Evidence of Magnetic Nulls in the Reconnection at Bow Shock

Abstract

Magnetic nulls are believed to play important roles in the energy dissipation during reconnection. Such nulls have been observed in reconnection at the magnetopause, magnetosheath, and magnetotail but have never been observed in reconnection at the bow shock. Recently, four reconnection events were reported at the terrestrial bow shock, by utilizing Magnetospheric Multiscale (MMS) data. We examine whether the magnetic nulls exist in these events. We successfully find radial nulls in three of the events, meaning that spacecraft were close to X points in these events; we, however, cannot find radial nulls in another event, which was actually a crossing of the reconnection exhaust region. The minimum distance between radial nulls and spacecraft is 8 km, about 0.3 ion inertial length. We reconstruct topologies of these nulls and subsequently resolve the reconnection rates in these events. We find that the resolved reconnection rates are comparable with those at the magnetopause and magnetotail. Surprisingly, we do not find electron heating at the radial nulls. Our results are useful to understand the reconnection at bow shock.

First Topology of Electron‐Scale Magnetic Hole

“…For the applications to current four‐spacecraft missions (e.g., the European Space Agency's Cluster mission, Escoubet et al, 2001; and NASA's Magnetospheric Multiscale Mission, Burch et al, 2016), we combine the four‐point field measurements from the SC tetrahedron at two different times into one set of simultaneous eight‐point measurements and use Ampere's theorem and Maxwell's divergence equation, as shown below. The simplified form of the SOTE method, the linear method which ignores all the second‐order terms in Equation , has been widely used in analyzing and reconstructing the magnetic nulls for its convenience in calculation (Chen, Fu, Wang, Cao, et al, 2019; Chen, Fu, Wang, Liu, & Xu, 2019; Fu et al, 2015, 2016, 2017, 2019; C. M. Liu, Chen, Wang, & Liu, 2019; C. M. Liu et al, 2018; Peng et al, 2017; Wang et al, 2019). Unfortunately, MHs will be proved to have nonlinear magnetic field topologies, and thus, linear method is not suitable here.…”

“…The simplified form of the SOTE method, the linear method which ignores all the second-order terms in Equation 1, has been widely used in analyzing and reconstructing the magnetic nulls for its convenience in calculation (Chen, Fu, Wang, Cao, et al, 2019;Chen, Fu, Wang, Liu, & Xu, 2019;Fu et al, 2015Fu et al, , 2016Fu et al, , 2017Fu et al, , 2019C. M. Liu et al, 2018;Peng et al, 2017;Wang et al, 2019).…”

First Observation of Magnetic Flux Rope Inside Electron Diffusion Region

“…These features, including electron demagnetization, energy dissipation, electron nongyrotropy (crescent distribution), and an X‐line, indicate that MMS encountered an EDR (Burch et al., 2016a; Chen et al., 2019a; Fu et al., 2019b; Wang et al., 2019b) in the region marked by gray shading in Figures 2a–2g. In the EDR, a small bipolar variation of the magnetic field B N component from ∼1 nT to ∼ −2.8 nT was detected at ∼12:15:44.75 UT (marked by cyan shading; Figure 2c), which is widely believed to be a signature of MFRs (Chen et al., 2019c; Huang et al., 2016a; Liu et al., 2019b; Wang et al., 2010).…”

“…The EDR is embedded in the ion diffusion region (IDR; Fu et al., 2019a; Øieroset et al., 2001) because of different motions between ions and electrons. Magnetic reconnection can lead to reconfiguration of magnetic field topology (Chen et al., 2019a; Fu et al., 2019b, 2020a) and formation of many structures, such as reconnection fronts (Fu et al., 2013b, 2019c, 2020b; Liu et al., 2018), magnetic nulls (Chen et al., 2018; Fu et al., 2020a), and magnetic flux ropes (MFRs; also called magnetic islands or plasmoids; Chen et al., 2019b, 2019c; Daughton et al., 2011; Drake et al., 2006a; Deng et al., 2004; Eastwood et al., 2016; Fu et al., 2016; Huang et al., 2016a; Slavin et al., 2003), which, in turn, play crucial roles in magnetic reconnection (Daughton et al., 2011; Huang et al., 2012; Oka et al., 2010; Wang et al., 2015).…”

Three-dimensional magnetic reconnection in astrophysical plasmas

“…He et al [96] extended the method to include an arbitrary number of null points (figure 11). As a result, 3D null points have been identified in the Earth’s magnetotail [71,97], magnetopause [99], turbulent magnetosheath [100] and bow shock [101]. The spatial scale for variations of the magnetic field near the observed magnetic null is of the order of an ion inertial length [98], implying that the Hall effect is important.…”