A common observation made by the toroidal imaging mass-angle spectrograph on the polar orbiting Polar Spacecraft is the acceleration of ionospheric O+ and H+ to similar velocities on open field lines in the low-latitude boundary layer (LLBL) and cusp regions. A detailed study of nine events made in the midaltitude cusp region is presented here and the acceleration can be split into two regions. On open field lines in the LLBL both H+ and O+ undergo rapid upward acceleration within approximately 0.75¿ invariant latitude. The acceleration aH+/aO+≈2 is inconsistent with the proposed electrostatic acceleration. Intense electrostatic waves often associated with the acceleration is consistent with energy transfer from the faster H+ to the slower O+ population via two-stream instabilities. In the cusp region injected magnetosheath plasma is a possible free energy source for the acceleration of ionospheric ion populations. The component temperature ratio T∥/T⊥ of the upgoing, mirrored injected magnetosheath ions is generally lower than the downgoing ions consistent with a two-stream instability driven energy transfer from the magnetosheath to the ionospheric ion populations. ¿ 2001 American Geophysical Union