Articles | Volume 35, issue 1
Regular paper
03 Jan 2017
Regular paper |  | 03 Jan 2017

On the response of quasi-adiabatic particles to magnetotail reconfigurations

Dominique C. Delcourt, Helmi V. Malova, and Lev M. Zelenyi

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.

Short summary
In a magnetic field reversal, the guiding center may not be valid due to large variation of the magnetic field on the length scale of the particle gyro-radius. Although they do not execute regular helical motion and temporarily meander inside the field reversal, quasi-adiabatic particles exit this reversal with a magnetic moment nearly identical to that at entry. We show that this behavior, which is a steady-state concept, can persist during dipolarization despite the induced electric field.