Electron magnetic reconnection without ion coupling in Earth’s turbulent magnetosheath

Abstract

Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region. On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed. Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region . In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales. However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth's turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling.

Observations of Magnetic Reconnection in the Transition Region of Quasi‐Parallel Shocks

“…We show that reconnecting current sheets are present within a disordered transition region close to the shock ramp, which is consistent with the appearance of these structures in recent hybrid and kinetic shock simulations (Bohdan et al, 2017;Gingell et al, 2017;Matsumoto et al, 2015). In contrast to magnetosheath observations reported by Phan et al (2018) and global hybrid simulations by Karimabadi et al (2014), structures discussed here appear within seconds of crossing the bow shock, suggesting a close association with shock processes and a rapid evolution. In contrast to magnetosheath observations reported by Phan et al (2018) and global hybrid simulations by Karimabadi et al (2014), structures discussed here appear within seconds of crossing the bow shock, suggesting a close association with shock processes and a rapid evolution.…”

“…Similarly to recent observations of magnetosheath reconnection (Phan et al, 2018), the outflow jet and particle heating appear limited to electrons. Similarly to recent observations of magnetosheath reconnection (Phan et al, 2018), the outflow jet and particle heating appear limited to electrons.…”

“…Thus, this event is appropriate for evaluating the predictions of recent hybrid simulations of reforming, quasi-parallel shocks with respect to reconnection (see Gingell et al, 2017, and Figure 1). For discussion of individual coherent structures, we introduce a new coordinate system derived by using a hybrid minimum variance analysis Phan et al, 2018). The current sheet normal N is determined using Although many magnetic directional discontinuities are visible within the transition region shaded in Figure 2, we must observe electron or ion jets in order to conclude that these current sheets are actively reconnecting.…”

Asymmetric Reconnection Within a Flux Rope‐Type Dipolarization Front

“…A third possibility which can only occur in a very turbulent plasma is electron-only reconnection as described in Phan and Eastwood (2018). Such a process requires a large B M , and seems like a very plausible explanation for the 8 September event.…”

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

“…These gyrotropic distributions may not survive either when a plasma/magnetic field boundary has a width comparable to the particle's gyroradius ( = mv ⟂ qB ) due to finite gyroradius effect or when the particles become decoupled from the magnetic field. However, the recently launched Magnetospheric Multiscale (MMS) mission , measuring charged particle orders of magnitude faster, provides a good opportunity to explore electron-scale physics (e.g., Chen et al, 2016;Lavraud et al, 2016;Phan et al, 2018;Khotyaintsev et al, 2016). On the other hand, due to electron's small mass, such agyrotropic distributions of electrons are difficult to investigate in space because in situ measurements RESEARCH LETTER 10.1029 in previous space missions are too slow to electron scales.…”