Preferential O+ heating in the topside ionosphere

We report observations of preferential heating of the ionospheric majority ion species O+. The heating took the form of a hot (few electron volts) tail in the upgoing thermal O+. No such hot tail was observed in the light ion distributions. An event observed at 700‐800 km altitude was highly anisotropic with a much hotter tail transverse to the local magnetic field. A series of events observed at 250‐375 km were more nearly isotropic with respect to the local magnetic field. Broadband plasma wave emission near the lower hybrid resonance was observed in conjunction with the high altitude event. The low altitude events were tightly correlated with auroral electron precipitation, but broadband lower hybrid emission was much reduced. The observed hot tail formation is sufficient to produce significant O+ ion transport to higher altitude auroral acceleration regions.