest that the strong electric fields observed by MMS do not represent a steady global reconnection rate. Key Points We motivate a theory of the guide field dependence of the location within the diffusion region of X line, stagnation points, and nonzero J · E′ Two‐dimensional PIC simulations of three MMS events confirm location of energy conversion moves toward electron stagnation point with increasing guide field Reconnection rate in 2‐D PIC simulations agrees well with AMPERE observations and global simulations but are far lower than MMS observations