Structure of a Perturbed Magnetic Reconnection Electron Diffusion Region in the Earth’s Magnetotail

We report in situ observations of an electron diffusion region (EDR) and adjacent separatrix region in the Earth’s magnetotail. We observe significant magnetic field oscillations near the lower hybrid frequency which propagate perpendicularly to the reconnection plane. We also find that the strong electron-scale gradients close to the EDR exhibit significant oscillations at a similar frequency. Such oscillations are not expected for a crossing of a steady 2D EDR, and can be explained by a complex motion of the reconnection plane induced by current sheet kinking propagating in the out-of-reconnection-plane direction. Thus, all three spatial dimensions have to be taken into account to explain the observed perturbed EDR crossing. These results shed light on the interplay between magnetic reconnection and current sheet drift instabilities in electron-scale current sheets and highlight the need for adopting a 3D description of the EDR, going beyond the two-dimensional and steady-state conception of reconnection.

Figure 1

Top: Four spacecraft (a) $B_L$, (b) $B_M$, and (c) $B_N$ measured by FGM [38]; (d) $v_{e,M}$ and (e) $v_{e,L}$ from FPI [39]; (f) $E_N$ from EDP [40, 41]; (g) $T_{e,\Vert }/T_{e,\perp }$; (h) $|\delta \mathbf{B}|$ computed high-pass filtering FGM data with $f>0.5\mathrm{Hz}$ (the high-pass filter allows us to separate the scale associated with the fluctuations from the scale associated with the magnetic field variations due to the current sheet crossing). The green shaded interval indicates the inner EDR. Bottom: (i) 2D PIC simulation data of $T_{e,\Vert }/T_{e,\perp }$ with the reconstructed MMS trajectory crossing the EDR. The magnetic flux contour lines are superposed; (j) spacecraft position relative to MMS1 in the $LN$ plane.

Figure 2

Four spacecraft MMS observations: (a) $B_L$; (b) $v_{e,L}$; (c) $E_N$; (d) $\mathrm{\Delta }{E}_N=E_{N,\mathrm{MMS}2}-E_{N,\mathrm{MMS}1}$ and $\mathrm{\Delta }{v}_{e,L}=v_{e,L,\mathrm{MMS}2}-v_{e,L,\mathrm{MMS}1}$. PIC simulation: contour plot of (e) $v_{e,L}$ and (f) $E_N$. The black (blue) line represents MMS1 (MMS4) trajectory.

Figure 3

Top: (a) three components of $\delta \mathbf{B}$. Offsets of 2.3 nT and 4.3 nT are added to $\delta B_M$ and $\delta B_N$, respectively; (b) $\mathbf{J}$ calculated from particle moments. Spectrum of (c) $\mathbf{B}$ wave power; (d) $\mathbf{E}$ wave power; (e) ${\log }_{10}[(E/B)(1/v_{\mathrm{ph}})]$. (f) ${\beta }_e$. The black line indicates $f_{\mathrm{LH}}$. Bottom: normalized power of magnetic field fluctuations $\delta B_N$ versus (g) $|k|{\rho }_e$ and frequency; (h) $k_L{\rho }_e$ and $k_M{\rho }_e$ ($0.5\mathrm{Hz}<f<2.5\mathrm{Hz}$); (i) $k_M{\rho }_e$ and $k_N{\rho }_e$ ($0.5\mathrm{Hz}<f<2.5\mathrm{Hz}$);. The dashed line in (g) corresponds to $f=1.4\mathrm{Hz}$.

Figure 4

Schematic representation of the kinking of the electron scale current sheet propagating in the out-of-reconnection-plane direction (not to scale).