a Simulation Study of Beam Driven Instabilities and Solitary Structures in the Auroral Acceleration Region.

Various problems associated with the model particle distributions and assumptions for auroral acceleration are considered using particle in cell plasma simulation techniques. We have considered the presence and effects of microinstabilities in two regions along auroral field lines. At low altitude, where ions which have been accelerated up out of the ionosphere have a small relative drift (due to mass ratio), the plasma is unstable to an ion two-stream instability. We will show that the heating due to the two-stream instability does not distort the distribution function sufficiently to prevent double layer formation at higher altitude. However, under certain circumstances heating by the oblique two-stream modes is capable of forming conic like distributions. At higher altitude, it has been suggested that the presence of a background ion population may have a significant effect on the formation of weak double layers. We will show that in such a system, a weak beam-plasma instability couples to the acoustic modes in the background ion population seeding the formation of ion phase space holes. Such structures propagate at the acoustic speed of the background ion population, in the direction of the ion beam (opposite of the direction of propagation for such structures in the standart two-species model) and, given a drift of the background electrons, they may develop into double layers. This may explain previous discrepancies between observations of such structures and theoretical prediction of their behavior.