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V.I. Pariev (University of Arizona, LANL), S.A. Colgate, J.M. Finn (LANL)
Magnetic fields comprise roughly equal energy density as the background radiation within galaxies, or fields of \approx 3 \times 10-6 G and about 10% of this energy density in the space between galaxies within galactic clusters. In order to obtain a satisfactory explanation of the origin of these magnetic fields one needs to consider magnetic dynamos within galaxies. Due to the large available binding energy, large shear velocities, and inherent high gain, accretion disks around supermassive central black holes in AGNs become a unique location where a dynamo can work most efficiently. We consider an \alpha-\Omega dynamo. The \Omega deformation is provided by the Keplerian rotation. The \alpha deformation is produced by star-- disk collisions. These collisions entrain gas and magnetic field, which is ejected from the disk, high above the disk plane. These episodic, off--axis, large out-of-the-disk plumes with their anticyclonic helical gas motion produce the \alpha deformation required for the dynamo. This dynamo mechanism is robust and cannot be quenched by the growing magnetic field until it reaches equipartition with the Keplerian motion, unlike turbulent disk dynamos. We present results of simulations of the star--disk collision dynamo, using our 3D~kinematic vector potential dynamo code. Estimates of the threshold values of Magnetic Reynolds number necessary for the dynamo growth are obtained. Growing modes of magnetic field are presented.
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