A Bifurcated Reconnecting Current Sheet in the Turbulent Magnetosheath

Abstract

We report the Magnetospheric Multiscale (MMS) observation of a bifurcated reconnecting current sheet in Earth’s dayside magnetosheath. Typical signatures of the ion diffusion region, including sub-Alfvénic demagnetized ion outflow, super-Alfvénic electron flows, Hall magnetic fields, electron heating, and energy dissipation, were found when MMS traversed the current sheet. The weak ion exhaust at the current sheet center was bounded by two current peaks in which super-Alfvénic electron flow directed toward and away from the X line were observed, respectively. Both off-center current peaks were primarily carried by electrons, one of which was supported by field-aligned current, while the other was mainly supported by current driven by electric field drift. The two current peaks also exhibit other differences, including electron heating, electron pitch angle distributions, electron nongyrotropy, energy dissipation, and magnetic field curvature. An ion-scale magnetic flux rope was detected between the two current peaks where electrons showed field-aligned bidirectional distribution, in contrast to field-aligned distribution parallel to the magnetic field in two current peaks. The observed current sheet was embedded in a background shear flow. This shear flow worked together with the guide field and asymmetric field and density to affect the electron dynamics. Our results reveal the reconnection properties in this special plasma and field regime which may be common in turbulent environments.

1. Introduction

Magnetic reconnection is a basic plasma process throughout the universe. It occurs in a current sheet and rapidly converts stored magnetic energy into plasma kinetic energy and generates energetic particles [1,2]. In Earth’s magnetosphere, magnetic reconnection may occur in the magnetopause and magnetotail [3]. When the interplanetary magnetic field (IMF) has a southward component, magnetic reconnection occurs between oppositely oriented IMF and geomagnetic field at the low-latitude magnetopause [4,5,6,7,8]. This magnetopause reconnection drives plasma convection of open-field lines to the nightside and finally causes magnetic reconnection in the magnetotail [9,10]. The magnetopause reconnection may also occur for the northward IMF condition but at the high-latitude magnetopause [11,12]. Magnetic reconnection in the magnetotail occurs between oppositely directed lobe field lines [13,14,15,16]. Recent studies have revealed that magnetotail reconnection is triggered on electron scales and then develops into traditional reconnection [17,18,19] and even evolves into a turbulent state [20,21,22].

Another possible region for the occurrence of reconnection is the magnetosheath [23,24,25,26,27], which lies between the bow shock and the magnetopause and is full of shocked solar wind. The magnetosheath is highly turbulent, with fluctuating flows and magnetic fields [28,29,30]. The turbulence is stronger in the magnetosheath downstream of the quasi-parallel shock (the angle between the upstream IMF and the shock normal ${\theta }_{Bn}<45°$), where current sheets are likely to form [31]. These current sheets may originate from solar wind discontinuities [32,33] or upstream foreshock waves [34,35]. Some studies suggest that thin current sheets can be formed locally in the magnetosheath, for example, due to velocity shears [36,37]. Recent in situ observations have demonstrated that fast magnetosonic waves excited upstream of the bow shock can evolve into intermittent current sheets in the downstream magnetosheath, where magnetic reconnection is triggered [38]. Magnetic reconnection in these current sheets is important for the dissipation of turbulent energy at kinetic scales and the resulting plasma heating and acceleration [39,40,41].

Different from magnetopause and magnetotail current sheets, magnetosheath current sheets generally have small dimensions with typical scales of the order of ion inertial length [39]. The width of these current sheets can be down to several times the electron inertial length [23,42]. High-resolution spacecraft observations found that reconnection in magnetosheath turbulence is a class of electron-only reconnection without the coupling of ions [23,26,42]. This new type of reconnection has also been observed in the magnetopause and magnetotail [43,44,45,46,47] and is attributed to the limited dimensions of these current sheets [48,49]. Some studies found that electron-only reconnection could develop into ion reconnection and that this could be the early stage of traditional reconnection [50,51]. However, this transition may not apply to electron-only reconnection in the magnetosheath owing to the small dimensions of the current sheets in the turbulent environment [52]. Except for different spatial scales, current sheets therein have various regimes, such as guide fields and asymmetries, as a result of different nonlinearities in the magnetosheath [38]. So the study of reconnection properties in these current sheets can be informative, for example, in testing the effect of different guide fields and plasma conditions on reconnection.

