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 400 km/sec. We show that the horseshoe type distribution is unstable to predominantly perpendicular ion waves with frequencies in the lower-hybrid range. ¿ 1999 American Geophysical Union