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, J→·(E→+v→e×B→)≡J→·E→′ 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 J→·E→′ was largest and positive near the magnetic field reversal point, though patchy lower-amplitude J→·E→′ also occurred on the magnetosphere-side EDR near the electron-crescent point. The current associated with the large J→·E→′ 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 J→·E→′ was observed in three other moderate-shear EDR events, whereas three high-shear events had a strong positive J→·E→′ near the electron-crescent point and one low-shear event had a strong positive J→·E→′ only near the BL=0 point. The dual-region J→·E→′>0 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 eVDFs, limiting the current carried near the stagnation point.