Electron-Scale Quadrants of the Hall Magnetic Field Observed by the Magnetospheric Multiscale spacecraft during Asymmetric Reconnection

An in situ measurement at the magnetopause shows that the quadrupole pattern of the Hall magnetic field, which is commonly observed in a symmetric reconnection, is still evident in an asymmetric component reconnection, but the two quadrants adjacent to the magnetosphere are strongly compressed into the electron scale and the widths of the remaining two quadrants are still ion scale. The bipolar Hall electric field pattern generally created in a symmetric reconnection is replaced by a unipolar electric field within the electron-scale quadrants. Furthermore, it is concluded that the spacecraft directly passed through the inner electron diffusion region based on the violation of the electron frozen-in condition, the energy dissipation, and the slippage between the electron flow and the magnetic field. Within the inner electron diffusion region, magnetic energy was released and accumulated simultaneously, and it was accumulated in the perpendicular directions while dissipated in the parallel direction. The localized thinning of the current sheet accounts for the energy accumulation in a reconnection.

Figure 1

(a)–(d) ${\mathrm{B}}_L$, ${\mathrm{B}}_M$, ${\mathrm{B}}_N$, and $|\mathbf{B}|$ at the four satellites, (e) ion flows $v_{iL}$, $v_{iM}$, and $v_{iN}$, and (f) electron flows $v_{eL}$, $v_{eM}$, $v_{eN}$, and $v_{iL}$ at mms1. The electron (g) temperature, (h) density, and (i) energy spectrum at mms1.

Figure 2

(a) A schematic illustration for the reconnection. The shadow region denotes the electron current layer, and the pink arrows mean the electron flows. The blue signs of “$+$” and “$-$” mean the positive and negative ${\mathrm{B}}_M$. (b) Relative position of the four satellites at 06:05:21 UT.

Figure 3

(a)–(c) $v_{eL}$, $v_{eM}$, and $v_{eN}$ at all four satellites, (d)–(g) ${\mathrm{B}}_L$, ${\mathrm{B}}_M$, ${\mathrm{B}}_N$, and $|\mathbf{B}|$, and (h) electron pitch angle distribution during 200–2000 eV at mms1. Panels (i)–(p) and (q)–(x) show the data in Jet_2 and Jet_1, respectively.

Figure 4

(a) $v_{eL}$, $v_{eM}$, and $v_{eN}$ at mms1, (b) the total (thick curve) and electron current (thin curve) density, (c) $j_L$, $j_M$, and $j_N$, (d) $j_{\parallel }$ (blue curve) and $|j_{\perp }|$ (yellow curve), (e) the electric field ($E_N^{\prime }$, blue curve; $E_M^{\prime }$, red curve; $E_L^{\prime }$, green curve) in the $X$-line frame ${\mathbf{E}}^{\prime }=\mathbf{E}+{\mathbf{V}}_{\text{xline}}\times \mathbf{B}$, (f) the drift velocity (${\mathbf{E}}^{\prime }\times \mathbf{B}/|\mathbf{B}{|}^2$) and ${\mathbf{V}}_{\perp }=[\mathbf{B}\times (\mathbf{E}\times \mathbf{B})/|\mathbf{B}{|}^2]$, (g) $\mathbf{J}·(\mathbf{E}+{\mathbf{V}}_e\times \mathbf{B})$ (black curve), ${\mathbf{J}}_{\parallel }·{\mathbf{E}}_{\parallel }$ (pale blue curve), ${\mathbf{J}}_{\perp }·{(\mathbf{E}+{\mathbf{V}}_e\times \mathbf{B})}_{\perp }$ (yellow curve), the shadow means the errors of $\mathbf{J}·(\mathbf{E}+{\mathbf{V}}_e\times \mathbf{B})$, and (h)$E_x$ (black curve) and $-{(v_e\times B)}_x$ (blue curve). Panels (i)–(p) and (q)–(x) show the data in Jet_2 and Jet_1, respectively.