Electron-only Reconnection in Ion-scale Current Sheet at the Magnetopause

In the standard model of magnetic reconnection, both ions and electrons couple to the newly reconnected magnetic field lines and are ejected away from the reconnection diffusion region in the form of bidirectional burst ion and electron jets. Recent observations propose a new model: electron only magnetic reconnection without ion coupling in electron scale current sheet. Based on the data from Magnetospheric Multiscale (MMS) Mission, we observe a long extension inner electron diffusion region (EDR) at least 40 di away from the X line at the terrestrial Magnetopause, implying that the extension of EDR is much longer than the prediction of the theory and simulations. This inner EDR is embedded in an ion scale current sheet (the width of 4 di, di is ion inertial length). However, such ongoing magnetic reconnection was not accompanied with burst ion outflow, implying the presence of electron only reconnection in ion scale current sheet. Our observations present new challenge for understanding the model of standard magnetic reconnection and electron only reconnection model in electron scale current sheet.


Introduction
Magnetic reconnection is a widespread important physical process that allows the rapid energy conversion of magnetic field into the plasmas, resulting in the particle's acceleration/heating and the changing of magnetic field topology (Priest et al., 2000;Yamada et al., 2010). Magnetic reconnection is frequently observed or thought to play a major role in the astrophysical and space plasmas, such as solar flares, solar and stellar coronae, solar wind, planetary magnetosphere, the interplanetary space, the interstellar medium, neutron star, accretion disks, astrophysical jets, galaxy clusters and black holes (Priest et al., 2000;Øieroset et al., 2001;Deng et al., 2001;Lin et al., 2005;Vaivads et al., 2004;Huang et al., 2010;Yamada et al., 2010). The crucial region of magnetic reconnection can be divided into ion diffusion region (IDR, where the ions are demagnetized) and electron diffusion region (EDR, where both the ions and the electrons are demagnetized) due to the different mass between the ion and the electron. Around or in the diffusion region, the significant phenomenon is Hall effect that includes Hall currents, bipolar Hall electric field pointing toward the center of the current sheet, and Hall quadrupolar out-of-plane magnetic field because of the relative motion between the ions and the electrons (Priest et al., 2000;Øieroset et al., 2001;Deng et al., 2001;Lin et al., 2005;Vaivads et al., 2004;Huang et al., 2010Huang et al., , 2012Yamada et al., 2010). EDR, which is embedded in the ion diffusion region, can extend several d i along the outflow direction and develop two sub-structures, i.e., inner EDR and outer EDR. The inner EDR containing the Xline is the core region during the magnetic reconnection, which features intense electron currents, nonzero E' = E+V e ×B, electron nongyrotropy or electron crescent distribution, super-Alfvénic electron flow, and electron dissipation J•E' > 0, etc. (Zenitani et al., 2012;Hesse et al., 2013); while the outer EDR can extend tens of d i (where d i is ion inertial length) along the outflow direction, where the electrons are remained decoupled from the magnetic field and form a super-Alfvénic outflow jet, electron nongyrotropy, strong electron currents and J•E' < 0 (Zenitani et al., 2012;Le et al., 2013).
The inner EDR has recently been in-situ identified at the terrestrial magnetopause and in the magnetotail by the unprecedented high-resolution measurements from the Magnetospheric Multiscale (MMS) mission (Burch et al., 2016;Zhou et al., 2017Zhou et al., , 2019Huang et al., 2018;Torbert et al., 2018;Fu et al., 2019). Outer EDR, with energy conversion from the particles to the fields (J•E'<0), has been identified in the magnetosheath (Phan et al., 2007), in the magnetotail (Zhou et al., 2014) and at the magnetopause (Hwang et al., 2017). We should point out that such outer EDR is only an electron jet with the violation of electron frozen-in condition and has a negative J•E'. Recently, an EDR with positive energy dissipation (J•E'>0) extended 20 d i away from the X-line embedding in the burst ion outflow was reported in the downstream of magnetic reconnection at the magnetopause (Zhong et al., 2020).
Recent studies have presented reconnection with burst of electron jets but no ions accompanied at turbulent magnetosheath  and quasi-parallel shocks Gingell et al., 2019), which challenges the standard model of EDR in the reconnection that the ions are ejected away from the diffusion region in the form of burst ion jets in the downstream.
It is revealed that the electron-scale current sheet could also produce turbulent energy transformation and dissipation without ion participation during magnetic reconnection .
In this study, we report a textbook inner EDR emerging in an ion-scale current sheet (the thickness up to 4 d i ) at the magnetopause boundary layer, which is characterized by super-Alfvén electron jet, electron nongyrotropy, and positive energy dissipation, clear parallel electric field. This EDR with a thickness of ~0.53 d i is extended about 40 d i away from the Xline in the downstream of magnetic reconnection but without burst ion outflow. Our observations demonstrate a new feature of reconnection in space, i.e., electron-only reconnection in the ion-scale current sheet, which is different from the traditional magnetic reconnection model, and challenges the previous observations as well.

