Wave generation by ion horseshoe distributions on auroral field lines

Recent observation by Freja satellite of ions in the auroral zone at the bo undary of the plasma sheet and tail lobe reveal the presence of a ring or h orseshoe type distribution function. The horseshoe type distribution functi on 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 velo city of about 400km/sec. We show that the horseshoe type distribution is un stable to predominantly perpendicular ion waves with frequencies in the low er-hybrid range.