CHAOTIC SCATTERING OF PITCH ANGLES IN THE CURRENT SHEET OF THE MAGNETOTAIL

The process of pitch angle scattering by a current sheet is studied using the modified Harris field model. The relationship between the incoming asymptotic pitch angle-alpha(in) and the outgoing asymptotic pitch angle-alpha(out) is studied from first principles by numerically integrating the equation of motion. We give evidence that charged particles undergo chaotic scattering by the current sheet. For fixed alpha(in), it is shown that alpha(out), exhibits sensitive dependence on the energy parameter H in certain energy ranges. For fixed H in the same energy ranges, alpha(out) sensitively depends on alpha(in). For other energy values, alpha(out) does not show sensitive dependence on alpha(in) for most phase angles. The latter energy values occur at the resonance values given by the previously identified relationship H1/4 approximately N + 0.6. Thus alpha(out) exhibits no significant sensitivity near these resonance energies. This conclusion applies to a range of alpha(in) centered at alpha(in) = 0 (i.e., field aligned in the asymptotic region). A distribution of alpha(in) is mapped from the asymptotic region to the midplane, and it is found that the resulting particle distribution should have beam structures with well-collimated pitch angles near each resonance energy value. Implications for the particle distribution functions in the Earth's magnetotail are discussed.