Dynamics of charged particles in bifurcated current sheets: The κ ≈ 1 regime

Abstract

We examine the nonlinear dynamics of charged particles in the double‐hump current sheets that develop during the late growth phase of substorms. We focus on particles for which the adiabaticity parameter κ (defined as the square root of the minimum curvature radius to maximum Larmor radius ratio) is of the order of unity. We show that as in the case of a simple parabolic field reversal, the magnetic moment scattering experienced by these particles may be described as the result of perturbation of the gyromotion by an impulsive centrifugal force. Here however the double‐hump structure of the current sheet leads to two successive centrifugal perturbations, which has a significant impact on the net change of magnetic moment. In a single‐hump current sheet, three distinct regimes of magnetic moment variations are obtained for a given value of κ, namely, systematic enhancement at small pitch angles, negligible change at large pitch angles, and in between either damping or enhancement depending upon gyration phase. In contrast, in a double‐hump current sheet, repeated application of this three‐branch pattern can lead to magnetic moment damping for particles that previously experience magnetic moment enhancement and vice versa. The gyrophase gain both during and between the centrifugal impulses is found to play an essential role in the net magnetic moment change. In particular, in contrast to single‐hump current sheets, the κ ≈ 1 limit may be characterized by quasi‐adiabatic behavior with negligible variations of the magnetic moment.