Ion velocity distribution function in the upper auroral F region: 1. Phenomenological approach

Abstract

We propose an expression for the O+ atomic oxygen ion velocity distribution in the upper part of the auroral F layer when the ions are subjected to an intense convection electric field. We assume that this layer is made up of neutral O atomic oxygen and atomic O+ ions. In order to obtain the O+ ion velocity distribution function, we use the Boltzmann equation with a modified relaxation collision term. The evolution of this function is governed by the convection electric ionospheric fields and by the ion‐neutral and ion‐ion collisions. To investigate these two collision effects, we use a new phenomenological and self‐consistent model. We obtain the non‐Maxwellian ion distribution function as a linear combination of a toroidal anisotropic contribution and of a Maxwellian isotropic contribution. The coefficients of this linear combination depend on the ion‐neutral and ion‐ion collision frequencies νii and νin. We self‐consistently calculate the expressions and values of these two frequencies. We note that the νii/νin ratio increases when the electric field increases and that this ratio is greater in our model than for the Maxwellian steady state. At 500 km the O+ ion velocity distribution function is almost isotropic, but at 300 km the distribution is anisotropic when the magnitude of the ionospheric electric field is very large.