ONSET OF NONADIABATIC PARTICLE MOTION IN THE NEAR-EARTH MAGNETOTAIL

The onset of nonadiabatic proton motion is studied using direct integr ation of the Lorentz force equation of motion in the T89c magnetic fie ld model with no electric field. Irreversible changes in the magnetic moment mu occur on traversals of the equator and give the gyrophase de pendence predicted by Birmingham [1984]. Birmingham's expression delta (B) and the semiemipirical centrifugal impulse model of Delcourt et al . [1996] delta(CIM2) have linear regression coefficients with Delta mu /mu Of 0.99 and 0.95, respectively, for Delta mu/mu less than or equal to 1. By contrast, epsilon = 1/kappa(2) where kappa is the kappa para meter, has a linear regression coefficient with Delta mu/mu of only 0. 5. To reliably estimate the onset of nonadiabatic behavior, one must t herefore use delta(B) or delta(CIM2) rather than kappa. Using isoconto urs of constant delta(B) we map the regions of nonadiabatic ion motion . For a given energy the transition to nonadiabatic motion occurs over a radial distance of similar to 2 R-E On the nightside and is closest to the Earth at midnight. At midnight the nonadiabatic regime for pro tons extends inward to similar to 11 R-E (similar to 7.5 R-E) for 1 ke V and to similar to 6 R-E (similar to 4.5 R-E) for 1 MeV with the Kp = 0 (Kp = 6) model. For O+ the nonadiabatic regime is 1.5 to 2 R-E clos er to the Earth than for protons. Drift trajectory calculations and an alytical estimates show that particles drifting through regions with d elta(B) > 0.01 suffer net Delta mu similar to mu. The net Delta mu is extremely sensitive to initial gyrophase and it is shown that for delt a(B) > 0.01 differences in gyrophase diverge exponentially with repeat ed equatorial crossings. Because the equatorial gyrophase determines t he mu scattering, this implies that the mu scattering is chaotic so th at no gyrophase-averaged invariant exists for the nonadiabatic drift m otion. Despite this, the average, nonadiabatic drift paths are fairly well defined. The resulting hybrid drift consists of dayside adiabatic and nightside nonadiabatic drift. A single nonadiabatic nightside dri ft path is associated with a family of adiabatic dayside drift paths. If some of the adiabatic drift paths are open to the magnetopause, all of the particles on the family of hybrid drift trajectories will be s ubject to loss on a timescale comparable to the drift period. Because the nonadiabatic behavior is due solely to field line curvature, the s ame behavior will be present with a nonzero convection electric field with the important difference that the lower-energy particles will be on open convection drift paths. The hybrid drift path-induced loss eff ects are therefore most important for higher-energy particles, > 50 ke V, whose adiabatic drift paths are closed in the presence of a convect ion electric field. The implications of nonadiabatic effects for ring current modeling based on Liouville's theorem apply equally well in th e zero and finite electric field cases.