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

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 ourrecent study (paper I) to a truly bipolar setup. We perform axisymmetric MHDsimulations of the disk-jet interaction on a computational domain covering bothhemispheres, in particular addressing the question of an intrinsicallyasymmetric origin of jet / counter jet systems. Treating both hemispheressimultaneously, we overcome the equatorial plane symmetry boundary conditionused 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 firstapprove the quality of our numerical setup by generating perfectly symmetricjets, lasting over a 1000s of dynamical time scales. We then disturb thehemispheric symmetry in the disk, and investigate the subsequent evolution ofthe outflow. The evolution first leads to a substantial disk warping withelectric 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 onehemisphere. In both cases the disk asymmetry results in asymmetric outflowswith mass fluxes differing by 10-20%. We find up to 30% difference in mass fluxbetween jet and counter jet for this setup, lasting over 1000s of dynamicaltime scales (i.e. lasting for the whole simulation). In summary, our resultssuggest that the jet asymmetries in protostellar and extragalactic jets canindeed be generated intrinsically and maintained over long time by diskasymmetries and the standard jet launching mechanism.

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