The modulation of 4‐ to 16‐MeV electrons in the outer heliosphere: Implications of different local interstellar spectra

Abstract

The modulation of low‐energy galactic and Jovian electrons is studied using a fully three‐dimensional, steady state modulation model and a two‐dimensional termination shock model, both based on Parker's transport equation. For this purpose the Jovian electron source and local interstellar electron spectrum (LIS) must be specified. The latter is not well known below ∼100 MeV so that four different local interstellar spectra for electrons are studied and compared to the ∼4‐ and ∼16‐MeV Pioneer 10 observations up to ∼70 AU. Assuring that the model represents these observations contributes to a better understanding of the heliospheric diffusion coefficients applicable to low‐energy electron modulation. These electrons respond directly to the energy dependence of the diffusion coefficients below ∼500 MeV in contrast to protons which experience large adiabatic energy losses. In addition, low‐energy electrons experience progressively less drifts below ∼100 MeV. The study illustrates that the different LIS scenarios require a different radial dependence of the diffusion coefficients throughout the heliosphere. By relating the parallel mean free path observations at Earth to the values needed to compute reasonable electron modulation at 16 MeV, a preferred LIS range could be established with ∼100 and ∼1000 particles m−2 s−1 sr−1 MeV−1, as lower and upper limits, respectively. On the basis of this preferred LIS range and the very small radial dependence observed for these electrons between 20 and 70 AU, the model predicts that very large modulation should occur beyond ∼70–80 AU in the heliosphere. This indicates that what happens in the outer most parts of the heliosphere is very important for cosmic ray modulation.