THE DIFFUSE AURORA - A SIGNIFICANT SOURCE OF IONIZATION IN THE MIDDLEATMOSPHERE

Energetic electrons can penetrate into the middle atmosphere causing e xcitation, dissociation, and ionization of neutral constituents, resul ting in chemical changes. In this paper, representative electron spect ra measured by the Upper Atmosphere Research Satellite particle enviro nment monitor are used to determine the relative contributions of brem sstrahlung X rays and direct electron impact on the energy deposition and ionization production rates for altitudes between 20 and 150 km. A bove 50 km most of the ionization comes from direct electron impact. H owever, in the stratosphere the energy contributed below 50 km is most ly due to bremsstrahlung X rays. In the diffuse aurora the ionization from the bremsstrahlung component exceeds that due to the galactic cos mic ray background to altitudes as low as 30 km during geomagnetically active periods. This paper demonstrates that a diffuse auroral source can input as much or more energy into the upper portion of the lower and middle atmosphere as previously reported for relativistic electron events. The effects of the diffuse aurora (including both the direct electron and the bremsstrahlung contributions) on atmospheric chemistr y may be significant.