Dynamos and angular momentum transport in accretion disks

• Published in: Physics of fluids. B, Plasma physics Plasma physics, 1991-08, Vol.3 (8), p.2374-2378
• Main Authors: Vishniac, Ethan T., Jin, Liping, Diamond, P. H.
• Format: Article
• Language: English
• Subjects: 640102 - Astrophysics & Cosmology- Stars & Quasi-Stellar, Radio & X-Ray Sources; ACCRETION DISKS; ANGULAR MOMENTUM; ANNULAR SPACE; ASTROPHYSICS; ASYMMETRY; CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; CLOSED CONFIGURATIONS; CONFIGURATION; DIMENSIONS; DOCUMENT TYPES; Earth, ocean, space; ENERGY TRANSFER; Exact sciences and technology; External geophysics; Geodesy and gravity; HELICITY; INSTABILITY; IONIZATION; MAGNETIC FIELD CONFIGURATIONS; MAGNETIC FIELDS; NONLINEAR PROBLEMS; PARTICLE PROPERTIES; RADIATION PRESSURE; REVIEWS; SATURATION; SPACE; TEMPERATURE EFFECTS; THICKNESS; TOROIDAL CONFIGURATION; TURBULENCE; VISCOSITY; WAVE FORMS
• ISSN: 0899-8221; 2163-503X
• DOI: 10.1063/1.859606

The transport of angular momentum in astrophysical disks is one of the major issues in modern astrophysics. Here, recent work [Astrophys. J. 3 4 7, 435 (1989); 3 6 5, 648 (1990)] will be reviewed that suggests that internal waves, analogous to deep ocean waves, play a critical role in transporting angular momentum in neutral disks and generating a magnetic dynamo in ionized disks. Previously, it was shown that low‐frequency, slightly nonaxisymmetric (‖m‖=1) waves in thin accretion disks could penetrate to small radii with a unique amplitude because of nonlinear saturation. Here, the ability of these waves to drive an α‐Ω dynamo in a disk of thickness H and radius r and keplerian rotational frequency Ω(r)∝r −3/2 is examined. The asymmetry in the wave distribution that 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 B r B φ. It is argued that the magnetic field saturates when its pressure is comparable to the thermal pressure and a crude model of the nonlinear transfer of power to small‐scale turbulence is presented. The dynamo process creates a large‐scale, axisymmetric toroidal field with B r ∼(H/r)3/2 B φ. Smaller‐scale waves create small‐scale fields with a maximum b r b φ∼(H/r)6/5 P. In this model, viscous and thermal instabilities in radiation pressure dominated, and electron scattering regions in accretion disks appear to be substantially suppressed.