The Effect of a Guide Field on Local Energy Conversion During Asymmetric Magnetic Reconnection: MMS Observations

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, varies with shear angle. We analyze 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 largest 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 X point was carried by electrons with a velocity distribution function (VDF) resembling the magnetosheath inflow, shifted in the −v∥ direction. At the magnetosphere side EDR, the current was carried by electrons with a crescent‐like VDF. We compare this 8 December event to 10 other EDRs with different guide field strengths. The dual‐region was observed in three other moderate‐shear EDR events, whereas three high‐shear events had a strong positive near the electron crescent point and one low‐shear event had a strong positive only near the BL=0 point. The dual‐region was seen for one of three “intermediate”‐shear EDRs with guide fields of ∼0.2–0.3. 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 electron VDFs, limiting the current carried near the stagnation point.