Mechanism of Reconnection on Kinetic Scales Based on Magnetospheric Multiscale Mission Observations

We examine the role that ions and electrons play in reconnection using observations from the Magnetospheric Multiscale (MMS) mission on kinetic ion and electron scales, which are much shorter than magnetohydrodynamic scales. This study reports observations with unprecedented high resolution that MMS provides for magnetic field (7.8 ms) and plasma (30 ms for electrons and 150 ms for ions). We analyze and compare approaches to the magnetopause in 2016 November, to the electron diffusion region in the magnetotail in 2017 July followed by a current sheet crossing in 2018 July. Besides magnetic field reversals, changes in the direction of the flow velocity, and ion and electron heating, MMS observed large fluctuations in the electron flow speeds in the magnetotail. As expected from numerical simulations, we have verified that when the field lines and plasma become decoupled a large reconnecting electric field related to the Hall current (1–10 mV m−1) is responsible for fast reconnection in the ion diffusion region. Although inertial accelerating forces remain moderate (1–2 mV m−1), the electric fields resulting from the divergence of the full electron pressure tensor provide the main contribution to the generalized Ohm's law at the neutral sheet (as large as 200 mV m−1). In our view, this illustrates that when ions decouple electron physics dominates. The results obtained on kinetic scales may be useful for better understanding the physical mechanisms governing reconnection processes in various magnetized laboratory and space plasmas.