Proton velocity distributions in the magnetotail: Theory and observations

The structure of a plasma sheet ion distribution is modeled by using the large-scale kinetic approach in which the trajectories of a distribution of plasma mantle particles are followed through the magnetotail. In the plasma sheet boundary layer, beamlets that originate from localized regions in the current sheet maintain their identities even after multiple interactions with the current sheet. Because of their origin, beamlet distribution functions appear to be phase bunched and azimuthally asymmetric, even far from the current sheet. Structures in the distribution of ions manifest themselves in both configuration space and velocity space. At the outer edge of the plasma sheet the particles experiencing the greatest acceleration in the distant magnetotail form a structure with multiple peaks in velocity space. When earthward and tailward beamlets are found in the same region, there is an increase in density and a decrease in bulk velocity. At the outer edge of the plasma sheet and thus near the boundary layer, counterstreaming distributions are observed. In the plasma sheet, two populations are found: beamlets with large drift velocity and a chaotic counterpart which has been thermalized by multiple crossings of the current sheet. In the vicinity of the earthward edge of the plasma sheet, the two populations merge to become an isotropic distribution function. A detailed comparison of these results with observations from the plasma instrument (PLS) on the Galileo spacecraft during the Earth 1 flyby is presented. The principal features of the observed and computed distributions are in substantial qualitative agreement.