Generalized non-linear Compton-Getting transformation for energetic particles: Applications throughout the heliosphere

Abstract

We develop the theoretical basis for a non-linear frame transformation of energetic ion intensity anisotropies (i.e., a generalized Compton-Getting transformation). Within the basic physics-motivated assumption that the anisotropies are gyrotropic in the transform frame, the rather general formulation allows us to examine a wide range of ion intensity distributions throughout the heliosphere. In addition to generalizing more familiar applications of the Compton-Getting transformation (that utilizes the bulk plasma velocity V for the transformation), we also apply the component (V^) transverse to the magnetic field (B). The physical justification is that (V^) is the ExB drift velocity with which the magnetic field lines “move”. In this paper, we have focused on two quite different areas: the elucidation of the strong field-aligned anisotropies and steep energy spectra being observed near the Sun from Parker Solar Probe (and Solar Orbiter), and the extraction of the bulk plasma velocity in the heliosheath at ∼100 AU using the anisotropies in energetic ion intensities observed on Voyager 1&2.