Stability and heating of magnetically driven jets from Keplerian accretion discs

We have performed 3-D numerical magnetohydrodynamic (MHD) jet experiments to study the instabilities associated with strongly toroidal magnetic fields. In contemporary jet theories, a highly wound up magnetic field is a crucial ingredient for collimation of the flow. If such magnetic configurations are as unstable as found in the laboratory and by analytical estimates, our understanding of MHD jet driving and collimation has to be revised. A perfectly conducting Keplerian disc with fixed density, rotational velocity and pressure is used as a lower boundary for the jet. Initially, the corona above the disc is at rest, permeated by a uniform magnetic field, and is in hydrostatic equilibrium in a softened gravitational field from a point mass. The mass ejection from the disc is subsequently allowed to evolve according to deviations from the initial pressure equilibrium between disc and corona. The energy equation is solved, with the inclusion of self-consistently computed heating by viscous and magnetic dissipation. We find that magnetic dissipation may have profound effects on the jet flow as: 1) it turns on in highly wound up magnetic field regions and helps to prevent critical kink situations; 2) it influences jet dynamics by re-organizing the magnetic field structure and increasing thermal pressure in the jet; and 3) it influences mass loading by increasing temperature and pressure at the base of the jet. The resulting jets evolve into time-dependent, nonaxisymmetric configurations, but we find only minor disruption of the jets by e.g. the kink instability.