Remote sensing of a magnetotail reconnection X-line using polar rain electrons

We report on electron phase space distributions (PSDs) observed near the plasma sheet (PS) boundary layer (PSBL) by the Cluster electron spectrometers when the northern lobe was occupied by significant fluxes of polar rain (PR) electrons. These observations reveal the spatial structure of the electron transition layer (TL) between the polar rain electrons and the PSBL electron population accelerated during reconnection. This TL comprises overlapping spatial dispersion signatures in both energy and pitch angle, which are caused by convection of flux tubes across the magnetic separatrix during the electron time-of-flight (TOF) from the X-line combined with acceleration in the reconnection region. Analysis of this structure allows us to estimate the location of the X-line. By assuming the PSBL population arises through acceleration of the PR electrons, comparison of their PSD indicates that the electrons gain energy proportional to their initial energy.