Energy Conversion and Electron Acceleration in the Magnetopause Reconnection Diffusion Region

Data are analyzed from a Magnetospheric Multiscale encounter with a dayside magnetopause reconnection region on 29 December 2016. The uniqueness of the event stems from the small (similar to 7 km) average spacecraft separation and the sequential sampling of an electron diffusion region with electron crescent distributions. We quantitatively investigate the earthward acceleration of magnetosheath electrons through the in-plane null by the polarization electric field E-N that points radially outward from the magnetopause. The results compare favorably with previous plasma simulations with one important difference that the reconnection electric field (E-M) extends throughout the region of strong E-N so that both fields energize electrons in the same region. This acceleration is quantified here for the first time. As the spacecraft penetrate deeper into the region of enhanced E-N, the magnetic reflection of lower-energy electrons produces a thinner crescent. Plain Language Summary Reconnection causes the conversion of magnetic energy to particle energy and the breaking and reconnection of solar wind and geomagnetic field lines. In asymmetric reconnection at the dayside magnetopause with different plasma densities and magnetic field strengths on the Earth and Sun sides of an X-line, this energy conversion is displaced to the earthward side of the current layer. An electric field pointing outward normal to the dayside magnetopause drives reconnection by accelerating solar wind electrons across the reconnection current sheet. These electrons take up dawnward-directed meandering orbits, and they are ultimately the carriers of the reconnection current. This paper reports the first direct measurement of this acceleration.