Electron anisotropies in the inner heliosphere

Abstract

A three-dimensional numerical model is used to study the propagation and modulation of a few MeV Jovian and galactic electrons in the inner heliosphere. The main aim of the work is to compute the three-dimensional electron intensity anisotropy as described by a standard approach to the modulation of cosmic rays in the heliosphere, with enhanced perpendicular diffusion. In order to accomplish this, the spatial dependence of the full diffusion tensor has to be specified and the three-dimensional spatial intensity gradients have to be computed in detail. The emphasis is placed on the role that polar perpendicular diffusion plays in order to establish how large the anisotropy vector components may get in the inner heliosphere, for both galactic and Jovian electrons. The modelling results should be seen as upper limits of what standard modulation theory predicts and may serve as reference for the interpretation of electron observations and to establish what percentage of the observed electron anisotropy can be attributed to Jovian electrons and to galactic electrons, respectively. It is found that the total anisotropy for electrons is dominated by the contribution of the Jovian electron anisotropy in the equatorial regions, as expected, close to the Jovian electron source during solar minimum conditions, whereas at high heliolatitudes the galactic electron anisotropy contribution dominates the total anisotropy. An equatorial approach of the Jovian electron source produces a much sharper anisotropy–time profile than a latitudinal approach, the latter interpreted as indicative of the role of enhanced perpendicular diffusion.