Stability of the polar wind

Abstract

The classical polar wind is an ambipolar outflow of thermal plasma from the terrestrial ionosphere at high latitudes. At altitudes above about 3000 km, the H+ flow becomes supersonic and collisionless, and the H+ velocity distribution becomes non‐Maxwellian. The non‐Maxwellian features include a temperature anisotropy, with the parallel H+ temperature greater than the perpendicular temperature (Ti∥ > Ti⊥), and an asymmetry, with an elongated tail in the upward direction. These distortions from a Maxwellian increase as the H+ gas escapes in the diverging geomagnetic field, and at 10 earth radii the parallel‐to‐perpendicular temperature ratio is about 50 and the elongated tail is sufficiently long to move the drift velocity point off the peak of the distribution function. The stability of these highly non‐Maxwellian H+ velocity distributions was studied with regard to the excitation of electrostatic waves and the plasma was found to be remarkably stable for a wide range of electron temperatures (0.1 ≤ Te/Ti∥ ≤ 10). This indicates that the various macroscopic formulations of the classical polar wind are valid. The stability of a perpendicularly heated polar wind was also studied, assuming bulk perpendicular heating of H+ in the cusp, followed by the subsequent convection of the heated plasma into the polar cap. Two regions of instability were found. For high electron temperatures (Te/Ti∥≳5), the plasma is unstable for Ti⊥/Ti∥ > 4, while for lower electron temperatures the plasma is unstable only for Ti⊥/Ti∥ > 30.