Here, we report an ion-scale reconnecting current sheet observed by Magnetospheric Multiscale (MMS) [53] in the turbulent magnetosheath. The current sheet exhibits a double-peaked current structure with two sharp magnetic field variations at the edges and a plateau near the center. This type of current sheet has been extensively studied in the magnetotail and solar wind but has rarely been observed in the magnetosheath. With high-resolution MMS measurements, we examine the properties of the two current peaks and discuss the effects of background conditions on them, especially the large shear flow that is intrinsic to magnetosheath turbulent nature.

2. Database

The MMS mission has four satellites that form a regular tetrahedron configuration and carry identical instruments. This study uses both fast survey mode and burst mode data from the field and plasma instruments onboard MMS. Magnetic field data are taken from Fluxgate Magnetometer, and electric field data are from Electric Double Probes. The plasma moments data are from Fast Plasma Investigation with 30 ms electron cadence and 150 ms ion cadence in burst mode. High-time resolution plasma measurements make the MMS mission capable of investigating small-scale reconnection events in the magnetosheath.

3. Observations

3.1. Event Overview

Figure 1 presents an overview of the plasma and magnetic field observations in the magnetosheath between 00:10 and 00:40 UT on 30 November 2015 by the MMS2 satellite. The average interspacecraft separation was 18.5 km, ~0.8 ion inertial lengths calculated with the average plasma density in the magnetosheath. Since the data from the four satellites are almost identical over large scales, only the MMS2 measurements are shown in Figure 1. Ion and electron energy spectra (Figure 1a,b) together with fluctuating magnetic field (Figure 1d), plasma density (Figure 1e), and flow velocity (Figure 1f) suggest that MMS2 was located in a highly turbulent magnetosheath. The plasma $\beta$ (Figure 1c) also varied dramatically, always being larger than 1, which is typical for the magnetosheath. Three components of the magnetic field all changed sharply between −100 and 100 nT over the scales of the order of ion inertial length (~23 km), implying some large current density spikes where magnetic reconnection could occur. In fact, several reconnection events have been reported during this interval [25,27,36]. In this paper, we mainly focus on a bifurcated current sheet with two pronounced current density peaks (~3 µA/m², Figure 1g) detected at 00:31 UT. The $\beta$ value around this current sheet was approximately 4. We note that there were some regions with decreased plasma $\beta$ and increased plasma density and velocity, such as 00:17–00:20 UT, 00:22–00:26 UT, and 00:28–00:32 UT, which could correspond to magnetosheath high-speed jets [54,55]. The studied current sheet was located near the boundary of one such jet region. MMS2 satellite detected this current sheet at approximately (9.0, −2.9, −0.5) $R_E$ in the Geocentric Solar Ecliptic (GSE) coordinate system. The zenith angle of the observations relative to the center of Earth is 93°.

3.2. Evidence for Reconnection in a Bifurcated Magnetosheath Current Sheet

Figure 2 shows MMS2 observations of the current sheet at 00:31 UT, which exhibits typical signatures of magnetic reconnection. The data are examined in a local current sheet (LMN) coordinate system, where $L$ is along the reconnecting magnetic field direction, $N$ is aligned with the current sheet normal, and $M=N\times L$ is the direction out of the reconnection plane. Here, the $N$ direction is obtained based on the maximum gradient direction of the magnetic field with the minimum directional derivative (MDD) method [56]; $M=N\times L^{\prime }$, where $L^{\prime }$ is the maximum variance direction from the minimum variance analysis (MVA) [57], and finally $L=M\times N$.

