Pitch angle scattering near energy resonances in the geomagnetic tail

In sharp field reversals where the particle Larmor radius is larger than th e magnetic field line curvature radius, particles may not conserve their ma gnetic moment (first adiabatic invariant). We examine the pitch angle scatt ering 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 i s characterized by systematic enhancements of magnetic moment at relatively small (up to similar to 30 degrees) pitch angles, negligible change at lar ge pitch angles and, in between, either enhancement or damping depending on phase. We show that this three-branch pattern emerges near the energy reso nances reported by Burkhart and Chen [1991], where particles escape after t ransient oscillations inside the reversal. Away from resonance, the three-b ranch pattern gradually expands and affects an increasing volume of the vel ocity space. Such a structuring of pitch angle scattering is similar to tha t obtained in weak field reversals (i.e., for Larmor radii comparable to th e field line curvature radius), even though the particle orbits in sharp an d 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 perturbatio n of the particle gyromotion by an impulsive centrifugal force. We demonstr ate 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 shar p reversals has significant implications for particle injection into the lo ss cone and gyrophase bunching near the magnetotail midplane.