Nightside sounding rocket study of precipitating ions in a Poleward Boundary Intensification

The sounding rocket Cascades-2 launched on 20 March 2009 from the Poker Flat Research Range at 11:04:00 UT. The rocket initially crosses a diffuse arc, then crosses a streamer, which is the equatorward extent of one PBI (poleward boundary intensification), and finally crosses the initiation of a separate PBI before entering the polar cap. In this presentation we examine the in situ ion signatures at various times in the flight. At the poleward edge of the electron precipitation, as the rocket crosses the PBI, medium energy (between 10 eV and 800 eV), precipitating ions are observed. This population is not present at the time of the streamer or earlier in the flight. We hypothesize that the precipitating ions are drifting into the auroral region from the polar cap, which is why they are observed at the poleward boundary of the auroral oval. There they encounter the Alfvenic activity which is creating the soft electron precipitation of the PBI. This Alfvenic activity accelerates them down the field to the observation point, with dispersion signatures indicating wave-particle interactions at similar altitudes to those of the field-aligned electron bursts. The precipitating ion population is depleted in the poleward boundary region, and thus not observed in the more equatorward streamer. Similar signatures have been observed on previous nightside auroral sounding rockets at the poleward boundary by Lynch et al. [Annales Geophysicae, 2007]. In this presentation we examine the rocket data along with various scenarios for ion source regions to check the plausibility of our hypothesis. In particular we consider the scenario by which the source of these ions is the dayside cusp/cleft ion fountain upflow region.