Energetic ion composition in the subsolar magnetopause and boundary layer

Energetic ion mass spectrometer data obtained on ISEE 1 have shown that the plasma in the subsolar magnetospheric boundary layer, magnetopause, and adjacent magnetosheath have an ionospheric component (He + and O + ) in addition to the solar wind component (H + and He ++ ). We have examined in detail nine intervals where the location of the subsolar magnetopause and boundary layer are well defined by the ISEE 1 fast plasma and magnetic filed measurements. In five of the identified boundary layer intervals, keV He + ions were observed; energetic O + ions were seen, above background, in two of the boundary layers where He + was observed. The temporal resolution of the ion mass spectrometer was too coarse to rule out the possibility of He + or O + ions being present in the four remaining intervals. Ionospheric He + ions were also observed in the magnetosheath in two (and O + in one) of the nine intervals studied.