Heating of Thermal Ions Near the Equatorward Boundary of the Mid-Altitude Polar Cleft

Intense, energetic oxygen ions are frequently observed to be heated near the equatorward boundary of the mid-altitude polar cleft on the Dynamics Explorer -1 satellite. These observations confirm reports from the VIKING satellite. In this paper we present examples of heating of oxygen ions equatorward of the ‘central’ cusp/cleft region and use simultaneously obtained low-frequency electric and magnetic field observations to quantitatively test resonant and non-resonant heating processes. We find that, within observational and model uncertainties, the heating rates inferred from observed electric field spectra are large enough to produce the observed perpendicular oxygen temperatures in time scales determined by the measured poleward drift rate of thermal oxygen ions. The examples presented provide a good comparison of basic plasma theory and observation.