The role of electrons and helium atoms in global modeling of the heliosphere

Abstract

In situ and remote observations of the outer regions of the heliosphere and in the local interstellar medium (LISM) by Voyager, New Horizons, and IBEX continue to present new challenging questions, reflecting the complex processes of the solar wind (SW) - local interstellar medium (LISM) interaction. We present a new version of our recent 3-D MHD-plasma/kinetic-neutrals model of the SW-LISM interaction, which self-consistently includes neutral hydrogen and helium atoms. The new model treats electrons as a separate fluid assumed to co-move with the plasma mixture. In addition, it includes the effect of Coulomb collisions between electrons, He+ ions, and protons. The properties of electrons in the distant SW and in the very local interstellar medium (VLISM) are mostly unknown due to the lack of in situ observations. In this study we discuss the implications of using different models for the electron pressure. A common assumption (model 0) in single-ion global models is to assume that electrons have the pressure of the ion mixture. In this case electrons become hot in the distant SW where plasma is energetically dominated by pickup ions. In the proposed new model, electrons in the SW are colder, which leads to a better agreement with New Horizons observations in the supersonic SW. In the VLISM, however, ions and electrons may be almost in thermal equilibrium due to Coulomb collisions. As far as the plasma mixture properties are concerned, the major differences between the models are in the inner heliosheath, where colder electrons result in hotter protons and induce cooling of the plasma mixture due to the increase in the charge exchange frequency. This makes the heliosheath thinner by ~5 AU along the upstream direction and up to 60 AU in the downwind region. The filtration of interstellar H and He atoms is also discussed. At 1 AU, in the model with separate electrons the H density increases by ~2%. However, the fraction of pristine H atoms decreases by ~12%, while that of atoms born in the IHS increases up to ~35%. While the density of He atoms in the SW remains essentially unchanged, the contribution from the warm breeze increases by ~3%.