Large‐scale characteristics of reconnection diffusion regions and associated magnetopause crossings observed by MMS

Abstract

The Magnetospheric Multiscale (MMS) mission was designed to make observations in the very small electron diffusion region (EDR), where magnetic reconnection takes place. From a data set of over 4500 magnetopause crossings obtained in the first phase of the mission, MMS had encounters near or within 12 EDRs. These 12 events and associated magnetopause crossings are considered as a group to determine if they span the widest possible range of external and internal conditions (i.e., in the solar wind and magnetosphere). In addition, observations from MMS are used to determine if there are multiple X‐lines present and also to provide information on X‐line location relative to the spacecraft. These 12 events represent nearly the widest possible range of conditions at the dayside magnetopause. They occur over a wide range of local times and magnetic shear angles between the magnetosheath and magnetospheric magnetic fields. Most show evidence for multiple reconnection sites.

Evidence of Magnetic Nulls in Electron Diffusion Region

“…During the third crossing (13:07:02 UT), MMS detected both the reversal of magnetic field (B L , Figure 1c) and ion flow velocity (V iL , Figure 1f) in L direction, the significant increase of electron flow velocity up to 1000 km/s (V e⊥ , Figure 1g), the demagnetization of electrons , and the agyrotropy of electron distributions . The plasma density used to calculate the ion inertial length is taken from Fuselier et al (2017). Indeed, this region has been suggested by a couple of scientists as an EDR hosting magnetic reconnection.…”

“…These EDR events, as listed in Table 1, have been suggested as the regions hosting magnetic reconnection (Fuselier et al, 2017), without any proof of X-lines. These EDR events, as listed in Table 1, have been suggested as the regions hosting magnetic reconnection (Fuselier et al, 2017), without any proof of X-lines.…”

“…Among these events, seven of them have guide field, while five of them do not have. Among these events, guide field exists in some of the events (a, c, and g-k), but not in others (b, d-f, and l), according to previous analyses (Fuselier et al, 2017). We artificially subtract the guide field, in order to find an in-plane null.…”

Crescent‐Shaped Electron Distributions at the Nonreconnecting Magnetopause: Magnetospheric Multiscale Observations

“…The crescent electrons have a cutoff energy, depending on the electric field in the normal direction and the penetration distance from the separatrix (Egedal et al, 2016), so that they are not filled in at lower energies. Therefore, agyrotropic electron crescents, along with other signatures (such as intense current density and enhanced energy dissipation), are taken as direct observational indicators of an EDR or near-EDR event (e.g., Fuselier et al, 2017;Webster et al, 2018). These electron crescents can become oblique due to the presence of the parallel electric field or the electron inertia if the magnetic field lines rotate rapidly (Egedal et al, 2018).…”

The MMS Dayside Magnetic Reconnection Locations During Phase 1 and Their Relation to the Predictions of the Maximum Magnetic Shear Model

“…So far 12 EDR regions have been identified and studied during phase 1a of the MMS (e.g., Burch, Torbert, et al, ; Burch & Phan, ; Chen et al, ; Eriksson et al, ; Khotyaintsev et al, ; Lavraud et al, ; Norgren et al, ). An overview of the global topology of all currently documented EDR events can be found in Fuselier et al (). To identify and study the physics within an EDR, it is important to know where reconnection will occur at the magnetopause and how local and external parameters influence these locations and ultimately influence the physics within the reconnection region.…”