Observations of transverse ion acceleration in the topside auroral ionosphere

The paper reports data obtained from a sounding rocket flight which reached an apogee of 927 km and passed through several auroral arcs. Therma/superthermal ions were sampled by charged particle analyzers which allowed for a rapid (about 1 s) sampling of their distribution function. During portions of the flight when the rocket was not in an energetic auroral structure, the ion data are fit to a Maxwellian function which yields the plasma parameters. Throughout the middle portion of the flight, above 700-km altitude, ion distributions having a superthermal tail were measured. When the rocket was immersed in energetic auroral electron precipitation, two other ion distributions were observed. Transversely accelerated ions which represented bulk heating of the ambient population were observed continuously in these arcs. The characteristic perpendicular energy of the transversely bulk heated ions reached as high as 3 eV compared to typically less than 0.4 eV during nonauroral times. The observations are discussed in terms of some current theories of transverse ion energization.