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Title: On the physical nature of accretion disc viscosity
Authors: Martin, RG
Nixon, CJ
Pringle, JE
Livio, M
First Published: 7-Jan-2019
Publisher: Elsevier
Citation: New Astronomy, 2019, 70, pp. 7-11
Abstract: We use well–established observational evidence to draw conclusions about the fundamental nature of the viscosity in accretion discs. To do this, we first summarise the observational evidence for the value of the dimensionless accretion disc viscosity parameter α, defined by Shakura and Sunyaev (1973, 1976). We find that, for fully ionized discs, the value of α is readily amenable to reliable estimation and that the observations are consistent with the hypothesis that α∼0.2−0.3. In contrast in discs that are not fully ionized, estimates of the value of α are generally less direct and the values obtained are generally < 0.01 and often ≪ 0.01. We conclude that this gives us crucial information about the nature of viscosity in accretion discs. First, in fully ionized discs the strength of the turbulence is always limited by being at most trans-sonic. This implies that it is necessary that credible models of the turbulence reflect this fact. Second, the smaller values of α found for less ionized, and therefore less strongly conducting, discs imply that magnetism plays a dominant role. This provides important observational support for the concept of magneto-rotational instability (MRI) driven hydromagnetic turbulence.
DOI Link: 10.1016/j.newast.2019.01.001
ISSN: 1384-1076
Embargo on file until: 7-Jan-2020
Version: Post-print
Status: Peer-reviewed
Type: Journal Article
Rights: Copyright © 2019 Elsevier. After an embargo period this version of the paper will be an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-No Derivatives License (, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
Description: The file associated with this record is under embargo until 12 months after publication, in accordance with the publisher's self-archiving policy. The full text may be available through the publisher links provided above.
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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