Particle Dynamics Near κ = 1 and the Centrifugal Impulse Model

Abstract

Charged particle motion in magnetic field reversals is well understood in two limiting regimes: the adiabatic regime where the guiding center theory and adiabatic invariance are valid, and the current sheet regime where one can separate timescales of the characteristic z-oscillation and gyromotion about the field normal to the reversal plane. Both approximations fail when the magnetic field curvature radius is on the order of the particle gyroradius, i.e. when the ratio of minimum curvature radius to maximum gyroradius, κ2, is near 1. Near κ = 1 the motion has been termed “strongly” or “globally” chaotic, and several proposed applications exploit this chaotic behavior. However, it has been shown recently that there is a characteristic “three-branched” behavior in this regime only part of which is related to nonadiabaticity and the phase dependence which leads to chaos. Moreover, a simple physical model attributing the nonadiabatic jumps in magnetic moment to an impulsive centrifugal force, is able to reproduce this behavior. We give further justification to this centrifugal effect and scrutinize the limits of applicability of this centrifugal impulse model. A phenomenological extension to higher κ values is developed, which extends the model's validity in pitch and phase angle as well.