Propagation of spiral density waves in a magneto-active disk

Details Propagation of spiral density waves in a magneto-active disk Propagation of spiral density waves in a magneto-active disk

Jan 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994ap%26ss.211...13c&link_type=abstract

Astrophysics and Space Science (ISSN 0004-640X), vol. 211, no. 1, p. 13-29

Astronomy and Astrophysics

Astrophysics

2

Density Wave Model, Magnetic Fields, Magnetoacoustic Waves, Spiral Wrapping, Wave Propagation, Asymptotic Methods, Continuum Flow, Inhomogeneity, Solar Corona, Star Formation, Wentzel-Kramer-Brillouin Method

Scientific paper

The propagation of spiral density waves in a differentially rotating, self-gravitating, magnetoactive and highly flattened disk is investigated by using the asymptotic theory for tightly wound spirals developed by Lin and his collaborators. We adopt the continuum fluid model as the primary basis, and our treatment will be largely analytical. The disk plasma is studied in the frozen field approximation and inhomogeneous magnetic fields in the plane of the disk are considered in detail. In a differentially rotating disk with strong magnetic fields, the field lines will be considerably distorted and the mutual influence of magnetic fields and differential rotation is by no means obvious. In this paper we present a new asymptotic dispersion relation for tightly wound spiral density waves with magnetic fields along the spiral arms Btheta(r). The effects of the magnetic fields lead to such terms like k2(a2+(VA)2), where k is the wave number, a represents the speed of sound, VA = (B2/4 pi rho)1/2 is the Alfven speed, B denotes the field strength, and rho is the plasma density. These terms depict the well-known magnetoacoustic waves and could have been anticipated without a detailed computation. However the interaction of magnetic fields and differential rotation may give rise to other significant terms which are not so easy to foresee. We also present a more exact local dispersion relation by using the Wentzel-Kramer-Brillouin (WKB) approximation and study the effects of magnetic fields on the growth rate through the parameters Q and J defined in the literature. Although the effects of the magnetic fields are rather insignificant for applications to Galactic dynamics, the effects of the magnetic fields are important for applications to star formation and problems related to the solar nebula.

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