Investigation of the Initial Stages of Ion Heating in the Topside Ionosphere

Simultaneous observations of ion distribution functions and plasma waves in the topside ionosphere during an auroral substorm near local midnight are presented. A low-energy (0 -- 20 eV) ion spectrograph was flown on a sounding rocket and observed over 100 small-scale heating structures with cross-field widths on the order of 50 m. However, the ambient ion temperature remained a few tenths of an eV throughout the flight. Several well-known mechanisms of ion heating in this region are investigated. For example, electromagnetic fields in resonance with ions at their cyclotron frequency have been shown to be capable of energizing ions to tens of eV in seconds (Chang et al., GRL 13,636,1986; Knudsen et al., JGR 103,4171,1998; André et al., JGR 103,4199,1998). On this flight both electric wave power at the local O+ cyclotron frequency and electron fluxes exceeded established thresholds for the onset of ion energization. However, the large plasma density (>2*E⁴ {cm}^-3) may play a role in the observed lack of ion heating. The implications of these observations for theories of ion energization will be discussed.