Gyrophase effects in the centrifugal impulse model of particle motion in the magnetotail

We investigate the dynamics of charged particles in a magnetic field reversal in the particular case where the Larmor radius is comparable to the magnetic field line curvature radius. In the current interpretation framework based upon the parameter κ (defined as the square root of the minimum curvature radius-to-maximum Larmor radius ratio), this situation corresponds to κ of the order of 1. We show that this nonadiabatic regime, which lies at the transition between adiabatic motion (κ ≫ 1) and the nonadiabatic one characterized by oscillations about the field minimum (κ < 1), results from prominent centrifugal effects in the field reversal. To model these effects, we develop a simple analytical description which is based upon an impulsive centrifugal force perturbing the particle gyromotion on the time scale of the cyclotron turn. Comparisons with numerical calculations of ion trajectories demonstrate that this centrifugal impulse model adequately describes both the magnetic moment variations and the gyrophase variations experienced by the particles. As for the magnetic moment, three distinct behaviors (negligible change, strong phase dependence with possible damping, and systematic enhancement) are identified depending upon pitch angle. As for the gyration phase, important bunching effects are obtained which are interpreted as being due to the action of the impulsive centrifugal force. These phase bunching effects are enhanced as the κ parameter approaches unity. In the limit κ ∼ 1, a strong imbalance is obtained between the two phase sectors corresponding to duskward and dawnward motions. This imbalance, which extends over a few tenths of an Earth radius in the Z direction, leads to the formation of a thin current sheet in the vicinity of the magnetotail midplane.