The Role of the Parallel Electric Field in Electron‐Scale Dissipation at Reconnecting Currents in the Magnetosheath

Abstract

We report observations from the Magnetospheric Multiscale satellites of reconnecting current sheets in the magnetosheath over a range of out‐of‐plane “guide” magnetic field strengths. The currents exhibit nonideal energy conversion in the electron frame of reference, and the events are within the ion diffusion region within close proximity (a few electron skin depths) to the electron diffusion region. The study focuses on energy conversion on the electron scale only. At low guide field (antiparallel reconnection), electric fields and currents perpendicular to the magnetic field dominate the energy conversion. Additionally, electron distributions exhibit significant nongyrotropy. As the guide field increases, the electric field parallel to the background magnetic field becomes increasingly strong, and the electron nongyrotropy becomes less apparent. We find that even with a guide field less than half the reconnecting field, the parallel electric field and currents dominate the dissipation. This suggests that parallel electric fields are more important to energy conversion in reconnection than previously thought and that at high guide field, the physics governing magnetic reconnection are significantly different from antiparallel reconnection.

Observational Evidence of Magnetic Reconnection in the Terrestrial Bow Shock Transition Region

“…The local energy conversion through the nonideal electric field J · E ′ = J · (E + V e × B) (Figure 2e) is enhanced mainly in the perpendicular component near the midplane (location 1), a feature consistent with laboratory (Yamada et al, 2016), space (Wilder et al, 2018), and simulation (Yamada et al, 2016) results of energy conversion in the diffusion region of reconnection with symmetric upstream conditions and negligible guide field. The enhancement of J · E ′ occurs in the parallel component near the separatrix in the Hall structure (location 2).…”

“…Reconnection is also demonstrated in particle‐in‐cell simulations relevant to astrophysical (high M A ~ 40) quasi‐perpendicular shocks ( θ Bn > 45°), where current sheets are generated through the Weibel instability, and the magnetic islands generated by reconnection accelerate electrons to suprathermal energies (e.g., Matsumoto et al, ). Evidence of reconnection has been reported to occur in the magnetosheath downstream of the Earth's bow shock (Eriksson et al, ; Phan et al, ; Retinò et al, ; Vörös et al, ; Wilder et al, , ). Features of current sheet structures consistent with magnetic reconnection were identified in the transition region of a quasi‐parallel shock in a recent study (Gingell et al, ).…”

Observations of a Kinetic‐Scale Magnetic Hole in a Reconnection Diffusion Region

“…This large parallel electric field led to a strong magnetic energy dissipation J · E ′ ~J || E || ~17 nW/m 3 (Figure i). It has been shown that parallel electric field contributed dominantly to the energy dissipation around the electron diffusion region (EDR) with guide field (Wilder et al, ; Zhou et al, , ). The intense parallel electric field can accelerate electrons efficiently, which resulted in a beam‐like electron population in the antiparallel direction with speed around 7,000 km/s (Figure k).…”

Energy Conversion at Kinetic Scales in the Turbulent Magnetosheath

“…We can speculate that in turbulent space plasmas the chaotic motions typically generate current sheets with significant guide fields. Both numerical simulations (Shay et al, 2014) and data analysis (Phan et al, 2013;Wilder et al, 2018) show that a guide field suppresses electron perpendicular heating and supports parallel heating. This could explain our observations of preferred parallel heating and energy conversion at magnetosheath current sheets.…”