Incoherent radar spectra in the auroral ionosphere in the presence of a large electric field: The effect of O⁺-O⁺ Coulomb collisions

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We have used Monte Carlo simulations of O⁺ velocity distributions in the high latitude F- region to improve the calculation of incoherent radar spectra in auroral ionosphere. The Monte Carlo simulation includes ion-neutral, O⁺-O collisions (resonant charge exchange and polarization interaction) as well as O⁺-O⁺ Coulomb self-collisions. At high altitudes, atomic oxygen O and atomic oxygen ion O⁺ dominate the composition of the auroral ionosphere and consequently, the influence of O⁺-O⁺ Coulomb collisions becomes significant. In this study we consider the effect of O⁺-O⁺ Coulomb collisions on the incoherent radar spectra in the presence of large electric field (100 mVm^-1). As altitude increases (i.e. the ion-to-neutral density ratio increases) the role of O⁺-O⁺ Coulomb self-collisions becomes significant, therefore, the one-dimensional, 1-D, O⁺ ion velocity distribution function becomes more Maxwellian and the features of the radar spectrum corresponding to non-Maxwellian ion velocity distribution (e.g. baby bottle and triple hump shapes) evolve to Maxwellian ion velocity distribution (single and double hump shapes). Therefore, O⁺-O⁺ Coulomb self-collisions act to isotropize the 1-D O⁺ velocity distribution by transferring thermal energy from the perpendicular direction to the parallel direction, however the convection electric field acts to drive the O⁺ ions away from equilibrium and consequently, non-Maxwellian O⁺ ion velocity distributions appeared. Therefore, neglecting O⁺-O⁺ Coulomb self-collisions overestimates the effect of convection electric field.