THE DYNAMICS OF FLUX TUBES IN A HIGH-BETA PLASMA .2. BUOYANCY IN STARS AND ACCRETION DISKS

We apply a new model for the structure of a magnetic field embedded in a turbulent plasma to stars and accretion disks. This model is based on the popular notion that the magnetic field tends to separate into i ndividual flux tubes. Realistic stellar plasmas are expected to either be in the ideal fluid limit where specific values of resistivity and viscosity can be ignored (e.g., the top of the solar convection zone) or the resistive limit (the bulk of the solar convection zone). In con trast, hot accretion disks are usually in the ideal fluid limit. We fi nd that with the exception of radiation pressure-dominated environment s, flux tubes are no more, and perhaps slightly less, buoyant than a d iffuse field of comparable energy density. However, in radiation press ure-dominated accretion disks the increased buoyancy of the magnetic f ield leads to the conclusion that the viscosity associated with magnet ic stresses scales with the gas pressure, rather than the total pressu re. Finally, if we assume that a stellar magnetic field is limited by the value that would seriously affect the bottom layer of the convecti on zone, then we can show that the large-scale poloidal field of the S un cannot be more than a few gauss.