Field‐aligned electron precipitation at the edge of an arc

Measurements of field‐aligned electrons at the edge of an arc are presented from a sounding rocket flight through a quiet afternoon auroral arc. High time resolution measurements show the evolution of the electron distribution function over the 2.3‐km width of field‐aligned precipitation. A nearly constant 1.2‐eV perpendicular temperature was found for these field‐aligned fluxes over a broad range of parallel energies (100–900 eV). The small perpendicular temperature indicates that the acceleration region is located at a low altitude. Two models of cold plasma convection into the edge of a V‐shaped potential structure are examined and found consistent with both the observed fluxes and spatial width of the edge precipitation. Both models predict an average source plasma density less than 130/cm³ to account for the observed field‐aligned fluxes and thus an acceleration region well above 1000 km.