An alternative interpretation of ion ring distributions observed by the S3‐3 satellite

Abstract

Recently, theoretical interest has grown in possible plasma wave instabilities arising from free energy in auroral ion distributions, and a number of ion‐mode instabilities have been identified. These instabilities typically rely on positive slopes in the ion's velocity distribution parallel (∂f/∂v∥ > O) or perpendicular (∂f/∂v⊥ > O) to the magnetic field. Upflowing ion beams generated by electrical potential drops along auroral field lines are obvious candidates for the parallel (beam) instability, while ion conics have been treated in the limit of the flute mode instability, assuming that a positive perpendicular gradient exists. However, ion conics are not responsible for the condition ∂f/∂v⊥ > O. Downflowing ion beam distributions can have ∂f/∂v⊥ > O and therefore might lead to flute mode instability. Examples of both conics and downflowing beams are presented showing that only the downflowing component leads to significant ∂f/∂v⊥ > O, while ion conics generally have ∂f/∂v⊥ < O and are themselves stable to the flute mode.