The current sheet crossing is characterized by a reversal of the magnetic field component $B_L$ from negative to positive (Figure 2a). The MDD and spatial-temporal difference (STD) analyses [58] suggest that the current sheet is a quasi-1D structure moving with a speed of 60 km/s along the $-N$ direction, matching well with the results (57 km/s) of the four-spacecraft timing method performed on the $B_L$ profile. A striking feature is that $B_L$ varied sharply at the two edges of the current sheet and exhibited a plateau near zero between them. In other words, this current sheet had a bifurcated structure. The magnetic field component $B_M$ outside the current sheet, which is usually referred to as the guide field, changed its sign on the two sides. Disregarding the sign of the guide field, its magnitude was roughly comparable to the reconnecting field component $B_L$, indicating that the current sheet had a large guide field. The bifurcation of the current sheet can also be seen from its current density profile (Figure 2e), which is calculated with high-resolution ion and electron moments data according to the formula $J=N_{e}e(V_i-V_e)$. Here, $N_e$ is the electron number density, $e$ is the elementary charge, and $V_i$ and $V_e$ are the ion and electron velocities, respectively. The current density $J_M$ had one peak (3.5 µA/m²) as MMS2 entered the current sheet and the other peak (2.0 µA/m²) as it exited. The widths of the two $J_M$ peaks were 27 km (~1.3 $d_i$) and 16 km (~0.8 $d_i$), respectively, according to the speed of the current sheet. The edges of the current peaks are defined as where the current density strength begins to increase from the background value. Here, $d_i=c/{\omega }_{pi}=21 \mathrm{km}$ is the hybrid ion inertial length, where ${\omega }_{pi}={(4\pi n{e}^2/m_i)}^{1/2}$ based on the hybrid density $n=(n_1{B}_2+n_2{B}_1)/(B_1+B_2)$ in asymmetric magnetic reconnection [59]. In contrast, the current density was weak in between, which covered a width of 44 km (~2.1 $d_i$). The plasma density was overall larger on the negative $B_L$ side than on the positive $B_L$ side (Figure 2b). Notably, the plasma density decreased in the first $J_M$ peak whereas it increased in the second $J_M$ peak. The variation in plasma density in the current sheet showed an anticorrelation with the magnetic field strength. Overall, this current sheet was an asymmetric current sheet with a large guide field.

Reconnection outflow jets in the $L$ direction are usually used to identify whether reconnection is occurring in the current sheet. From the ion velocity shown in Figure 2c, the current sheet was embedded in a large-scale background shear flow in both the $L$ and $M$ directions. The shear flows $\Delta V_{iL}$ and $\Delta V_{iM}$ between the two sides of the current sheet were about 140 km/s and 100 km/s, respectively. In addition, MMS2 detected an ion flow bump in the $L$ and $M$ directions within the current sheet (between two vertical dashed lines, approximately 00:31:00–00:31:01 UT) in the context of the large-scale shear flow. The positive ion flow $V_{iL}$ enhancement between the current sheet and the adjacent region was 50 km/s, ~0.34 $V_{AL}$ on the negative $B_L$ side and 20 km/s, ~0.33 $V_{AL}$ on the positive $B_L$ side. Here, $V_{AL}$ is the $L$ component of the local ion Alfvén speed calculated with $B_L$ and ion number density at corresponding sides, respectively. Using the asymmetric hybrid Alfvén speed $V_{ALh}$ [59], the positive outflow velocities relative to the adjacent flows at two sides were 0.6 $V_{ALh}$ and 0.2 $V_{ALh}$ respectively. According to the positive ion outflow, the approximate trajectory of the MMS2 across the current sheet is shown by the dashed red arrow in Figure 3.

