On the minimum length for magnetic waves in molecular clouds

Details On the minimum length for magnetic waves in molecular clouds On the minimum length for magnetic waves in molecular clouds

Mar 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...270..397e&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 270, no. 1-2, p. 397-400.

Astronomy and Astrophysics

Astrophysics

21

Interstellar Magnetic Fields, Interstellar Matter, Magnetohydrodynamic Waves, Molecular Clouds, Cosmic Dust, Wave Propagation

Scientific paper

The minimum wavelength for the propagation of magnetic waves in molecular clouds is derived, considering both ion-neutral and grain-neutral coIlisional viscosity. For the Mathis, Rumpl, and Nordsieck grain size distribution and an Alfven speed v(A) = 1 km/s, the rate of wave damping from collisions between neutral molecules and charged grains exceeds the damping rate for ion-neutral collisions when the gas density is in the range about 5 x 10 exp 3 - 4 x 10 exp 10/cu cm; the upper limit is where the smallest grains stop gyrating around the field. For the Mathis and Whiffen distribution, which has fewer small grains, the ions always dominate the viscosity because at a high enough density for the total ion cross section to drop below the total grain cross section, the grains stop gyrating around the field. In either case, the molecular gas is strongly coupled to magnetic waves on all scales where the density is less than 10 exp 6/cu cm for v(A) = 1 km/s and it is strongly coupled on all scales independent of density if B = constant, which seems likely for small, nonself-gravitating clumps. Such coupling allows the formation of even the smallest observed cloud structures by non-linear hydromagnetic processes.

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