Wave generation by ion horseshoe distributions on auroral field lines

Recent observation by Freja satellite of ions in the auroral zone at the boundary of the plasma sheet and tail lobe reveal the presence of a ring or horseshoe type distribution function. The horseshoe type distribution function measured by the satellites has a field aligned component which is moving in the case of the Freja data in the down‐going direction with a peak velocity of about 400km/sec. We show that the horseshoe type distribution is unstable to predominantly perpendicular ion waves with frequencies in the lower‐hybrid range.