Simulating turbulence in galactic halos and in the outer heliosphere

Abstract

First, simulations of the wind-filled halos of starburst galaxies are performed in the framework of single-fluid magnetohydrodynamics, suitably extended to also track the self-consistent evolution of additional turbulence-related quantities. These quantities comprise the turbulent energy density, the cross-helicity, and the turbulent length scale. After a brief discussion of these extended equations and the employed numerical approach, we present selected simulation results, both for non-magnetized benchmark runs as well as for tests using the full system of equations. The dominant and unexpected feature of the former is a macroscopic flow instability near the rotational axis that prevents the outflow from reaching a steady state. Methods to determine the cause and nature of this instability are presented, followed by a preliminary analysis of the resulting turbulent properties. Second, the above framework is extended further to account for a non-constant energy difference (or residual energy), a quantity not conserved in the absence of dissipation, in addition to the Elsasser energies and by allowing each of these quantities its own characteristic correlation length scale. This setting is then applied to the outer heliosphere beyond the termination shock, where the solar wind expands both sub-Alfveńically and nonradially. The resulting solutions of this six-equation model are illustrated and studied in some detail.