In situ spacecraft observations of a structured electron diffusion region during magnetopause reconnection

The electron diffusion region (EDR) is the region where magnetic reconnection is initiated and electrons are energized. Because of experimental difficulties, the structure of the EDR is still poorly understood. A key question is whether the EDR has a homogeneous or patchy structure. Here we report Magnetospheric Multiscale (MMS) spacecraft observations providing evidence of inhomogeneous current densities and energy conversion over a few electron inertial lengths within an EDR at the terrestrial magnetopause, suggesting that the EDR can be rather structured. These inhomogenenities are revealed through multipoint measurements because the spacecraft separation is comparable to a few electron inertial lengths, allowing the entire MMS tetrahedron to be within the EDR most of the time. These observations are consistent with recent high-resolution and low-noise kinetic simulations.

Figure 1

(a) Magnetic field components as measured by WIND and propagated to the magnetopause; (b) MMS1 magnetic field components; (c) MMS1 ion density and (d) MMS1 ion velocity components; (e) Zoom-in of the MMS1 magnetic field components and strength; (f) Zoom-in of the electron velocity components. Data are shown in GSE. The yellow shaded region in panels (a)–(d) indicates the EDR crossing.

Figure 2

Four spacecraft measurements of (a) $B_L$; (b) $B_M$; (c) $B_N$; (d) $J_L$; (e) $J_M$. (f) Curvature radius of the magnetic field lines; (g) illustration of the encounter. The red line represent the trajectory of the barycenter of MMS constellation. Since the velocity of the magnetopause is much larger than the spacecraft velocities, the MMS path shown is produced entirely by the motion of the magnetopause in the LN plane. The three tetrahedra represent MMS location at different times along the trajectory; (h) projection of the MMS tetrahedron in the LN and in the MN plane. The tetrahedron quality factor is 0.84.

Figure 3

(a) Magnetic field components and strength; (b) electron velocity components; (c) current density components; (d) M component of electric field $(30\text{ms}$ resolution), ${({\mathbf{v}}_e\times \mathbf{B})}_M(30\text{ms}$ resolution), ${({\mathbf{v}}_i\times \mathbf{B})}_M(150\text{ms}$ resolution); (e) agyrotropy parameter $\sqrt{Q}$; (f) parallel and perpendicular electron temperature; (g) electron pitch angle distribution in the energy range $[20,200]\text{eV}$. The black vertical dashed-line indicates the time of the $\left|\mathbf{B}\right|$ minimum. Data from MMS1.

Figure 4

Four spacecraft (a) $B_L$; (b) time-shifted $B_L$. (c) Time-shifted $J_M$; (d) time-shifted $J_N$; (e) time-shifted $E_M$ (8192 samples/s); (f) time-shifted $E_N$ (8192 samples/s); (g) time-shifted $E_M^{\prime }{J}_M$; (h) time-shifted $E_N^{\prime }{J}_N$; (i) time-shifted ${\mathbf{E}}^{\prime }·\mathbf{J}$; (j) ${\mathbf{E}}_{\text{FPI}}^{\prime }·\mathbf{J},E_{\text{FPI},M}^{\prime }{J}_M,E_{\text{FPI},N}^{\prime }{J}_N$. The $\alpha ,\beta$, and $\gamma$ lines correspond to the times of the $\alpha ,\beta$, and $\gamma$ distribution functions in panels (l)–(t) shifted accordingly to the timing method. (k) Illustration of $J_M$ and of the energy conversion and $v_{e,N}$ flow. (l–t) Electron distributions by MMS4 projected on $({\mathbf{v}}_{\perp ,1},{\mathbf{v}}_{\perp ,2}),({\mathbf{v}}_{\left|\right|},{\mathbf{v}}_{\perp ,2})$ and $({\mathbf{v}}_{\left|\right|},{\mathbf{v}}_{\perp ,1})$ planes at three different times $t_{\alpha }=12$:05:43.269, $t_{\beta }=12$:05:43.299, $t_{\gamma }=12$:05:43.389.