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E.T. Vishniac (Johns Hopkins University)
Accretion disks have the potential to accrete poloidal flux from their environment. It is generally understood that as long as the turbulent diffusion coefficient for mixing magnetic flux through the disk is comparable to the effective "viscosity" for angular momentum transport, this process will be very weak. The poloidal flux will move a short distance towards the disk center before radial diffusion outward balances the inward motion of the accreting plasma. However, magnetic helicity conservation implies that the turbulent diffusion coefficient is multiplied by a correction factor, roughly equal to the ratio of the poloidal field energy density to the total magnetic field energy density. Consequently, magnetic flux accretion will be very efficient until the accreted flux reaches a critical strength, somewhere close to the inner edge of the disk. At this strength the poloidal field is capable of carrying a significant fraction of the total angular momentum flux in the disk, and of creating a strong magneto-centrifugally driven jet. Whether or not this happens in a real system depends on the ambient magnetic flield, the radius of the inner disk edge, the mass flux through the disk, and the mass of the accreting object. Lower accretion rates, larger central masses, and smaller inner disk radii all favor jet formation. For similar accretion rates and environments, black holes are much more likely to form jets than cataclysmic variables.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.