Particle energization near an <i>X</i> line in the magnetotail based on global MHD fields

Abstract

Particle acceleration near an X line in the magnetotail and the resulting ion distribution functions in the central plasma sheet are studied by using 2½‐dimensional test particle simulations. The electric and magnetic fields are taken from a three‐dimensional global magnetohydrodynamic (MHD) simulation model describing the solar wind‐magnetosphere interaction. A southward interplanetary magnetic field is chosen as the input solar wind condition. Test particles are generated from a model ion source distribution consistent with the macroscopic MHD parameters in the lobe region. The particles are calculated earthward and tailward of the X line. It is found that ion distributions consist of two main components: a low‐energy population similar to the input distribution and a high‐energy population. The former corresponds to particles that do not cross the region in which the B χ field reverses direction, while the latter consists of particles which cross the midplane, gaining energies on the order of keV. The spatial dependence of the relative population of low‐energy versus high‐energy components is discussed. The details of the acceleration process are determined from the motion of test particles in the simulation.