Julia Stawarz

The region of shocked solar wind plasma downstream of Earth’s bow shock is filled with complex turbulent fluctuations that generate many thin current sheets, which can be sites for magnetic reconnection. Recent observations from NASA’s Magnetospheric Multiscale (MMS) mission have revealed that a novel type of magnetic reconnection, known as electron-only reconnection, is present in Earth’s turbulent magnetosheath. A diagram of reconnection in a turbulent environment is shown in Figure 1. In contrast to standard ion-coupled reconnection, electron-only reconnection, occurs at thin scale current sheets approaching the electron scales and the ions in the plasma never couple into the newly reconnected magnetic field lines before the field lines fully relax. Whether ions couple into the reconnection dynamics can potentially have an impact on both the nonlinear dynamics of the turbulence and how the energy dissipated by the turbulence is partitioned between ions and electrons. Therefore, in this study, we examine how the behaviour of reconnection events in a turbulent environment depend on the properties of the turbulence.

We identify 60 intervals of turbulence observed by MMS across the dayside magnetosheath, an example of which is shown in Figure 2. In a turbulent environment, the length of the current sheets is expected to be related to the typical size of the twisted magnetic structures that are formed by the turbulence, which can be quantified in an average sense by the magnetic correlation length. Idealised simulations, show that ion-coupled reconnection is expected to transition to electron-only reconnection if the length of the current sheets is less than 40 ion inertial lengths. As shown in Figure 3, estimates of the correlation length for the magnetosheath intervals examined in this study, show the correlation length systematically varies across the magnetosheath, with values near 10 ion inertial lengths near the sub-solar point and several tens to hundreds of ion inertial lengths toward the flanks.

The unique high-resolution, multi-spacecraft measurements from MMS allow the examination of the small-scale current sheets in the turbulent plasma in greater detail than previously possible. As illustrated for an example interval in Figure 4, we use this data to systematically identify reconnection events in each of the intervals, identifying several hundred reconnection events across the 60 intervals. Most of the reconnection events show no clear evidence of ion jets being formed, consistent with electron-only reconnection, with only 18 events having clear ion jets. Two example reconnection events, one without an ion jet and one with an ion jet, are shown in Figures 5 and 6.

In Figure 7, we examine the properties of all the reconnection events and find that the speed of the electron jets is correlated with the thickness of the current sheets, with fast super-Alfvénic electron jets occurring at thin sub-ion scale current sheets, consistent with expectations for electron-only reconnection, and electron jet speeds closer to the Alfvén speed occurring at ion-scale current sheets. Furthermore, the ion jets that are observed only occur at current sheets near the ion inertial length, consistent with ion coupled reconnection. When the reconnection events are sorted by the magnetic correlation length a clear tendency for thinner sub-ion scale current sheets and a slight tendency for faster electron jets is present for the intervals with shorter correlation lengths near 10 ion inertial lengths.

The results observationally demonstrate that reconnection events, either with or without ion jets, are a common feature in the turbulent magnetosheath and that electron-only reconnection may be more prevalent in turbulent collisionless plasmas with relatively small magnetic correlation lengths.