O+ ion temperature partition coefficients β∥ and β⊥: The Effect of O+–O+ Coulomb self‐collisions

Abstract

We have computed altitude profiles for O+ ion temperature partition coefficients β∥ and β⊥ in the auroral ionosphere for different values of electric field with the use of a Monte Carlo simulation. The Monte Carlo model includes the effect of E × B drift, O+–O (resonant charge exchange and polarization interaction) collisions, and O+–O+ Coulomb self‐collisions. These effects were included self‐consistently in the computations. At low altitudes, the O+ ion concentration is small compared with the O concentration; thus the role of O+–O+ Coulomb self‐collisions in isotropizing the O+ ion velocity distribution is negligible and hence non‐Maxwellian features (β⊥ > β∥) are obtained due to the effect of O+–O collisions. However, as altitude increases, the O+ concentration increases and consequently, the role of O+–O+ Coulomb collisions becomes significant in transferring energy from the field‐perpendicular direction to the field‐parallel direction. This explains the increase of β∥ and the decrease of β⊥ with altitude. Also, we have investigated the variation of β∥ and β⊥ with the electric field and found that as electric field increases, O+ ion temperature increases, β∥ decreases, and β⊥ increases due to the interplay between E × B drift and O+–O collisions. In other words, the effect of O+–O+ Coulomb collisions becomes less important because these Coulomb collisions are in turn dependent on O+ ion temperature. Therefore the combined effects of E × B drift, O+–O collisions, and O+–O+ Coulomb collisions determine the altitude profiles of β∥ and β⊥. Finally, a comparison has been made between the Monte Carlo calculations obtained in this paper and observations of the O+ ion temperature partition coefficient β∥. The comparison showed a remarkably close agreement in the corresponding results for the altitude variation of β∥. This close agreement provides further evidence that the Monte Carlo model described in this paper is a powerful tool for studying O+ ion behavior in the auroral ionosphere, especially when O+–O+ Coulomb self‐collisions are included. As a result of the comparison, we were able to predict the real values of the convection electric field in the auroral ionosphere for three ion heating events.