First Resolved Observations of the Demagnetized Electron-Diffusion Region of an Astrophysical Magnetic-Reconnection Site

Spatially resolved, diagnostic signatures across the X-line and electron-diffusion region (EDR) by the Polar spacecraft are reported at Earth's magnetopause. The X-line traversal has a local electron's skin depth scale. First, resolved EDR profiles are presented with peak electron thermal Mach numbers >1: 5, anisotropy >7, calibrated electron agyrotropy >1, and misordered expansion parameters indicative of demagnetization and strong (150 eV) increases in electron temperature. The amplitude and phase of these profiles correlate well with a guide geometry kinetic simulation of collisionless magnetic reconnection. Such high resolution diagnosis has been made possible by data processing techniques that afford an 11-fold reduction in the aliasing time for the electron moments.