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Motivation and challenge to capture both large-scale and local transport in next generation accretion theory

Part of: Macroscopic Randomness Focus on Plasma Astrophysics

Published online by Cambridge University Press:  20 August 2015

Eric G. Blackman  and
Farrukh Nauman
Eric G. Blackman*
Affiliation:
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA School of Natural Sciences, Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA
Farrukh Nauman
Affiliation:
Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
*
†Email address for correspondence: [email protected]
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Abstract

Accretion disc theory is less developed than stellar evolution theory although a similarly mature phenomenological picture is ultimately desired. While the interplay of theory and numerical simulations has amplified community awareness of the role of magnetic fields in angular momentum transport, there remains a long term challenge to incorporate the insights gained from simulations into improving practical models for comparison with observations. What has been learned from simulations that can lead to improvements beyond SS73 in practical models? Here, we emphasize the need to incorporate the role of non-local transport more precisely. To show where large-scale transport would fit into the theoretical framework and how it is currently missing, we review why the wonderfully practical approach of Shakura & Sunyaev (Astron. Astrophys., vol. 24, 1973, pp. 337–355, SS73) is necessarily a mean field theory, and one which does not include large-scale transport. Observations of coronae and jets, combined with the interpretation of results from shearing box simulations, of the magnetorotational instability (MRI) suggest that a significant fraction of disc transport is indeed non-local. We show that the Maxwell stresses in saturation are dominated by large-scale contributions and that the physics of MRI transport is not fully captured by a viscosity. We also clarify the standard physical interpretation of the MRI as it applies to shearing boxes. Computational limitations have so far focused most attention toward local simulations, but the next generation of global simulations should help to inform improved mean field theories. Mean field accretion theory and mean field dynamo theory should in fact be unified into a single theory that predicts the time evolution of spectra and luminosity from separate disc, corona and outflow contributions. Finally, we note that any mean field theory, including that of SS73, has a finite predictive precision that needs to be quantified when comparing the predictions to observations.

Type
Research Article
Information
Journal of Plasma Physics , Volume 81 , Issue 5 , October 2015 , 395810505
Copyright
© Cambridge University Press 2015 

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