The transport of interstellar pickup ions

A new approach to the time-dependent anisotropic propagation of interstella r pickup ions in the interplanetary medium is presented. The model includes the effects of adiabatic focusing in a radial magnetic field, adiabatic de celeration, anisotropic pitch angle scattering, convection in the solar win d, and the continual injection of newly ionized particles. It is assumed th at pickup ions experience difficulty in scattering through 90 degrees. A tw o-timescale scattering operator is introduced together with a generalized h emispherical model for the transport of pickup ions. The approach described here significantly extends the previous studies by Isenberg (1997) and Sch wadron (1998) in that the pitch angle dependence of the pickup ions is not assumed to be of the form f(r, v, t. mu) f-(r, v, t)H(mu) + f(+)(r, v, t)H( -mu) (H(mu) is the Heaviside step function) from the outset. Specifically, (1) a higher-order truncation of the underlying Boltzmann equation is used here, thus allowing a more careful analysis of the evolving pickup ion dist ribution; (2) we include a finite scattering rate for particles within each hemisphere and therefore present a more accurate treatment of pitch angle evolution; and (3) we do not assume instantaneous "isotropization" of the n ewborn pickup ion distribution within the sunward hemisphere but instead al low it to evolve into a scattered distribution on a timescale <(<tau>)over bar>, thus preserving the pitch angle characteristics of the ring beam. The anisotropic pitch angle scattering is found to result in the sunward accum ulation of pickup ions, and particles moving sunward suffer more efficient cooling than those moving antisunward. Compared with the steepness at 90 de grees pitch angle, the pitch angle dependence is not important within each hemisphere for moderately anisotropic scattering. However, for highly aniso tropic scattering, the particle distribution is dominated by particles movi ng sunward, adiabatic cooling is more efficient, and the deviation of sunwa rd moving particle distribution from a homogeneous hemisphere may be large at high velocities.