Dynamics of pick-up ions in collisionless velocity shears

We study the motion of charged particles in large-scale velocity shears that are produced in the interaction of magnetized plasma winds and plasma obstacles. The purpose of the analysis is to account for the observation of strongly energetic contaminant ions in the region of interaction of the solar wind with planetary/cometary non-magnetic ionospheric obstacles (Venus, Mars, comets). The convective electric field set up by the streaming plasma is incorporated to the equation of motion of ions born in a velocity shear. Neglecting collisions and the back reaction of the contaminant particles on the wind, the trajectories of the particles are computed as a function of the shear properties as well as of the mass of the ions and the magnetic field configuration. For a linear dependence of the wind velocity across the shear, the problem is solved analytically and we find that the particle velocity can have either a purely oscillatory behavior or grow exponentially with time depending on the value of a dimensionless parameter proportional to the product of the velocity gradient and the cyclotron frequency of the ion trajectories. In the latter case a strong acceleration of the contaminant ions can be achieved. Adopting magnetic field and flow properties appropriate for cometary and planetary environments, we explore the potential importance of the mechanism discussed to explain the presence of superthermal ions and filamentary structures in such regions.