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

Abstract 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 sheetand reconnected fluxtube, which moves tailward. This fluxtube creates normal component of the magnetic field and the transverse electric field component. Initial current sheetgeometry includes tangential and shear components of the magnetic field. Interaction of cold magnetosheathions with the reconnected current sheetresults in particle accelerationand 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.