Application of the centrifugal impulse model to particle motion in the near‐Earth magnetotail

Abstract

Particles traveling in the geomagnetic tail do not conserve their magnetic moment (first adiabatic invariant) due to significant field variations on the length scale of their Larmor radius. We examine the possibility of describing these magnetic moment changes by the action of an impulsive centrifugal force on the timescale of the particle cyclotron turn. Trajectory calculations demonstrate that such a centrifugal impulse model adequately describes the nonadiabatic particle behavior for situations where the κ parameter (defined as the square root of the minimum curvature radius‐to‐maximum Larmor radius ratio) is of the order of 1 to 2. In particular, it is shown that this behavior can be organized by another parameter (referred to as κα), which is proportional to κ but depends upon the particle pitch angle, namely, one obtains: systematic magnetic moment enhancements for κα ≪ 1, large gyrophase effects with possibly prominent damping of the magnetic moment for κα ∼ 1, and nearly constant magnetic moment for κα ≫ 1. More generally, we show that the centrifugal impulse approximation applies to an intermediate orbit regime at the transition between the fully adiabatic (magnetic moment conserving) regime and that regime where particles experience meandering motions about the field minimum. It applies to ion transport in the near‐Earth magnetotail where the magnetic field lines evolve from dipolar to taillike configurations and where the κ parameter nears unity. In this region of space the model predictions are in agreement with numerical results, revealing both enhanced trapping (due to magnetic moment enhancement) and possible precipitation (due to magnetic moment damping) of plasma sheet ions depending upon their pitch and phase angles.