A NUMERICAL STUDY OF MAGNETIC BUOYANCY IN AN ACCRETION DISK

We follow the evolution of a closed loop of relatively weak poloidal m agnetic field, originally embedded somewhat above the midplane in a hy drostatic accretion disk. The equations of magnetohydrodynamics are so lved on a numerical grid in axisymmetric geometry. Viscous heating, ra diative transfer, and the horizontal and vertical components of the gr avity of the central star are taken into account. As the evolution pro ceeds, toroidal field is built up as a result of shear in the disk, an d the field becomes buoyant as a result of interchange modes of the Pa rker instability. The effective wavelength of the buoyant instability, and its dependence on the strength of the initial held loop, are foun d to be consistent with a linear stability analysis. The buoyancy resu lts in turbulent motions and expulsion of some field from the disk. Ev entually, a saturation state is reached, in which the field assumes a patchy structure, and the ratio of gas to magnetic pressure stabilizes in the range 1-5. Outward angular momentum transfer and an accompanyi ng radial expansion of the magnetized region occur as a result of magn etic torques, and an equivalent alpha-viscosity parameter is estimated . The implications of these results on the generation of a magnetic dy namo in a disk are discussed.