ISOTROPIC AND PERPENDICULAR ENERGIZATION OF OXYGEN IONS AT ENERGIES BELOW 1 EV

We present observations of low-energy O+ ion distributions obtained by the Freja satellite at altitudes between 1100 and 1600 km in the days ide high-latitude magnetosphere. The particle observations are obtaine d by the Freja ion mass spectrometer designed to observe ion energies up to several keV. We show that such instruments can also be used to a ccurately measure temperatures less than 1 eV, by operating the instru ment at a fixed energy level of a few eV. When the mass spectrometer i s operated at an energy approximately equal to the energy of O+ ions m oving with the satellite velocity (several km/s), the satellite motion and the detector viewing directions become important. The idea is tha t the satellite ram effect together with different instrument viewing directions related to the satellite spin can allow sampling of differe nt regions of ion velocity space. We find that isotropic O+ energizati on dominates at mean energies below about 0.4 eV, while energization p erpendicular to the geomagnetic field dominates at higher energies. Th e isotropic ion distributions may exhibit a bulk motion up or down alo ng the geomagnetic field, while the perpendicularly heated distributio ns are moving upward. The perpendicular energization occurs in the day side auroral region including the cusp/cleft, the source region of the so-called cleft ion fountain. The perpendicular heating to a few eV c an be associated with weak broadband low-frequency electric wave field s, similar to the stronger emissions causing O+ energization to hundre ds of eV. However, at low energies other mechanisms map also be import ant.