ASTROPHYSICAL JOURNAL, cilt.404, sa.1, ss.185-196, 1993 (SCI İndekslerine Giren Dergi)
Magnetic fields with an ordered component parallel to the plane permeate the disks of spiral galaxies and may do the same in the gaseous disks of protostellar and mass-transfer binary star systems. Magnetic buoyancy makes such configurations prone to the Parker instability, which, combined with the magnetorotational instability recently rediscussed by Balbus & Hawley in this journal, may lead to sustained dynamo action and other interesting magnetohydrodynamic phenomena. As a prelude to more complete studies, we analyze the simpler problem of Parker's instability in a disk with a realistic vertical structure, but without including the effects of rotation and shear in the horizontal directions. In addition to the continuum modes found by previous authors for the idealized case when the vertical gravitational field is taken to be a simple step function, we discover the possibility for discrete modes whose power is more spatially confined in z. When these discrete modes prove unstable, they favor condensations that are placed antisymmetrically with respect to the midplane, a feature found also in numerical simulations carried into the nonlinear regime by Matsumoto et al. Transport motions across the midplane would help to alleviate one criticism directed against the convective model of accretion disk viscosity developed by Lin and his colleagues. For the continuum modes, we note that the characteristic length scale for instability is typically half of the conventionally estimated value, yielding growth rates that are approximately double previous estimates.