Abstract
Abstract. We investigate the response of quasi-adiabatic particles to dynamical reconfigurations of the magnetotail field lines. Although they travel through a sharp field reversal with a characteristic length scale smaller than their Larmor radii, these quasi-adiabatic particles experience a negligible net change in magnetic moment. We examine the robustness of such a quasi-adiabatic behavior in the presence of a large surging electric field induced by magnetic field line reconfiguration as observed during the expansion phase of substorms. We demonstrate that, although such a short-lived electric field can lead to substantial nonadiabatic heating, quasi-adiabaticity is conserved for particles with velocities larger than the peak ExB drift speed. Because of the time-varying character of the magnetic field, it is not possible to use the adiabaticity parameter κ in a straightforward manner to characterize the particle behavior. We rather consider a κ parameter that is averaged over equatorial crossings. We demonstrate that particles intercepting the field reversal in the early stage of the magnetic transition may experience significant energization and enhanced oscillating motion in the direction normal to the midplane. In contrast, particles interacting with the field reversal in the late stage of the magnetic transition experience weaker energization and slower oscillations about the midplane. We show that quasi-adiabatic particles accelerated during such events can lead to energy–time dispersion signatures at low altitudes as is observed in the plasma sheet boundary layer.
Highlights
IntroductionCharged particles traveling in a sharp field reversal may not conserve their magnetic moment (first adiabatic invariant) because of significant variations of the magnetic field within a cyclotron turn
Charged particles traveling in a sharp field reversal may not conserve their magnetic moment because of significant variations of the magnetic field within a cyclotron turn
We investigate the response of quasi-adiabatic particles in the presence of a timevarying magnetic field
Summary
Charged particles traveling in a sharp field reversal may not conserve their magnetic moment (first adiabatic invariant) because of significant variations of the magnetic field within a cyclotron turn. For κ between 3 and 1, the motion may become chaotic (Büchner and Zelenyi, 1989) and pitch angle scattering can lead to prominent filling of the loss cone (Sergeev et al, 1983). In this 1 < κ < 3 range, deviation from adiabaticity may be described by considering a perturbation of the particle gyro-motion with a centrifugal impulse (Delcourt et al, 1996). As a result of this centrifugal perturbation, damping or enhancement of the particle magnetic moment (denoted by μ hereinafter) may be obtained depending upon pitch angle and gyration phase; in the long term, this may result in a possibly chaotic behavior with prominent dependence upon initial conditions (see Anderson et al, 1997). For κ smaller than 1, a distinct dynamical regime is obtained since the particle Larmor radius becomes larger than the field line curvature radius and the particle may oscillate on either side of the field reversal midplane
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