ON THE IMPROVEMENT OF ANALYTICAL CALCULATIONS OF COLLISIONAL AURORAL ION VELOCITY DISTRIBUTIONS USING RECENT MONTE-CARLO RESULTS

The combined effect of ion-neutral collisions and strong dc electric f ields is known to drive marked departures from a simple Maxwellian sha pe in the supersonic ions of the auroral ionosphere. With the current strong interest in investigating highly disturbed periods, an accurate knowledge of the ion distribution function is gaining in importance. It is known that the shape of the ion velocity distribution below 500 km can have a pronounced toroidal character in the case of O+ ions whi le looking more like an ellipsoid of revolution in the NO+ case. Calcu lations of these effects have been made using analytical expansion tec hniques as well as numerical Monte Carlo simulations. While the two ap proaches yield results that are in qualitative agreement with each oth er, we show that there can at times be some important quantitative dif ferences between the two techniques. This should not be surprising in view of the fact that analytic expansions are based on collision, mode ls that are independent of the relative energy between colliding parti cles, while the more recent Monte Carlo calculations do not suffer fro m this limitation. This drawback notwithstanding, the advantage of ana lytical techniques lies with the speed of the computations; these tech niques are also free of the statistical noise that can clutter the Mon te Carlo results. The latter can, however, be used to improve the accu racy of the analytical calculations and therefore their usefulness. Th is is done here by obtaining a new model of collision cross section in tegrals which is extracted from the more recent Monte Carlo results. F or strong dc electric field conditions, the new model can, in all case s, greatly enhance the accuracy of the analytical temperature calculat ions. The new model also produces overall shapes that are in better ag reement with the numerical simulations in the NO+ case. However, even with the new generalized cross section model, the analytical descripti on of O+ ions remains problematic above 100 mV/m in an atmosphere with a strong atomic oxygen content.