Recent Observational Advances in our Understanding of Magnetic Reconnection in Earth's Magnetosphere

Abstract:

The availability of multi-point observations through the ion diffusion region has provided a wealth of information on the workings of magnetic reconnection at the dayside magnetopause and at the nightside plasma sheet. Ion inertia and finite gyroradius result in clear Hall effects on ion inertial length scales that have been observed by single and multi-point spacecraft resulting in a fairly detailed understanding of that physics. The role of Hall dynamics on establishing the reconnection rate, and the outflow conditions is well understood. Electron violation of the frozen-in condition is much more subtle and rarely observed at the X-point; indirect evidence has been more frequently observed in parallel electric fields. The waves responsible for this violation and for resultant energy conversion are at the forefront of observational studies. The contraction of islands centered at multiple O-points could provide for rapid energy conversion. However, observations of in vivo FTEs at the dayside and classical plasmoids at the nightside, are rare even near the expected most probable location of reconnection. Rather, asymmetric proto-plasmoids and dipolarization (a.k.a reconnection) fronts at the nightside signify distinctly different evolution of the reconnection geometry from that of X and O points. Operating within meso-scale (1-3Re) flows the inherently kinetic (at electron inertial scales embeded in ion inertial scale current systems) processes at such fronts play a significant role in the dynamics and drive global magnetospheric energy conversion and flux transport. Recent studies suggest that the entire magnetosphere is, in fact, connected front-to-back by reconnection pulses resulting in meso-scale flows. The upcoming Heliophysics System Observatory, motivated by the launch of MMS and the coordinated observations of THEMIS, ARTEMIS, Van Allen probes and other space and ground assets will answer key questions on this subject locally, regionally and globally.