Beyond the same flow shear across the current sheet, electron flow shows more complicated variations (Figure 2d). Inside the two current density peaks, electron flow was significantly enhanced in the $-M$ direction, opposite to that of the ion flow, which is consistent with the acceleration by the inductive electric field around the X line [60]. $V_{eL}$ shows a negative jet in the first $J_M$ peak and a positive jet in the second $J_M$ peak, corresponding to electrons flowing toward and away from the X line in the separatrix region [61,62,63]. Electron pitch angle distributions at energies 0–200 eV (Figure 4i) showing field-aligned streaming populations parallel to the magnetic field in two $J_M$ peaks also support this point [64]. The electron inflow and outflow speed were comparable, about 300 km/s, much faster than the ion outflow speed. The electron flow streamlines are represented by magenta and orange arrows in Figure 3. This type of electron flow variation constituted a Hall current loop which can generate a negative Hall magnetic field $B_M$ according to Ampere’s law. As the MMS2 traversed the current sheet, a negative $B_M$ variation relative to the guide field was indeed observed. The pattern of such a Hall electron current system and the resultant Hall magnetic field were influenced by both the guide field and the density asymmetry [65,66,67,68,69], of which the guide field dominated in our event.

During this current sheet crossing, the electric field showed large-amplitude fluctuations, especially in the $M$ component (Figure 2f). The electric field $E_L$ was essentially positive within the current sheet, pointing outward from the X line. Simultaneously, the normal component $E_N$ was overall positive, bounded by two negative dips at the edges of the current sheet. Such an $E_N$ structure has been reported in observations and was attributed to the effect of the guide field [70]. Figure 2g shows the electron temperature parallel and perpendicular to the magnetic field. Electrons were predominantly heated in the parallel direction in the lower part of the current sheet (Figure 3), while in the upper part, they were heated mainly in the perpendicular direction. Electron heating exhibited an obvious asymmetry in two current peaks.

The observed positive ion outflow implies that MMS2 crossed the $+L$ side of the X line, as depicted in Figure 3. The positive $B_N$ up to 20 nT detected between 00:30:59.6 and 00:31:00.1 UT also supports this inference. We note, however, that $B_N$ actually exhibited a bipolar variation, with positive $B_N$ followed immediately by negative $B_N$. The negative $B_N$ was about 4 nT. Furthermore, $B_M$ and the magnetic field strength peaked near the point where $B_N$ changed its sign. These magnetic field signatures were observed by four MMS satellites and seem to suggest that MMS crossed a magnetic flux rope in the current sheet [71,72]. To further confirm the existence of flux rope, we examined the magnetic field line curvature in Figure 2h. The curvature in the $L$ and $N$ directions both reversed from positive to negative in the flux rope (denoted by the purple bar at the bottom of Figure 2), which strongly supports the idea that MMS crossed a closed magnetic field configuration in the L-N plane, denoted by an ellipse in Figure 3. The fact that the curvature in the $L$ and $N$ directions did not reverse simultaneously means that MMS did not cross the flux rope center. The field-aligned bidirectional distribution (Figure 4i,j) and increased parallel electron temperature could be caused by the electron acceleration in the flux rope [73,74]. The radius of the flux rope is calculated to be 60 km, ~2.9 $d_i$ along the $N$ direction, which suggests that the flux rope occupied approximately 60% of the width of the current sheet.

