Derivation of the Two‐Dimensional Distribution of Low‐Energy Electron Precipitation From 630‐nm All‐Sky Auroral Images and Its Application to the Polar Cap Boundary Near Midnight

Abstract We present a methodology for deriving the horizontal two‐dimensional distribution of low‐energy electron precipitation, specifically the possible lower bound of the differential energy flux of electron precipitation at 100 eV, from a 630‐nm auroral image obtained with a ground‐based all‐sky imager. The electron energy flux required to reproduce the auroral intensity distribution can be obtained using the GLobal airglOW (GLOW) model for the magnetic field lines within the field of view of the imager. The distribution of the electron precipitation occurring in a roughly circular region with a diameter of approximately 1,400 km centered on the observation point can be determined every 10 s or so. This methodology is implemented using the data obtained during the early recovery phase of a substorm from an imager operating in Longyearbyen, Norway. A characteristic spatio‐temporal variation of the electron precipitation in the polar cap boundary near midnight is revealed; the region of enhanced differential energy flux repeatedly expanded during the 20‐min interval, reaching nearly 30,000 km 2 on three occasions when evaluated at an altitude of 250 km.