The effect of instrument limitations on the derivation of plasma flows from energetic ion anisotropies, with an application to Ulysses observations at Jupiter

Plasma velocities determined from the anisotropies of energetic ions via the Compton–Getting effect have been important in studies of magnetospheric flows, particularly with regard to the magnetospheres of Jupiter and Saturn. In this paper we consider a range of issues concerned with the practical limitations of such measurements, and their effect on the velocities deduced. First, however, we consider the differing approaches to ion data analysis which have been employed, via fitting to a spherical harmonic expansion or directly to a model distribution function. We show that these approaches are formally identical when corresponding terms are included. The other issues considered are (a) the effect of flow and gradient contributions to the anisotropy and how and when they can be separately determined; (b) finite detector energy channel widths, telescope opening cones, and azimuthal sweep on spinning spacecraft; (c) lack of complete coverage of the unit sphere; (d) misidentification of the ion species detected; (e) telescope cross-calibration errors; and (f) contamination by energetic electron counts. The effect of these data limitations are systematically examined and quantified. The discussion is illustrated by consideration of the characteristics of energetic ion instruments carried by the Ulysses spacecraft, and an analysis of data obtained by the Anisotropy Telescopes instrument during the inbound pass of the spacecraft through the outer Jovian magnetosphere.