Nonadiabatic ion acceleration at the nightside highlatitude magnetopause: Fine structure of the velocity distribution function

In this paper we use Interball-tail observations and numerical modeling to investigate ion acceleration in reconnected nightside magnetopause. We consider magnetic field configuration corresponding to the superposition of the magnetopause current sheet and reconnected fluxtube, which moves tailward. This fluxtube creates normal component of the magnetic field and the transverse electric field component. Initial current sheet geometry includes tangential and shear components of the magnetic field. Interaction of cold magnetosheath ions with the reconnected current sheet results in particle acceleration and transition through the magnetopause. Reflected and transited ions form velocity distributions with a halo in the (v‖,v⊥) plane. Numerical modeling reproduce these velocity distributions quite well. Comparison of numerical results and spacecraft observation indicates that nonadibatic ion acceleration plays essential role in formation of such velocity distributions.