Modulation of Jovian and galactic electrons in the heliosphere: 2. Radial transport of a few MeV electrons

Abstract

Previously, the latitudinal transport of ∼7 MeV electrons was studied by comparing the results of a newly developed three‐dimensional model, based on the numerical solution of Parker's transport equation including the Jovian source, with Ulysses observations. Here, the radial transport of ∼16 MeV electrons is studied by comparing the model results with the −16 MeV electron intensities observed by Pioneer 10 up to ∼70 AU. Electron modulation responds directly to the energy dependence of the diffusion coefficients below ∼500 MeV, in contrast to protons, which experience large adiabatic energy losses below this energy, and in addition, low‐energy electrons do not experience significant drifts. The modulation of low‐energy electrons is therefore a handy tool to find a diffusion tensor suitable to the heliospheric modulation of electrons. It is illustrated that the computed electron intensities are sensitive to the radial dependence of the diffusion coefficients in the inner heliosphere and that compatibility between the model and observations gives an indication as to the radial dependence of the diffusion coefficients. The effects of different perpendicular diffusion coefficients in the radial direction are also shown, and an upper limit for the value of this coefficient is proposed. Finally, the relative contributions of the Jovian and galactic electrons to the total electron intensity are shown along the Pioneer 10 trajectory, illustrating that the Jovian electrons dominate the total electron intensity at these low energies in the inner equatorial regions only up to ∼9 AU. From 15 AU outward, the Jovian contribution becomes less than 20%, decreasing rapidly as a function of increasing distance.