Self‐consistent pitch angle diffusion of newborn ions

Abstract

It is well known from the study of ion pickup process by the solar wind that hydromagnetic turbulence can cause the newborn ions to undergo rapid pitch angle diffusion or scattering, thus forming a partial or complete velocity shell distribution. In most of the recent discussions based on quasi‐linear theory it is assumed that the spectral wave energy density associated with the hydromagnetic turbulence is constant in time, implying a saturated turbulence level. In contrast, in this work the effect of self‐consistently generated waves on the ion dynamics is discussed on the basis of a simple theoretical model, and it is shown both analytically and numerically that the self‐consistent diffusion process leads to a time‐asymptotic partial shell distribution which extends approximately from the initial pitch angle cos−1 µ0 to ∼π/2 in pitch angle space. Particularly, the role of resonant versus nonresonant diffusion processes is discussed in detail. In addition, the effect of continuous ion source term is also incorporated in the numerical analysis since in cometary environments the ions are continuously created.