Numerical simulations of astrophysical jets from Keplerian discs - III. The effects of mass loading

We present 2.5D time-dependent simulations of the non-linear evolution of n on-relativistic outflows from the surface of Keplerian accretion discs. The gas is accelerated from the surface of the disc (which is a fixed platform in these simulations) into a cold corona in stable hydrostatic equilibrium . We explore the dependence of the resulting jet characteristics upon the m ass loading of the winds. Two initial configurations of the threading disc magnetic field are studied: a potential field and a uniform vertical field configuration. We show that the nature of the resulting highly collimated, jet-like outflo ws (steady or episodic) is determined by the mass load of the disc wind. Th e mass load controls the interplay between the collimating effects of the t oroidal field and the kinetic energy density in the outflow. In this regard , we demonstrate that the onset of episodic behaviour of jets appears to be determined by the quantity N = B-phi(2)/4 pi pv(p)(2) which compares the s peed for a toroidal Alfven wave to cross the diameter of the jet, with the flow speed v(p) along the jet. This quantity decreases with increasing load . For sufficiently large N (small mass loads), disturbances appear to grow leading to instabilities and shocks. Knots are then generated and the outfl ow becomes episodic. These effects are qualitatively independent of the ini tial magnetic configuration that we employed and are probably generic to a wide variety of magnetized accretion disc models.