MAGNETIC INTERCHANGE INSTABILITY IN ACCRETION DISKS

We investigate the stability of a disk to magnetic interchange in the disk plane, when a poloidal magnetic field provides some radial suppor t of the disk. The disk is assumed to be geometrically thin and may po ssess rotation and shear. We assume the unperturbed magnetic field ver tically threads the disk and has a comparable radial component at the disk surface. We formulate the linear stability problem as an initial value problem in shearing coordinates and ignore any effects of winds. Shear stabilizes the interchange instability strongly compared to the uniformly rotating case studied previously and makes the growth algeb raic rather than exponential. A second form of instability with long w avelengths is identified, whose growth appears to be transient. If the field strength is measured by the travel time tau(A) of an Alfven wav e across the disk thickness, significant amplification for both forms of instability requires (tau(A) Omega)(-2) greater than or similar to L/H, where L is the radial length scale of the field gradient and H is the disk thickness. Field strengths such that 1 less than or equal to (tau(A) Omega)(-2) less than or similar to L/H are stable to these in stabilities as well as the instability recently investigated by Balbus and Hawley (1991). The results suggest that in disk environments in w hich the magnetic energy density is greater than the thermal energy de nsity, disks are stable over a substantial range of parameter space, w ith radial advection of magnetic flux limited by the interchange insta bility possibly near the disk center. Such environments may be relevan t for the production of magnetic winds or jets in young stars or activ e galactic nuclei.