Dissipation of magnetic fields in very dense interstellar clouds – I. Formulation and conditions for efficient dissipation

• Published in: Monthly notices of the Royal Astronomical Society Vol. 218; no. 4; pp. 663 - 684
• Main Authors: Nakano, Takenori, Umebayashi, Toyoharu
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford University Press 01-02-1986; Blackwell Science
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Interstellar medium (ism) and nebulae in milky way; Stellar systems. Galactic and extragalactic objects and systems. The universe; Dense cloud; Interstellar cloud; Magnetic field; Drift velocity; Interstellar matter
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/218.4.663

The drift velocity VB of magnetic fields relative to the fluid is obtained by solving the motions of many kinds of charged particles. Any closed curve moving with vB has a constant magnetic flux. This velocity contains the effects of both plasma drift and Joule dissipation. At the hydrogen density ${n}_\text{H}\gtrsim{10}^{11}\,\text{cm}^{-3}$, υB is at least 10 times smaller than the free-fall velocity uf, and then field dissipation is inefficient. At ${n}^{H}\gtrsim{10}^{12}\,\text{cm}^{-3}$ and temperature $T\lt{10}^{3}\,\text{K}$ where thermal ionization is inefficient, however, υB exceeds uf unless the characteristic curvature radius of field lines is much greater than the characteristic length of the cloud. In such a situation, charged grains are more abundant than ions, and the field decays mainly through Joule dissipation. Thus the magnetic field is decoupled from the gas and only nearly current-free fields can exist in a part of the cloud with ${n}_\text{H}\gtrsim{10}^{12}\,\text{cm}^{-3}\,\text{and}\,\,T\lt{10}^{3}\,\text{K}$.