The Parker Instability in a Thick Gaseous Disk. II. Numerical Simulations in Two Dimensions

Abstract

We present 2D, ideal-MHD numerical simulations of the Parker instability in a multi-component warm disk model. The calculations were done using two numerical codes with different algorithms, TVD and ZEUS-3D. The outcome of the numerical experiments performed with both codes is very similar, and confirms the results of the linear analysis for the undular mode derived by : the most unstable wavelength is about 3 kpc and its growth timescale is between 30-50 Myr (the growth rate is sensitive to the position of the upper boundary of the numerical grid). Thus, the time and length scales of this multicomponent disk model are substantially larger than those derived for thin disk models. We use three different types of perturbations, random, symmetric, and antisymmetric, to trigger the instability. The antisymmetric mode is dominant, and determines the minimum time for the onset of the nonlinear regime. The instability generates dense condensations and the final peak column density value in the antisymmetric case, as also derived by , is about a factor of 3 larger than its initial value. These wavelengths and density enhancement factors indicate that the instability alone cannot be the main formation mechanism of giant molecular clouds in the general interstellar medium. The role of the instability in the formation of large-scale corrugations along spiral arms is briefly discussed.

Dawes Review 4: Spiral Structures in Disc Galaxies

“…Density enhancements solely due to Parker instabilities are finite, and thus likely to be overwhelmed by gravitational instabilities (Elmegreen 1982;Kim, Ryu, & Jones 2001;Santillán et al 2000;Kim et al 2002). However there is some evidence of loops caused by Parker instabilities in the Galactic Centre, where magnetic fields are strong (Fukui et al 2006).…”

The Parker Instability with Cosmic-Ray Streaming

“…Modal analysis shows that the growth time of the most unstable mode is of order the freefall time (Parker 1966), while nonlinear simulations show the growth of Rayleigh-Taylor like "mushrooms" (e.g. Santillán et al (2000)). Thus, any assessment of the Parker Instability in a numerical model must address whether the instability can be resolved in length and time, as well as considering the input physics, the main sensitivities being the equation of state of the gas and the transport model for the cosmic rays.…”

Corrugations in the Disk of the Edge‐On Spiral Galaxy IC 2233

“…Suggestions have included: gravitational instabilities (e.g., Nelson 1976); tidal interactions ( Edelsohn & Elmegreen 1997); collisions of high-velocity clouds with the disk ( Franco et al 1988;Santillán et al 1999); interaction of spiral waves with the gaseous disk (Alfaro et al 2001); and the undular mode of the Parker instability (Franco et al 2002). Depending on their origin, the presence or absence of corrugations can provide important clues into the role of magnetic fields on galaxy disk structure (e.g., Spicker & Feitzinger 1986;Santillán et al 2000;Franco et al 2002), the formation of molecular clouds (e.g., Nelson & Matsuda 1980), the degree of self-gravity of galaxy disks (e.g., Revaz & Pfenniger 2004), or the timescales of recent interactions (e.g., Edelsohn & Elmegreen 1997). Moreover, current evidence suggests that whatever the physical process(es) responsible for corrugations, they are closely linked to the mechanisms responsible for the formation of the dense gas condensations and therefore with the regulation of star formation (e.g., Nelson & Matsuda 1980;Alfaro et al 1992;Alfaro & Efremov 1996;Alfaro 2003).…”