Dynamics of pick-up ions in turbulent electromagnetic fields in the magnetic polar regions of Mars ionosheath

We present results of the numerical simulation of the dynamics of newly born O+ ions in conditions representative of the magnetic polar regions of the martian ionosheath. The magnetic field configuration is constructed as a superposition of a uniform component and a strongly fluctuating, turbulent component characterized by a power law spectrum. The dynamics of a large number of particles, at least 10^4, is calculated to estimate the average properties of the O+ ion population. Results are obtained for different fluctuation amplitudes and spectrum properties. In addition, we study the effect of a solar wind velocity shear, with velocity increasing with height over the magnetic poles of the planet, as suggested from in situ measurements of the Mars Express spacecraft. We find that depending on the properties of the fluctuating magnetic field component, the average velocity of O+ ions over the magnetic poles may differ significantly from the simple cycloidal motion obtained in the case of a uniform, not fluctuating magnetic field. Particular cases are shown in which the properties of the average O+ ion velocity appear to be consistent with in situ measurements with the ASPERA-3 instrument onboard the Mars Express spacecraft.