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

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.