DYNAMO ACTION BY INTERNAL WAVES IN ACCRETION DISKS

In a previous paper, Vishniac and Diamond showed that low frequency, slightly nonaxisymmetric (\m\ = 1) waves in thin accretion disks could penetrate to small radii with an amplitude uniquely determined by nonlinear saturation. Here we examine the ability of these waves to drive an α-Ω dynamo in a disk of thickness H, radius r, and Keplerian rotational frequency Ω(r) ∝ r-3/2. The asymmetry in the wave distribution which creates a nonzero helicity follows from the fact that the fundamental waves all have a positive angular momentum flux. As a result there will be a large-scale magnetic field driven by an α-Ω dynamo. It is also likely that small-scale fields, driven by higher order wave modes, will contribute significantly to the local value of Br Bφ. We argue that the magnetic field saturates when its pressure is comparable to the thermal pressure, and present a crude model of the nonlinear of transfer of power to small-scale turbulence. The dynamo process creates a large-scale, axisymmetric, toroidal field with Br ∼ (H/r)3/2Bφ. The corresponding value of "α" is (H/r)3/2. The dynamo number is of order (r/H). The field Bφ has an odd symmetry in z. Smaller scale waves create small-scale fields with a maximum brbφ ∼ (H/r)6/5P. We note that in this model viscous and thermal instabilities in radiation pressure-dominated, electron-scattering regions in accretion disks appear to be substantially suppressed.