MODELING MHD ACCRETION-EJECTION—FROM THE LAUNCHING AREA TO PROPAGATION SCALES

Abstract

We present results of axisymmetric magnetohydrodynamic (MHD) simulations investigating the launching of jets and outflows from a magnetically diffusive accretion disk. The time evolution of the disk structure is self-consistently taken into account. In contrast to previous works we have applied {\em spherical} coordinates for the numerical grid, implying substantial benefits concerning the numerical resolution and the stability of the simulation. Thanks to the new setup we were able to run simulations for more than 150,000 dynamical times on a domain extending 1500 inner disk radii with a resolution of up to 24 cells per disk height in the inner disk. Depending on the disk magnetization, jet launching occurs in two different but complementary regimes - jets driven predominantly by centrifugal or magnetic forces. These regimes differ in the ejection efficiency concerning mass, energy and angular momentum. We show that it is the {\em actual} disk magnetization and not so much the initial magnetization which describes the disk-jet evolution best. Considering the actual disk magnetization we also find that simulations starting with different initial magnetization evolve in a similar - typical - way as due to advection and diffusion the magnetic flux in the disk evolves in time. Exploring a new, modified diffusivity model we confirm the self-similar structure of the global jet-launching disk, obtaining power laws for the radial profiles of the disk physical variables such as density, magnetic field strength, or accretion velocity.