Using an isothermal MHD code, we have performed three-dimensional, high-resolution simulations of the Parker instability. The initial equilibrium system is composed of exponentially decreasing isothermal gas and a magnetic field (along the azimuthal direction) under a uniform gravity. The evolution of the instability can be divided into three phases: linear, nonlinear, and relaxed. During the linear phase, the perturbations grow exponentially with a preferred scale along the azimuthal direction but with the smallest possible scale along the radial direction, as predicted from linear analyses. During the nonlinear phase, the growth of the instability is saturated and flow motion becomes chaotic. Magnetic reconnection occurs, which allows the gas to cross field lines. This, in turn, results in the redistribution of the gas and the magnetic field. The system approaches a new equilibrium in the relaxed phase, which is different from the one seen in two-dimensional works. The structures formed during the evolution are sheetlike or filamentary, whose shortest dimension is radial. Their maximum density enhancement factor relative to the initial value is less than 2. Since the radial dimension is too small and the density enhancement is too low, it is difficult to regard the Parker instability alone as a viable mechanism for the formation of giant molecular clouds.
Bibliographical noteFunding Information:
We are grateful to Doctors B.-C. Koo and Y. Lee for comments on the manuscript. Computations in the present work were carried out by using CRAY T3Es at SERI in Korea and at the University of Minnesota Supercomputing Institute in the US. The work by J. K. was supported in part by the Ministry of Science and Technology through Korea Astronomy Observatory grant 97-5400-000. The work by S. S. H. was supported in part by a grant from the Korea Research Foundation made in the year of 1997. The work by D. R. was supported in part by KOSEF through grant 981-0203-011-2. The work by T. W. J was supported in part by NSF grants AST-9619438 and INT-9511654 and by the University of Minnesota Supercomputing Institute.
- ISM: clouds
- ISM: magnetic fields
- ISM: structure