GIANT MOLECULAR CLOUD FORMATION THROUGH THE PARKER INSTABILITY IN A SKEWED MAGNETIC-FIELD

The effect of the magnetic skew on the Parker instability is investigated by means of the linear stability analysis for a gravitationally stratified gas layer permeated by a horizontal magnetic field. When the magnetic field is skewed (i.e., the field line direction is a function of the height), the wavelength of the most unstable mode is lambda approximately 10H where H is the pressure scale height. The growth rate of the short-wavelength modes is greatly reduced when the gradient in the magnetic field direction exceeds 0.5 rad per scale height. Our results indicate that the Parker instability in a skewed magnetic field preferentially forms large-scale structures like giant molecular clouds.