Magnetospheric Multiscale (MMS) Observations of magnetic reconnection in foreshock transients

Magnetic reconnection is a fundamental process of energy conversion in plasmas between electromagnetic fields and particles. Magnetic reconnection has been observed directly in a variety of plasmas in the solar wind and Earth's magnetosphere. Most recently, electron magnetic reconnection without ion coupling was observed for the first time in the turbulent magnetosheath and within the transition region of Earth's bow shock. In the ion foreshock upstream of Earth's bow shock, there may also be magnetic reconnection especially around foreshock transients that are very turbulent and dynamic. With observations from the National Aeronautics and Space Administration's Magnetospheric Multiscale mission inside foreshock transients, we report two events of magnetic reconnection with and without a strong guide field, respectively. In both events, a super-ion-Alfvenic electron jet was observed within a current sheet with thickness less than or comparable to one ion inertial length. In both events, energy was converted from the magnetic field to electrons, manifested as an increase in electron temperature. Weak or no ion coupling was observed in either event. Results from particle-in-cell simulations of magnetic reconnection with and without a strong guide field are qualitatively consistent with observations. Our results imply that magnetic reconnection is another electron acceleration/heating process inside foreshock transients and could play an important role in shock dynamics.