"Smoking-Gun" Observables of Magnetic Reconnection: Spatiotemporal Evolution of Electron Characteristics Throughout the Diffusion Region

Abstract:

Electron distribution functions can provide "smoking-gun" evidence for the detection of electron diffusion regions in collisionless magnetic reconnection. Knowledge of the spatiotemporal evolution of electron distributions during reconnection is significantly lacking, and will further elucidate the outstanding questions of how, where, and when electrons are energized during reconnection. Based on spacecraft observations and PIC simulations of symmetric reconnection, electrons in the inflow region are known to exhibit a temperature anisotropy Te// > Te⊥. Studies of exhaust electrons have reported hot and isotropic electrons, while others have reported anisotropic exhaust structures. Electron distributions in the vicinity of the X-line have a triangular, 3D velocity space structure with distinct striations corresponding to the number of times electrons reflect within the electron current layer. Here, we report the spatial and temporal evolution of electron distributions from the vicinity of the X-line to the end of the electron outflow jet, with the discovery that the discrete striations swirl and rotate as electrons re-magnetize, forming arc and ring structures. Highly structured, time-dependent electron anisotropy develops in the exhaust distributions only near or after the peak reconnection rate, explaining the previous discrepancy concerning the degree of electron anisotropy in the exhaust, and suggesting a technique to infer the evolution stage of reconnection using spacecraft measurements. We also present a theory for predicting the spacing of the striations of electron distributions in the vicinity of the X-line based on local measurements, which could be directly tested by spacecraft observations. Electron data from Cluster magnetotail reconnection inflows and exhausts exhibit many anisotropic structures as predicted by simulation. Observed distributions near the reconnection mid-plane (Bx ~ 0 nT) are often highly structured with populations exhibiting Te⊥ > Te// in addition to lower energy field-aligned beams. Our work advances the understanding of electron distribution evolution, setting a foundation to successfully interpret the high resolution electron data anticipated from NASA’s upcoming Magnetospheric Multi-Scale Mission.