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

Abstract

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. DOI: 10.1103/PhysRevLett.118.175101 Magnetic reconnection, as an effective mechanism for converting magnetic energy into plasma kinetic and thermal energy, has been believed to be responsible for a large number of explosive phenomena in space, astrophysics, and laboratory plasmas [1,2]. Because of the Hall effect, ions are unmagnetized and electrons are magnetized in the ion diffusion region [3,4], and the relative motion between ions and electrons results in a quadrupole pattern of the outof-plane magnetic field and a bipolar pattern of the Hall electric field pointing to the center at both sides of the reconnecting current sheet [5][6][7][8][9][10][11][12][13][14]. The unique signature of the Hall quadrupolar structure has now been regarded as one critical criterion to identify the ion diffusion region [9,10,[15][16][17][18]. The quadrupolar structure is symmetric in an antiparallel reconnection (i.e., the shear angle α ¼ 180°) and can be distorted by a guide field (α < 180°) [19,20]. It is well known that the width of each quadrant is of the ion scale [3][4][5][6]15]. If the asymmetric distribution of the reconnecting field and plasma was considered, the quadrupole pattern was replaced by a bipolar pattern, as predicted in simulations [21,22]. However, a recent measurement shows that the quadrupole pattern might still exist in an asymmetric reconnection [23].In this Letter, we present an asymmetric magnetic reconnection event encountered at the magnetopause by the Magnetospheric Multiscale (MMS) spacecraft, which was designed to explore electron physics in a reconnection [24]. Since the MMS crossed the ion diffusion region both north and south of the X line, all four Hall quadrants can be measured very well. Such a situation should be frequently found from the MMS measurement, based on the preliminary analysis.The data from several instruments are used. The mission and its instruments are described in several pape...

Interaction of Magnetic Flux Ropes Via Magnetic Reconnection Observed at the Magnetopause

“…The Hall electric field pointing to the south was observed and the counterpart directed to the north was not found. The observed asymmetric distribution of the Hall magnetic field and the unipolar Hall electric field here are consistent with the previous observations at the magnetopause (Wang et al, ) and the prediction of numerical simulations (Hesse et al, ; Huang et al, ; Pritchett & Mozer, ; Shay et al, ). Furthermore, the expected reconnection electron outflows, comparable to the local Alfven speed, were bounded by the inflowing electrons, and the parallel electric field was detected in the separatrix region.…”

“…The energy dissipation quantity J ⋅ ( E + V e × B ) in the electron frame (Zenitani et al, ) is displayed in Figure h. The values of J ⋅ ( E + V e × B ) were very small (~0.2 nW/m 3 ), in comparison with the observation near the electron diffusion region (~10 nW/m 3 ) (Burch, Torbert, et al, ; Wang et al, ). It means that the mms spacecraft did not observe the inner electron diffusion region of the coalescence.…”

An Electron‐Scale Current Sheet Without Bursty Reconnection Signatures Observed in the Near‐Earth Tail

“…In the ECS, the electric field was as strong as 20 mV/m, mostly in the perpendicular component. Inside the ECS, the energy dissipation was insignificant on average (Figure 3f), in comparison with those near the electron diffusion region Wang et al, 2017) and the coalescence point of two flux ropes (Wang et al, 2016). The most intense E // fluctuations were observed at~20:24:08 UT, corresponding to the outer boundary of the Hall magnetic field.…”

“…Commonly, it is attributed to the current sheets with a plateau in B L during reconnection (e.g., Karimabadi et al, 2005;Phan et al, 2006). The ECSs are often associated with the electron diffusion region and the separatrices during magnetic reconnection (Baumjohann et al, 2007;Burch et al, 2016;Ji et al, 2008;Phan et al, 2016;Wang et al, 2017). The ECSs are often associated with the electron diffusion region and the separatrices during magnetic reconnection (Baumjohann et al, 2007;Burch et al, 2016;Ji et al, 2008;Phan et al, 2016;Wang et al, 2017).…”

“…According to the MMS measurements at the magnetopause (Wang et al, 2017), the thickness of the quadrants of the Hall magnetic field can be compressed to be a few electron inertial lengths near the reconnection X-line. In this magnetotail event, the thicknesses of the detected two Hall quadrants were estimated.…”