On the nonadiabatic precipitation of ions from the near‐Earth plasma sheet

Abstract

We examine the precipitation of ions which result from nonadiabatic pitch angle scattering in the near‐Earth magnetotail. We focus on dynamical situations between the adiabatic limit where the particle magnetic moment is conserved and the current sheet limit where particles experience meandering motion about the midplane. Defining the κ parameter as the square root of the minimum curvature‐radius‐to‐maximum Larmor radius ratio, the situations we consider correspond to κ between 1 and 3. We demonstrate that throughout this range of κ values, magnetic moment variations are systematically organized into three categories: (1) at small (a few degrees) equatorial pitch angles, large magnetic moment enhancements regardless of gyration phase; (2) at large equatorial pitch angles (typically, above 30°), negligible magnetic moment changes; and (3) in between, a prominent dependence upon gyration phase with either magnetic moment enhancement or damping. We show that these three distinct regimes can be understood in terms of relative magnitude of the centrifugal force experienced by the particles with regard to that of the Lorentz force. In agreement with previous studies, we show that detrapping of plasma sheet ions due to nonadiabatic pitch angle scattering is effective throughout the 1‐ to 3‐κ range. However, the degree of loss cone filling critically depends upon the angular characteristics of the incident population. It is significantly greater for initially isotropic populations than for initially field‐aligned ones, because only particles away from the magnetic field direction experience strong damping of magnetic moment.