Pitch angle scattering near energy resonances in the geomagnetic tail

In sharp field reversals where the particle Larmor radius is larger than the magnetic field line curvature radius, particles may not conserve their magnetic moment (first adiabatic invariant). We examine the pitch angle scattering which affects the particles under such conditions. We show that this scattering alternatively;leads to a clear three-branch pattern of magnetic moment variations or to unstructured variations. The three-branch pattern is characterized by systematic enhancements of magnetic moment at relatively small (up to similar to 30 degrees) pitch angles, negligible change at large pitch angles and, in between, either enhancement or damping depending on phase. We show that this three-branch pattern emerges near the energy resonances reported by Burkhart and Chen [1991], where particles escape after transient oscillations inside the reversal. Away from resonance, the three-branch pattern gradually expands and affects an increasing volume of the velocity space. Such a structuring of pitch angle scattering is similar to that obtained in weak field reversals (i.e., for Larmor radii comparable to the field line curvature radius), even though the particle orbits in sharp and weak reversals are significantly different. This fact allows us to extend the centrifugal impulse model developed for weak reversals to sharp ones. In this model, nonadiabatic behavior is viewed as the result of perturbation of the particle gyromotion by an impulsive centrifugal force. We demonstrate that this model reproduces essential features of the particle dynamics in sharp reversals and, in particular, the energy resonance phenomenon. We show that in a like manner to weak reversals, three-branch dynamics in sharp reversals has significant implications for particle injection into the loss cone and gyrophase bunching near the magnetotail midplane.