Using a TVD MHD code, we have simulated the nonlinear development
of the Parker instability in an isothermal magnetized gas disk ,
which is under the influence of self-gravity. Our objective is to
investigate how the Jeans and Parker instabilities compete with
disruptive tendency of the convection in nonlinear stage of
evolution, and to know whether the Parker-Jeans instability can be
a mechanism for the formation of the larger scale structures in the
When the perturbation wavelengths are larger than a Jeans critical wavelength, a cooperative action of the Parker and Jeans instabilities can
completely suppress the disruptive behavior of the convective
instability and leads the ISM gas material into large-scale
structures of high density, whose masses and sizes correspond to HI
superclouds rather than GMCs. The gas disk develops
the vertical filamentary structures near the dense core instead of the
chaotic sheet structures that are often seen from simulations of
the classical Parker instability. The low density filaments
connect the dense part to the diffuse region far from the disk central plane.
The filamentary structure is similar to galactic diffuse vertical structure.
When the wavelength of given perturbations is so short that the Jeans
instability may not get triggered, the self-gravitating, magnetized gas disk seems to reach an equilibrium state different from the initial one.