Kinetic Structure of Phase Space Density Gradients in and around the Electron Diffusion Region of Magnetopause Reconnection

We present Magnetospheric Multiscale (MMS) four-spacecraft observations of the spatial gradient term in the electron Vlasov equation measured within the electron diffusion region (EDR) of magnetic reconnection occurring at Earth's magnetopause. We compare the MMS observations to particle-in-cell (PIC) simulations of asymmetric reconnection suitable for modeling dayside reconnection. A highly-structured, smile-shaped gradient distribution in ∇fe is discovered that corresponds to demagnetized electron crescent distributions specific to the central EDR. The intricate velocity-space features of the electron gradient distributions found in both the MMS data and the PIC simulations are useful for (1) distinguishing reconnection crescent signatures from non-reconnection diamagnetic crescent distributions that develop more generally at magnetized electron-scale boundary layers, (2) precisely determining the location of the MMS tetrahedron in relation to EDR sub-structures that are otherwise difficult to identify, and (3) understanding how spatial variations in the electron ensemble self-consistently support the reconnection electric field via net contributions to the bulk electron pressure divergence ∇⋅Pe. These results are relevant to recent studies of kinetic entropy and overarching questions in plasma physics research regarding how processes like Landau damping and magnetic reconnection appear to effect irreversible, dissipative phenomena even in the collisionless regime.