Monte Carlo calculations of the O+ velocity distribution in the auroral ionosphere

The O+ velocity distribution in the high latitude F region has been calculated by using a Monte Carlo technique. The calculations were restricted to the altitudes where O+ −O collisions dominate, i.e., between about 200–350 km. The Monte Carlo calculations confirm the earlier predictions, which were based on a simple relaxation collision model, that the O+ velocity distribution takes a toroidal shape for large convection electric fields. However, the toroidal features appear only for electric fields greater than about 80–100 mV/m when a more rigorous ion neutral collision model is used, whereas the relaxation collision model predicted toroidal features for electric fields greater than about 40 mV/m. Since a toroidal velocity distribution is unstable, ionospheric irregularities can be expected at altitudes between 200–350 km when the convection electric field exceeds 80–100 mV/m.