Mechanisms for auroral precipitation: A review

Abstract

Theories for and observations relating to auroral precipitation are reviewed for purposes of assessing our understanding of the aurora and of identifying those processes which play the most important role in auroral precipitation. For purposes of the review, four categories of auroral forms are defined: (1) the diffuse aurora, which usually forms the equatorward boundary of the auroral oval, (2) auroral patches and pulsations, which appear most commonly within the morning sector of the diffuse aurora, (3) the discrete aurora, most prominently observed in the evening and poleward sectors of the auroral oval, and (4) the ‘inverted‐V’ aurora, which appears in the same regions as discrete aurora but with much larger size scales. One principal conclusion is that the diffuse aurora, likely caused by both proton and electron precipitation, is the result of pitch angle scattering by electrostatic cyclotron waves. Another is that the auroral pulsations and patches are the result of pitch angle scattering of more energetic electrons by electromagnetic whistler mode waves. The patches may represent flux tubes of enhanced ionization tied to the lower ionosphere. The discrete aurora is the result of electron acceleration parallel to the magnetic field by the quasi‐static electric field of a current‐driven laminar electrostatic V shock. The theory for inverted‐V precipitation is not well developed, but observations strongly suggest that it is related to ion cyclotron anomalous resistivity.