The transport of interstellar pickup ions

Abstract

A new approach to the time‐dependent anisotropic propagation of interstellar pickup ions in the interplanetary medium is presented. The model includes the effects of adiabatic focusing in a radial magnetic field, adiabatic deceleration, anisotropic pitch angle scattering, convection in the solar wind, and the continual injection of newly ionized particles. It is assumed that pickup ions experience difficulty in scattering through 90°. A two‐timescale scattering operator is introduced together with a generalized hemispherical model for the transport of pickup ions. The approach described here significantly extends the previous studies by Isenberg (1997) and Schwadron (1998) in that the pitch angle dependence of the pickup ions is not assumed to be of the form ƒ(r, v, t, μ) = ƒ_(r, v, t)H(μ) + ƒ+(r, v, t)H(−μ) (H(μ) 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 distribution; (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 newborn pickup ion distribution within the sunward hemisphere but instead allow it to evolve into a scattered distribution on a timescale , thus preserving the pitch angle characteristics of the ring beam. The anisotropic pitch angle scattering is found to result in the sunward accumulation of pickup ions, and particles moving sunward suffer more efficient cooling than those moving antisunward. Compared with the steepness at 90° pitch angle, the pitch angle dependence is not important within each hemisphere for moderately anisotropic scattering. However, for highly anisotropic scattering, the particle distribution is dominated by particles moving sunward, adiabatic cooling is more efficient, and the deviation of sunward moving particle distribution from a homogeneous hemisphere may be large at high velocities.