Phase bunching during substorm dipolarization

We investigate the dynamics of near-Earth plasma sheet ions during storm time dipolarization of the magnetospheric field lines. We more specifically examine the behavior of O+ ions that are trapped in the equatorial vicinity. We show that during field line dipolarization, these particles may be transported in a nonadiabatic manner and experience: large magnetic moment enhancement together with prominent bunching in gyration phase. An analytical estimate is obtained that is in good agreement with numerical trajectory calculations and allows detailed analysis of the adiabatic-nonadiabatic transition. We focus on the gyrophase bunching effect and demonstrate that it occurs below some threshold energy at the dipolarization onset, this threshold energy being controlled by the amplitude of the magnetic transition and by the injection depth in the magnetotail. In the near-Earth tail, the phase-bunched particles possibly experience intense (up to the hundred of keV range) nonadiabatic energization and are radially distributed in a well-structured manner. As a result of this, systematic trajectory computations reveal that phase bunching during storm time dipolarization can lead to significant though localized variations of the cross-tail current and may thus be responsible for large fluctuations of the magnetic field.