PITCH-ANGLE SCATTERING OF COMETARY IONS - COMPUTER-SIMULATIONS

Pitch-angle evolution of newborn cometary ions is studied by means of one-dimensional electromagnetic hybrid computer simulations of homogeneous plasmas. Newborn ions are injected into the simulations at a constant rate, with a velocity relative to the solar wind which makes an angle alpha with respect to the ambient magnetic field. The simulations are done with relatively weak ion injection rates commensurate with those in the distant environment of comet Halley. In response to the linear temporal growth of the fluctuating magnetic field energy, the injected ions pitch-angle scatter toward isotropy in both the quasiparallel (0-degree less-than-or-equal-to alpha less-than-or-similar-to 60-degrees) and quasiperpendicular (60-degrees < alpha less-than-or-equal-to 90-degrees) regimes. For the injection of cometary oxygen ions the simulations show pitch angle 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 xygen ion pitch angle scattering rate at perpendicular injection is approximately twice that in the quasiparallel regime, in qualitative agreement with observations at comet Halley.