Event overview
The overview observations from 13:31:10 to 13:34:00 UT on September 07, 2015 when the MMS were located at ~ [3.85, 10.9, -0.12] R E (R E is the Earth's radius) are shown in Figure 1.
The magnetic field measured by the fluxgate magnetometer (FGM) instrument (Russell et al., 2016), the electric field from the electric double probes (EDP) instruments (Lindqvist et al., 2016), and the particle data measured by the fast plasma investigation (FPI) instrument (Pollock et al., 2016) onboard MMS in burst mode are used in this study. MMS traveled through firstly magnetosphere (positive B z in Figure 1(a), low speed plasma flow, high temperature and low density in Figure 1(b)-1(e)), then crossed magnetopause boundary layer, and finally entered the magnetosheath (negative B z in Figure 1(a), high speed flow, low temperature and high density in Figure 1(b)-1(f)). These two black dashed lines mark the magnetopause boundary layer which is characterized by the mixed particles: high energy particles from the magnetosphere and low energy particles from the magnetosheath (Figure 1(g) and 1(h)). One reconnecting current sheet marked by yellow shadow has most intense current density (Figure 1(i)), up to ~ 2 µA/m 2 , which will be investigated in the following part. one can infer that MMS detected one reconnection diffusion region with well-known Hall current, Hall quadrupolar out-of-plane magnetic field (small guide field ~ -8 nT here). Due to the large convective ion flow V i , the convection term will dominate the electric field. Thus, the convective term V i´B removed from the electric field E N , i.e., (E+V i´B ) N is shown in Figure   2(f). One can see that (E+V i´B ) N has a bipolar variation from positive to negative except one small pulse during the crossing of the current sheet, indicating that (E+V i´B ) N points toward the center of the current sheet. We also calculate the different terms of the general Ohm's law.
It can be seen that Hall term (J´B) N can well balance the electric field (E+V i´B ) N , which means that the bipolar change of (E+V i´B ) N is Hall electric field caused by the Hall term. The electron velocity V eL and V eM are up to -300 km/s and 400 km/s respectively after subtracting the background flow, which are much larger than the local Alfvén speed V A ~ 126 km/s, implying that MMS detected a super-Alfvénic electron flow in this diffusion region. In addition, MMS also measured one peak in electron density and the increase in electron temperature dominated by parallel temperature. It is interesting that the non-zero electric field E¢ in the electron frame ( Figure 2(g)), large parallel electric field (up to 5 mV/m in Figure 2(h)), and very strong energy dissipation from the fields to the plasmas (J•E¢>0, up to 7 nW/m 3 in Figure 2(k)) are observed during this crossing. All these features suggest the existence of inner EDR in this reconnection diffusion region. It is noticeable that the ion bulk velocity at LMN coordinate does not have obvious increase signature as Figure 1b  The trajectory of the MMS crossing the reconnection region is illustrated in Figure 4. In order to determine where the EDR is, we identify the separatrix and the center of EDR using B M -B g = 0 (where guide field B g ~ -8 nT) marked by three vertical dashed lines in Figure 2. The EDR extension slightly deviates from the center of current sheet due to the presence of guide field (Le et al., 2013). The distance between two separatrices and the center of EDR is estimated as 33 km and 55 km respectively based on the Timing analysis, contributing to ~2 d i width between two separatrices. Given the hypothesis that the separatrices are straight lines start from X line as well as the magnetic field lines are parallel to the separatrices nearby, one can obtain the cone angle of the spacecraft crossing point at separatrices from the following equation: tan~' where is the distance between the separatrix and the center of EDR, D is the extension of EDR from the X-line. The cone angle q can be derived by the magnetic field in L and N direction at the intersection of the trajectory of MMS and the separatrices. Thus, the cone angles q are ~1.19° and ~1.94° corresponding to the lower and upper crossing point at separatrices respectively (shown in Figure 4). Based on the triangle theory, the extension length of EDR is estimated to 1604 km and 1621 km away from the X-line resulting from the distance as 33 km and 55 km between the center of EDR and lower and upper separatrices respectively.
In roughly, therefore, the EDR extension from the X-line in the downstream is at least ~ 40 d i , which is the first time in-situ observation of inner EDR for such a long extension in space. In addition, the average reconnection rate R = 0.021 ~ 0.034 calculated by the equation given in Liu et al. (2017) and Nakamura et al. (2018), consistent with the previous predictions and observations (Xiao et al., 2007;Liu et al., 2017;Chen et al., 2019;Zhong et al., 2020).

Conclusion
Thanking for the unprecedented high-resolution data from the MMS mission, the inner EDR is successfully and definitely identified at the magnetopause (Burch et al., 2016;Zhou et al., 2017), in the magnetotail (Huang et al., 2018;Torbert et al., 2018;Chen et al., 2019;Zhou et al., 2019) by electron nongyrotropy or electron crescent distribution, strong energy dissipation J•E' > 0, super-Alfvénic electron flow, parallel electric field, and electron demagnetization, etc. In addition, the outer EDR with electron demagnetization and super-Alfvénic outflow jet is also identified in the magnetosphere (Xiao et al., 2007;Chen et al., 2019). In present study, we     The cone angles between the separatrix and EDR extension are 1.94° (upper) and 1.19° (lower).
The horizontal dashed line is the center of current sheet (i.e., the region with B L =0). The width between upper separatrix and lower separatrix along the trajectory of MMS is ~2 d i .