Toward a unified view of diffuse auroral precipitation

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Abstract
A theoretical description of the mechanisms responsible for diffuse aurora is presented. Emphasis is placed on explaining the origin of the quiet time proton strong diffusion precipitation and its approximate coincidence with the zone of more intense electron precipitation. During quiet geomagnetic conditions, resonant scattering by electrostatic waves is considered the most viable mechanism. It is shown that the broadband electrostatic noise recently detected on auroral field lines by Gurnett and Frank can adequately account for the observed precipitation of ions up to 100 keV. Near the equatorial plane these waves can be destabilized by the normal loss cone pitch angle distribution of plasma sheet protons. Closer to the earth the waves are more readily excited by the field‐aligned currents which are a permanent feature of the auroral flux tubes. A numerical simulation of the convective growth rate in these physically distinct regions is presented, and examples are given of the anticipated polarization and spectral characteristics of the ensuing turbulence. An analysis is also made of the resonant and nonresonant diffusion of plasma on the auroral field lines. Of particular importance is the rapid Landau resonant heating of ionospheric electrons by the ion mode turbulence. The ultimate demise of the outflowing ionospheric electrons is of crucial importance, since a small concentration of cold electrons can quench the loss cone instability of ion cyclotron waves near the equator. This is invoked to explain the restriction of intense broadband electrostatic noise and the concomitant plasma sheet proton precipitation to auroral flux tubes which carry sufficient field‐aligned current to excite ion mode waves (and thus heat electrons) in the topside ionosphere.