Comments on Alfven Waves and Plasma Drift in a Slightly Ionized Gas

Details Comments on Alfven Waves and Plasma Drift in a Slightly Ionized Gas Comments on Alfven Waves and Plasma Drift in a Slightly Ionized Gas

Jun 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...500..257k&link_type=abstract

Astrophysical Journal v.500, p.257

Astronomy and Astrophysics

Astronomy

5

Diffusion, Ism: Clouds, Ism: Magnetic Fields, Magnetohydrodynamics: Mhd, Stars: Formation, Waves

Scientific paper

It is well known that there exist two critical wavelengths for Alfven waves in a slightly ionized medium. One of the two is lambda 1 = pi VA tau ni, where VA is the Alfven speed in the medium and tau ni is the friction timescale for neutral particles due to ions. The other is lambda 2=4 pi V^{*}_{{A}} tau _{{in}} , where V^{*}_{{A}} is the Alfven speed in the ions and tau in is the friction timescale for the ions, respectively. In typical molecular clouds, lambda 2 is much smaller than lambda 1. We let lambda * denote the intermediate range: lambda 2 < lambda * < lambda 1, where the so-called ion-neutral damping of the Alfven waves occurs. In this paper, we show that magnetic forces affect the neutral fluid even at lambda *. This is possible when lambda 2 is much larger than the mean free path Lamda of the neutral particles for neutral-neutral collision. When tau in is much longer than the collision timescale among the neutral particles, tau nn, the communication timescale of the magnetic fields and the neutral fluid is tau in. If tau in >> tau nn, the neutral fluid can adjust itself to the magnetic forces during tau in. For typical molecular clouds, it is estimated that Lamda << lambda 2 and tau nn << tau in(<< tau ni). It may suggest that magnetically subcritical cloud cores contract quasi-statically with magnetic flux loss, even if the size is lambda *. On the other hand, if a disturbance scale of the magnetic field is smaller than lambda 2, the magnetic stress can escape as Alfven waves that couple almost exclusively with the ions. In addition, we point out that it is difficult for the Jeans length to become smaller than lambda 2 in isolated molecular clouds or cores. Observational implications are also presented.

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