A quasi-linear theory to explain ion acceleration in the distant cometary environment

Spacecraft observations at comet Halley as well as computer simulations have shown that pitch angle scattering of newborn cometary ions proceeds at a relatively fast rate, leading to relatively isotropic shell-like velocity distribution functions. Energization processes whereby shell distributions become more Maxwellian and a few ions are accelerated to high energies appear to proceed more slowly. The research on the latter, slower process is described, in which the scattering is assumed to be due to the resonant, growing magnetic fluctuations driven by the non-Maxwellian nature of the ion distribution. An ion shell distribution which is isotropic in the wave frame begins the process and a Fokker-Plank derived from quasilinear plasma theory is used to describe the broadening and energization of the cometary ion distribution.