Gary et al . ' Pitch - Angie Scattering of Cometary Ions

Pitch-angle volution of newborn cometary ions is studied by means of one-dimensiona,1 electromagnetic hybrid computer simulations ofhomogeneous plasmas. Newborn ions are injected into the simulations at a constant rate, with a velocity relative to the solar wind which makes an angie a with respect to the ambient magnetic field. The simulations are done with relatively weak ion injection rates commensurate with those in the disrant environment of comet Halley. In response to the linear temporal growth of the fluctuating magnetic field energy, the injected ions pitch-angie scatter toward isotropy in both the quasiparallel (0 ø _< a • 60 ø) and quasiperpendicular (60 ø < a < 90 ø) regimes. For the injection of comefury oxygen ions the simulations how pitch angie scattering rates that are essentially independent of the solar wind ion/cometary ion relative speed, and increase as the square root of the injection rate. Furthermore, the oxygen ion pitch angie scattering rate at perpendicular injection is approximately twice that in the quasipar'Mlel regime, in qualitative agreement with observations at comet Halley.