The Disk-Halo Interaction Via the Parker Instability

Abstract

Using a collection of superbubbles (SB) and High Velocity Clouds (HVC) as perturbative agents, we model the disk-halo interaction in magnetized gaseous disks of spirals with 2-D numerical MHD simulations. The gaseous disk is assumed in magnetohydrostatic equilibrium, with the B-field oriented parallel to the disk (the x-axis). The gas properties and the gravitational acceleration within the disk are defined similar to those found at the solar circle in the Milky Way (see Martos & Cox 1994). Each perturbation is introduced individually, at time intervals of Δt = 32 Myr, and at random locations in the disk. The energy injected by each SB is 10⁵³ erg, and the explosion centers are located within 100 pc from the midplane (z = 0). The impinging HVCs, on the other hand, have a variety of different densities and velocities (see Franco 1986, and Tenorio-Tagle et al. 1987), and collide with the disk at different angles. Thus, the energy and momentum injection per collision varies in each event. The numerical calculations were performed with the MHD code ZEUS-3D (Stone & Norman 1992a, 1992b), using the SGI ORIGIN-2000 of the Supercomputer Center at UNAM. The total extent of the computational grid, with 400 x 100 zones, is 16 kpc and 4 kpc in the x and z directions, respectively. The upper and lower boundaries are open, but the right and left boundaries are periodic. At midplane, the gas density and temperature are n _o = 1 cm⁻³ and T = l.l × 10⁴ K, respectively, and the field intensity is B ₀ = 5 G. Initially, gas is in hydrostatic equilibrium supported by magnetic and thermal pressures. The ratio of the magnetic-to-thermal energies, however, is not held constant with height but increases with increasing values of z (i. e., the disk is unstable to the Parker instability). The perturbations are introduced within a central box of the computational grid. Each SB has an initial energy density of 2.2 × 10¹³ erg/cm (equivalent to a spherical bubble with 10⁵³ erg). HVCs have 120 pc in height and 200 pc long, a mass density of 8.6 × 10⁻² g/cm, and different approaching velocities and incident angles. The B-field lines are distorted and compressed with each perturbation, increasing the field tension at the location of the event, and driving MHD waves that propagate to the rest of the disk and into the halo. All perturbations, then, create a complex (turbulent) network of flows that eventually trigger the Parker instability (Franco et al. 1995). The SBs expand mainly in the x-direction because the compressed B-field lines provide a strong pressure, reducing the SB growth along the z-axis. Hence, the SB energy is distributed by the MHD waves in a huge volume, but the SB mass is maintained within the disk in a well defined and small volume. In the case of HVC-disk collisions, the magnetic field prevents the cloud material from penetrating into the disk. Thus, the B-field provides an adequate coupling for the energy and momentum exchange between the disk and the halo but, for this restricted field geometry, also represents an effective shield that prevents a direct gas flow between the disk and the halo. Such a gas flow, however, can indeed occur via the Parker instability (see Figure 1), and the mass exchange between the disk and the halo is probably a complex two-step process. This work has been partially supported by grants from DGAPA-UNAM, CONACyT, and by a R&D grant from Cray Research Inc.