The effect of a guide field on local energy conversion during asymmetric magnetic reconnection: MMS observations
arXiv:1706.08404 · doi:10.1002/2017JA024247
Abstract
We compare case studies of Magnetospheric Multiscale (MMS)-observed magnetopause electron diffusion regions (EDRs) to determine how the rate of work done by the electric field, , and electron dynamics vary with magnetic shear angle. We provide an in-depth analysis of an MMS-observed EDR event with a guide field approximately the same size as the magnetosheath reconnecting field, which occurred on 8 December 2015. We find that was large and positive near the magnetic field reversal point, though patchy lower-amplitude also occurred on the magnetosphere-side EDR near the electron crescent point. The current associated with the large near the null was carried by electrons with a velocity distribution function (VDF) resembling that of the magnetosheath inflow, but accelerated in the anti-parallel direction by the parallel electric field. At the magnetosphere-side EDR, the current was carried by electrons with a crescent-like VDF. We compare this 8 December event to four others with differing magnetic shear angles. This type of dual-region was observed in another intermediate-shear EDR event, whereas the high-shear events had a strong positive near the electron crescent point and the low-shear event had a strong positive near the in-plane null. We propose a physical relationship between the shear angle and mode of energy conversion where (a) a guide field provides an efficient mechanism for carrying a current at the field reversal point (streaming) and (b) a guide field may limit the formation of crescent eVDFs, limiting the current carried near the stagnation point.