Improved gyrotropic transport equations for fully ionized magnetized gases: modelling the solar wind

Abstract

We apply a newly developed set of gyrotropic fluid transport equations to the solar wind. The transport equations are based on a perturbed bi-Maxwellian velocity distribution function and describe number density, flow speed, temperature along andacross the magnetic field and heat flow. The equations are designed to provide an improved description of collisions in fully ionized plasmas, leading to a better description of heat conduction and thermal forces than previously developed gyrotropicequations, while retaining the description of collisionless plasma flow provided by gyrotropic equations. The equations are implemented in a solar wind model extending from the chromosphere to 1 AU and are solved for ahydrogen-proton-electron plasma. The new model solutions are compared with other solar wind models. We find that in a rapidly expanding coronal-flow geometry, the new equations lead to a solar wind solution very different from previous gyrotropic models, with a solar wind mass flux seven times larger, and the asymptotic-flow speed reduced by a factor 4. Thisdifference is caused by the different description of heat flow in the two models, which affects the energy transport between the corona and lower layers of the solar atmosphere. In a radially expanding geometry, the differences between new and oldsolutions are much smaller. The proton temperature anisotropy in the collisionless outer solar wind behaves very similarly with the new and old gyrotropic equations, with both providing a good description of the effect of the Lorentz force.