Structure and Dissipation Characteristics of an Electron Diffusion Region Observed by MMS During a Rapid, Normal‐Incidence Magnetopause Crossing

Abstract

On 22 October 2016, the Magnetospheric Multiscale (MMS) spacecraft encountered the electron diffusion region (EDR) when the magnetosheath field was southward, and there were signatures of fast reconnection, including flow jets, Hall fields, and large power dissipation. One rapid, normal‐incidence crossing, during which the EDR structure was almost stationary in the boundary frame, provided an opportunity to observe the spatial structure for the zero guide field case of magnetic reconnection. The reconnection electric field was determined unambiguously to be 2–3 mV/m. There were clear signals of fluctuating parallel electric fields, up to 6 mV/m on the magnetosphere side of the diffusion region, associated with a Hall‐like parallel current feature on the electron scale. The width of the main EDR structure was determined to be ~2 km (1.8 de). Although the MMS spacecraft were in their closest tetrahedral separation of ~8 km, the divergences and curls for these thin current structures could therefore not be computed in the usual manner. A method is developed to determine these quantities on a much smaller scale and applied to compute the normal component of terms in the generalized Ohm's law for the positions of each individual spacecraft (not a barocentric average). Although the gradient pressure term has a qualitative dependence that follows the observed variation of E + Ve × B, the quantitative magnitude of these terms differs by more than a factor of 2, which is shown to be greater than the respective errors. Thus, future research is required to find the manner in which Ohm's law is balanced.

A Survey of Plasma Waves Appearing Near Dayside Magnetopause Electron Diffusion Region Events

“…Near the minimum in |B|, there is an enhanced current out of the reconnection plane (J Y ) that reaches amplitudes in excess of 2 μA/m 2 (Figure 1i), which is consistent with other observations of EDR events at the dayside magnetopause Chen et al, 2016;Norgren et al, 2016;Torbert et al, 2017). spectra, (c) ion and electron number density, (d) ion and electron parallel and perpendicular temperature, (e) ion bulk velocity in GSE coordinates, (f) electron bulk velocity in GSE coordinates, (g) magnetic field vector in GSE coordinates, (h) magnetic field magnitude, (i) electric field power spectral density, (j) magnetic field power spectral density, (k) electric current density in GSE coordinates, (l) J · E′ decomposed into the contributions from parallel and perpendicular currents, (m) agyrotropy in electrons with energies between 50 and 250 eV.…”

“…Near the minimum in | B| , there is an enhanced current out of the reconnection plane ( J Y ) that reaches amplitudes in excess of 2 μA/m 2 (Figure i), which is consistent with other observations of EDR events at the dayside magnetopause (Burch, Torbert, et al, ; Chen et al, ; Norgren et al, ; Torbert et al, ). At this time, near the stagnation point where the ion bulk velocity Z ‐component is near the magnetosheath reference velocity and on the magnetosphere side of the minimum in |B|, there is an enhancement in J · E ′ in excess of 10 nW/m 3 .…”

“…To show examples of the types of waves that are most likely to occur near the EDR, we will use the EDR event on 22 October 2016 as a case study. This event has been reported in other studies (Torbert et al, ). Figure shows MMS data from this magnetopause crossing.…”

Structure of Electron‐Scale Plasma Mixing Along the Dayside Reconnection Separatrix

“…MMS1 and MMS2 are nearly aligned in the direction, suggesting that they see the same jet traveling along what is likely the magnetospheric separatrix. This feature is similar to the structured CSs of Phan et al (), Torbert et al (), and Wang et al () although in this case v e, L only reverses cleanly on MMS2. Subsequent velocity reversals on MMS2 appear more complex than a single current channel or boundary between inflow and outflow, although none are as clear as the jet at the time of A2.…”

Asymmetric Reconnection Within a Flux Rope‐Type Dipolarization Front

“…The electrons were then turned toward the M-direction by B L and formed crescent-shaped velocity distributions similar to those first observed by . Such events have been observed either with minimal guide field (e.g., Torbert et al, 2017) or with a moderate guide field (e.g., Genestreti et al, 2017). This change is apparent in Figures 3j-3l as the nonthermal, crescent-shaped distribution of electrons which accelerated and also became more sparse until MMS passed the stagnation point at about 14:51:31.4 UT.…”