O+ and H+ above the polar cap: Observations and semikinetic simulations

Abstract

AbstractA one‐dimensional direct simulation Monte Carlo model is used to study the outflow of O+ and H+ ions from 1.2 RE to 15.2 RE along two flight trajectories originating from the polar cap, namely, the central polar cap (CPC) and the cusp. To study the effect of varying geophysical conditions and to deduce the proper set of parameters, several parameters were varied, and the results were compared to corresponding data from Cluster spacecraft. First, several sets of diffusion coefficients were considered based on using diffusion coefficients calculated by Barghouthi et al. (1998), Nilsson et al. (2013), and Abudayyeh et al. (2015b) for different altitude intervals. It was found that in the central polar cap using the diffusion coefficients reported by Barghouthi et al. (1998) for altitudes lower than 3.7 RE, zero diffusion coefficients between 3.7 and 7.5 RE and diffusion coefficients from Nilsson et al. (2013) for altitudes higher than 7.5 RE provide the best fit for O+ ions. For O+ ions in the cusp the best fit was obtained for using Barghouthi et al. (1998) diffusion coefficients for altitudes lower than 3.7 RE and Nilsson et al. (2013) diffusion coefficients for altitudes higher than that. The best fit for H+ ions in both regions was obtained by using the diffusion coefficients calculated by Abudayyeh et al. (2015b). Also, it was found that along an ion's trajectory the most recent heating dominates. Second, the strength of centrifugal acceleration was varied by using three values for the ionospheric electric field, namely, 0, 50, and 100 MV/m. It was found that the value of 50 MV/m provided the best fit for both ion species in both regions. Finally, the lower altitude boundary conditions and the electron temperature were varied. Increasing the electron temperature and the lower altitude O+ parallel velocity were found to increase the access of O+ ions to higher altitudes and therefore increase the density at a given altitude. The variation of all other boundary conditions only affected the densities of the ions and not the other moments due to the overwhelming effect of wave‐particle interaction. Furthermore, varying the parameters of one ion species has no effect on the other ion species. We also compared the energy gain per ion due to wave‐particle interaction, centrifugal acceleration, and ambipolar electric field and found that wave‐particle interaction is the most important mechanism, while ambipolar electric field is relatively unimportant especially at higher altitudes.