Structure of the Current Sheet in the 11 July 2017 Electron Diffusion Region Event

Abstract

The structure of the current sheet along the Magnetospheric Multiscale (MMS) orbit is examined during the 11 July 2017 Electron Diffusion Region (EDR) event. The location of MMS relative to the X‐line is deduced and used to obtain the spatial changes in the electron parameters. The electron velocity gradient values are used to estimate the reconnection electric field sustained by nongyrotropic pressure. It is shown that the observations are consistent with theoretical expectations for an inner EDR in 2‐D reconnection. That is, the magnetic field gradient scale, where the electric field due to electron nongyrotropic pressure dominates, is comparable to the gyroscale of the thermal electrons at the edge of the inner EDR. Our approximation of the MMS observations using a steady state, quasi‐2‐D, tailward retreating X‐line was valid only for about 1.4 s. This suggests that the inner EDR is localized; that is, electron outflow jet braking takes place within an ion inertia scale from the X‐line. The existence of multiple events or current sheet processes outside the EDR may play an important role in the geometry of reconnection in the near‐Earth magnetotail.

Reconstruction of the Electron Diffusion Region of Magnetotail Reconnection Seen by the MMS Spacecraft on 11 July 2017

“…The L and N components of the structure velocity are V 0 L = –231.7 km/s and V 0 N = –59.3 km/s, respectively, resulting in an angle θ between the x axis and the LM plane of 14.3°. The resultant speed of V 0 is roughly twice higher than that from the STD method and is substantially higher than an estimate (170 km/s) from four‐spacecraft timing method (Torbert et al, ) but agrees well with an estimate assuming 1‐D or 2‐D structure of the current sheet (Nakamura, Genestreti, Nakamura, et al, ).…”

“…Following the procedure described in sections and , we found the optimal structure velocity to be V 0 = (−255.3, −142.8, 51.7) km/s and the optimal LMN axes to be e L = (0.9605, −0.1735, −0.2177), e M = (0.0985, 0.9432, −0.3272), and e N = (0.2604, 0.2832, 0.9230) in GSE. The N direction remains the same as before, and the M axis is only 5° away from, and is within the error of, those estimated by Genestreti et al () and used by Nakamura, Genestreti, Nakamura, et al (). The L and N components of the structure velocity are V 0 L = –231.7 km/s and V 0 N = –59.3 km/s, respectively, resulting in an angle θ between the x axis and the LM plane of 14.3°.…”

“…It indicates that v z cannot completely be expressed as a function of A only. This suggests that electron inertia effects, neglected in the present reconstruction, are not fully negligible, as also revealed by Nakamura, Genestreti, Nakamura, et al () in a different way, and/or that the current density profile is different between the tailward and earthward sides of the X point.…”

“…Here the coordinate axes x ′ and y ′ lie in the xy plane, and the x ′ axis is parallel to the local tangent to the current sheet in the xy plane. We note that the nongyrotropic electron pressure term, as expressed by , has been shown to contribute dominantly to the reconnection electric field in the present EDR event (Nakamura, Genestreti, Nakamura, et al, ).…”

Electron Vorticity Indicative of the Electron Diffusion Region of Magnetic Reconnection

“…The latter are likely to be electrons doing the Speiser motion across the current sheet. Nakamura et al (2019) confirmed that the observed distributions are in good agreements with the theory presented by Bessho et al (2014). Nakamura et al (2019) confirmed that the observed distributions are in good agreements with the theory presented by Bessho et al (2014).…”