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Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability

Squire, J. and Quataert, E. and Kunz, M. W. (2017) Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability. Journal of Plasma Physics, 83 (06). Art. No. 905830613. ISSN 0022-3778. PMCID PMC5893179.

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In collisionless and weakly collisional plasmas, such as hot accretion flows onto compact objects, the magnetorotational instability (MRI) can differ significantly from the standard (collisional) MRI. In particular, pressure anisotropy with respect to the local magnetic-field direction can both change the linear MRI dispersion relation and cause nonlinear modifications to the mode structure and growth rate, even when the field and flow perturbations are very small. This work studies these pressure-anisotropy-induced nonlinearities in the weakly nonlinear, high-ion-beta regime, before the MRI saturates into strong turbulence. Our goal is to better understand how the saturation of the MRI in a low-collisionality plasma might differ from that in the collisional regime. We focus on two key effects: (i) the direct impact of self-induced pressure-anisotropy nonlinearities on the evolution of an MRI mode, and (ii) the influence of pressure anisotropy on the ‘parasitic instabilities’ that are suspected to cause the mode to break up into turbulence. Our main conclusions are: (i) The mirror instability regulates the pressure anisotropy in such a way that the linear MRI in a collisionless plasma is an approximate nonlinear solution once the mode amplitude becomes larger than the background field (just as in magnetohyrodynamics). This implies that differences between the collisionless and collisional MRI become unimportant at large amplitudes. (ii) The break up of large-amplitude MRI modes into turbulence via parasitic instabilities is similar in collisionless and collisional plasmas. Together, these conclusions suggest that the route to magnetorotational turbulence in a collisionless plasma may well be similar to that in a collisional plasma, as suggested by recent kinetic simulations. As a supplement to these findings, we offer guidance for the design of future kinetic simulations of magnetorotational turbulence.

Item Type:Article
Related URLs:
URLURL TypeDescription CentralArticle
Squire, J.0000-0001-8479-962X
Quataert, E.0000-0001-9185-5044
Kunz, M. W.0000-0003-1676-6126
Additional Information:© 2017 Cambridge University Press. Received 14 September 2017; revised 26 November 2017; accepted 27 November 2017. We would like to thank P. Sharma for kindly providing the modified version of the ZEUS code, which formed the basis for the nonlinear results of § 4.2. J.S. was funded in part by the Gordon and Betty Moore Foundation through grant GBMF5076 to L. Bildsten, E.Q. and E. S. Phinney. E.Q. was supported by Simons Investigator awards from the Simons Foundation and NSF grants AST 13-33612 and AST 17-15054. M.W.K. was supported in part by NASA grant NNX17AK63G and US DOE Contract DE-AC02-09-CH11466. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1548562. Some numerical calculations were carried out on the Comet system at the San Diego supercomputing center, through allocation TG-AST160068.
Funding AgencyGrant Number
Gordon and Betty Moore FoundationGBMF5076
Simons FoundationUNSPECIFIED
NSFAST 13-33612
NSFAST 17-15054
Department of Energy (DOE)DE-AC02-09-CH11466
Subject Keywords:astrophysical plasmas, plasma instabilities, plasma nonlinear phenomena
Issue or Number:06
PubMed Central ID:PMC5893179
Record Number:CaltechAUTHORS:20180423-092944632
Persistent URL:
Official Citation:Squire, J., Quataert, E., & Kunz, M. (2017). Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability. Journal of Plasma Physics, 83(6), 905830613. doi:10.1017/S0022377817000940
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:86003
Deposited By: Tony Diaz
Deposited On:23 Apr 2018 18:36
Last Modified:14 Apr 2020 22:11

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