The effect of the random magnetic field component on the Parker instability

The Parker instability is considered to play important roles in the evoluti on of the interstellar medium. Most studies on the development of the insta bility so far have been based on an initial equilibrium system with a unifo rm magnetic field. However, the Galactic magnetic field possesses a random component in addition to the mean uniform component, with comparable streng th of the two components. Parker and Jokipii have recently suggested that t he random component can suppress the growth of small wavelength perturbatio ns. Here we extend their analysis by including gas pressure, which was igno red in their work, and study the stabilizing effect of the random component in the interstellar gas with finite pressure. Following Parker and Jokipii , we model the magnetic field as a mean azimuthal component, B(z), plus a r andom radial component, epsilon (z)B(z), where epsilon (z) is a random func tion of height from the equatorial plane. We show that for the observationa lly suggested values of [epsilon (2)](1/2), the tension due to the random c omponent becomes important, so that the growth of the instability is either significantly reduced or completely suppressed. When the instability still works, the radial wavenumber of the most unstable mode is found to be zero . That is, the instability is reduced to be effectively two-dimensional. We discuss briefly the implications of our finding.