SPATIAL STRUCTURE AND GRADIENTS OF ION-BEAMS OBSERVED BY FAST

High time resolution measurements of ion distributions by the FAST sat ellite have revealed kilometer scale spatial structure in the low alti tude auroral acceleration region. The low altitude edge of the acceler ation region appears to contain fingers of potential that extend hundr eds of kilometers along B but are only a few to tens of kilometers wid e. These fingers of potential do not appear to be strongly correlated with the local current or total potential drop. Gradients in the ion b eam energy are found to be consistent with the electric field signatur es expected in the quasi-static potential drop model of auroral accele ration. Typical ion beams show gradients of 0.5-1.0 keV/km, with some events as large a 3 keV/km. Integrations of the electric field along t he spacecraft velocity are used to calculate parallel potential below FAST and are found to agree well with the ion beam energy for most eve nts. One event is shown where an apparent temporal change in the auror al configuration occurs at the edge of the ion beam producing a disagr eement between the beam energy and inferred potential.