3.3. Nonideal Ion and Electron Behaviors

Sub-Alfvénic ion outflow in Figure 2c suggests that MMS most likely crossed the ion diffusion region where demagnetized ions have not been fully accelerated. Figure 4d–f compare the observed electric field $E$ with $-(V_i\times B)$ and $-(V_e\times B)$. The error bar of the electric field measurement is superposed as well. The ions were demagnetized in most of the current sheet, especially in two $J_M$ peaks where $E$ and $-(V_i\times B)$ had significant deviations. Overall, the electrons remained magnetized throughout the ion exhaust, which is roughly bounded by the two $J_M$ peaks (separatrix regions) if disregarding small systematic constant offsets. However, there were some local deviations between $E$ and $-(V_e\times B)$ in the two $J_M$ peaks, such as in the $L$ and $N$ components at ~00:30:59.8 UT in the first $J_M$ peak and in the $N$ component at ~00:31:00.9 UT in the second $J_M$ peak. The break of the electron frozen-in condition therein was coincident with the increase in the electron nongyrotropy $Q^{1/2}$ [75] as shown in Figure 4g. The simultaneous presence of non-ideal electric fields $E^{\prime }=E+V_e\times B$ and the current in two $J_M$ peaks caused nonnegligible energy dissipation $J\cdot E^{\prime }$ therein (Figure 4h). In the first $J_M$ peak where parallel current $J_{\left|\right|}$ dominated (Figure 4c), energy dissipation was mainly caused by the parallel current and electric field. In contrast, the contribution from parallel and perpendicular currents to the second $J_M$ peak is comparable and energy dissipation was also seen from both parallel and perpendicular directions.

4. Discussion and Conclusions

In this paper, we report a reconnecting current sheet in the turbulent magnetosheath. The width of the observed current sheet is about 90 km, ~4.3 ion inertial lengths, which is consistent with the predictions in previous kinetic simulations of coherent current sheet structures formed in the turbulence [76,77,78]. The observations of sub-Alfvénic demagnetized ion outflow jet, super-Alfvénic electron flows in the separatrix region, Hall magnetic fields, and electron heating indicate that MMS crossed the ion diffusion region dominated by Hall effects. An ion-scale flux rope was detected inside the diffusion region. This flux rope did not span the whole current sheet and could be caused by the secondary tearing instability [79,80]. Considering that the flux rope was embedded between the electron inflow and outflow, we cannot rule out the possibility of flux rope generation by electron Kelvin–Helmholtz instability [81].

The current sheet in our event exhibits a typical bifurcated structure which had a pair of current peaks away from the current sheet center where $B_L$ was nearly zero. The bifurcated current sheet is frequently observed in the magnetotail [82,83,84], flank magnetopause [85], and solar wind [86,87]. Some such current sheets are associated with fast plasma flows and thus have been interpreted as Petschek-type reconnection exhausts [86,88]. However, this type of bifurcation may not be applicable to current sheets close to the X line [89]. Some other conditions for current sheet bifurcation like ion or electron temperature anisotropy [90,91] were also not observed in our event. La Belle-Hamer et al. [92] found that shear flow and density asymmetry on the two sides of current sheets can work together to affect the magnitude and location of the currents that bound the reconnection outflow regions. The competition of the two effects will determine the location of the stronger boundary current. In our event, the stronger $J_M$ peak occurred at the low-density edge of the current sheet where the shear flow between the ion outflow and the background flow is larger than that on the other side. This asymmetry of current sheet bifurcation is consistent with previous observations [93].

The current sheet bifurcation in our event is associated with the electron current. The two electron current peaks were coincident with super-Alfvénic electron inflow and outflow, respectively, and formed the boundaries of a weak ion exhaust. The current peaks at two sides of the current sheet exhibit noticeable differences. In the bottom current peak (Figure 3), the current was mainly antiparallel to the magnetic field, the plasma density was decreased, and the electrons were heated in the parallel direction. On the contrary, in the upper current peak, the current had comparable components in the parallel and perpendicular directions, the plasma density was enhanced, and the electrons were heated in the perpendicular direction. The upper current peak displays some features of the electron diffusion region. Another striking difference is the magnetic field topology. The magnetic field curvature was mainly along the $+N$ direction in the bottom peak while heavily skewed to the $+L$ and $+M$ directions in the upper peak. This result reveals a three-dimensional magnetic field configuration of the reconnecting current sheet.

In conclusion, this paper presents a detailed study of a bifurcated reconnecting current sheet in the magnetosheath. The event is an asymmetric magnetic reconnection with a large guide field and is embedded in a shear flow. Such background condition may be common in the magnetosheath turbulence, and our results are important supplements to the study of reconnection under different magnetic field and plasma environments.