Bipolar jets launched from accretion disks. II. Formation of symmetric and asymmetric jets and counter jets

We investigate the jet launching process from accretion disks extending our recent study (paper I) to a truly bipolar setup. We perform axisymmetric MHD simulations of the disk-jet interaction on a computational domain covering both hemispheres, in particular addressing the question of an intrinsically asymmetric origin of jet / counter jet systems. Treating both hemispheres simultaneously, we overcome the equatorial plane symmetry boundary condition used in most previous studies which naturally fosters a symmetric evolution. For the magnetic diffusivity prescription we apply an alpha-parametrisation, considering both, globally models of diffusivity, and local models. We first approve the quality of our numerical setup by generating perfectly symmetric jets, lasting over a 1000s of dynamical time scales. We then disturb the hemispheric symmetry in the disk, and investigate the subsequent evolution of the outflow. The evolution first leads to a substantial disk warping with electric currents intersecting the equatorial plane. We investigate two models, i) a disk with (initially) different thermal scale height in both hemispheres, and ii) a symmetric disk into which a local disturbance is injected in one hemisphere. In both cases the disk asymmetry results in asymmetric outflows with mass fluxes differing by 10-20%. We find up to 30% difference in mass flux between jet and counter jet for this setup, lasting over 1000s of dynamical time scales (i.e. lasting for the whole simulation). In summary, our results suggest that the jet asymmetries in protostellar and extragalactic jets can indeed be generated intrinsically and maintained over long time by disk asymmetries and the standard jet launching mechanism.