Characteristics of Scatter-free Behavior of Heliospheric Pickup Ions

Theoretical treatments of heliospheric pickup ion behavior generally focus on the solution of various forms of the Boltzmann or transport equation. Like their cosmic-ray counterparts, these approaches often presume a radial geometry for the interplanetary magnetic field, and focus on diffusive processes acting on already energetic particles. At the opposite extreme lies the test particle picture of pickup ions, wherein one can treat the problem from the birth of the ions through their motion in a Parker Spiral field with a radially flowing solar wind. This approach thus explicitly includes the initial pickup process by the convection electric field, and the effects of the radial evolution of the latter in the presence of the focusing effects of the radial expansion of the solar wind. Scattering processes can be added to the test particle approach in the manner described by Giacalone (e.g. Adv. Space Res. 23, p. 581, 1999), but it is first instructive to gain perspective from the first order, scatter-free calculations. The results suggest how the combination of the spatial distribution of ion production and initial velocity of the parent neutral might combine to produce characteristic features in heliospheric pickup ion distributions from different sources.