Species dependent energies in upward directed ion beams over auroral arcs as observed with FAST TEAMS

Abstract

Upward flowing field‐aligned ion beams over auroral arcs have been observed with the 3‐dimensional ion mass spectrograph TEAMS on FAST. We have performed a statistical study on a sample of 77 ion beams from the auroral campaign in early 1997. All observed beams contain substantial amounts of H + , He + and O + . A clear ordering of the total energies according to mass is found, with H + having the lowest and O + the highest energy. The composition varies significantly from beam to beam, with O + /H + ratios ranging from ≈ 0.1 to 10. No variation of the energy ratio between species is observed as a function of relative abundance. These results are discussed in the light of earlier observations of higher energies for O + in statistical studies of beams during solar minimum and attempts to explain this behavior in terms of beam instabilities.