TY - JOUR
T1 - Shearing-box simulations of MRI-driven turbulence in weakly collisional accretion discs
AU - Kempski, Philipp
AU - Quataert, Eliot
AU - Squire, Jonathan
AU - Kunz, Matthew Walter
N1 - Funding Information:
This work was supported in part by NSF grants AST 13-33612, AST 1715054, and a Simons Investigator award from the Simons Foundation. JS would like to acknowledge the support of a Rutherford Discovery Fellowship and the Marsden Fund, administered by the New Zealand Royal Society Te Aparangi. MWK was supported by NASA grant NNX17AK63G. This work was also made possible by computing time granted by UCB on the Savio cluster.
Funding Information:
We thank C. Gammie, F. Foucart, A. Schekochihin, and J. Stone for useful discussions, as well as all the members of the horizon collaboration, http://horizon.astro.illinois.edu, for their advice and encouragement. This work was supported in part by NSF grants AST 13-33612, AST 1715054, and a Simons Investigator award from the Simons Foundation. JS would like to acknowledge the support of a Rutherford Discovery Fellowship and the Marsden Fund, administered by the New Zealand Royal Society Te Apa¯rangi. MWK was supported by NASA grant NNX17AK63G. This work was also made possible by computing time granted by UCB on the Savio cluster.
Publisher Copyright:
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.
PY - 2019/7/1
Y1 - 2019/7/1
N2 - We present a systematic shearing-box investigation of magnetorotational instability (MRI)-driven turbulence in a weakly collisional plasma by including the effects of an anisotropic pressure stress, i.e. anisotropic (Braginskii) viscosity. We constrain the pressure anisotropy (Δp) to lie within the stability bounds that would be otherwise imposed by kinetic microinstabilities. We explore a broad region of parameter space by considering different Reynolds numbers and magnetic-field configurations, including net vertical flux, net toroidal-vertical flux, and zero net flux. Remarkably, we find that the level of turbulence and angular-momentum transport are not greatly affected by large anisotropic viscosities: the Maxwell and Reynolds stresses do not differ much from the MHD result. Angular-momentum transport in Braginskii MHD still depends strongly on isotropic dissipation, e.g. the isotropic magnetic Prandtl number, even when the anisotropic viscosity is orders of magnitude larger than the isotropic diffusivities. Braginskii viscosity nevertheless changes the flow structure, rearranging the turbulence to largely counter the parallel rate of strain from the background shear. We also show that the volume-averaged pressure anisotropy and anisotropic viscous transport decrease with increasing isotropic Reynolds number (Re); e.g. in simulations with net vertical field, the ratio of anisotropic to Maxwell stress (αA/αM) decreases from ∼0.5 to ∼0.1 as we move from Re ∼103 to Re ∼104, while-4$\pi$Δp/B2 → 0. Anisotropic transport may thus become negligible at high Re. Anisotropic viscosity nevertheless becomes the dominant source of heating at large Re, accounting for ${\gtrsim } 50 {{\\rm per\cent}}$ of the plasma heating. We conclude by briefly discussing the implications of our results for radiatively inefficient accretion flows on to black holes.
AB - We present a systematic shearing-box investigation of magnetorotational instability (MRI)-driven turbulence in a weakly collisional plasma by including the effects of an anisotropic pressure stress, i.e. anisotropic (Braginskii) viscosity. We constrain the pressure anisotropy (Δp) to lie within the stability bounds that would be otherwise imposed by kinetic microinstabilities. We explore a broad region of parameter space by considering different Reynolds numbers and magnetic-field configurations, including net vertical flux, net toroidal-vertical flux, and zero net flux. Remarkably, we find that the level of turbulence and angular-momentum transport are not greatly affected by large anisotropic viscosities: the Maxwell and Reynolds stresses do not differ much from the MHD result. Angular-momentum transport in Braginskii MHD still depends strongly on isotropic dissipation, e.g. the isotropic magnetic Prandtl number, even when the anisotropic viscosity is orders of magnitude larger than the isotropic diffusivities. Braginskii viscosity nevertheless changes the flow structure, rearranging the turbulence to largely counter the parallel rate of strain from the background shear. We also show that the volume-averaged pressure anisotropy and anisotropic viscous transport decrease with increasing isotropic Reynolds number (Re); e.g. in simulations with net vertical field, the ratio of anisotropic to Maxwell stress (αA/αM) decreases from ∼0.5 to ∼0.1 as we move from Re ∼103 to Re ∼104, while-4$\pi$Δp/B2 → 0. Anisotropic transport may thus become negligible at high Re. Anisotropic viscosity nevertheless becomes the dominant source of heating at large Re, accounting for ${\gtrsim } 50 {{\\rm per\cent}}$ of the plasma heating. We conclude by briefly discussing the implications of our results for radiatively inefficient accretion flows on to black holes.
KW - MHD
KW - accretion, accretion discs
KW - instabilities
KW - plasmas
KW - turbulence
UR - http://www.scopus.com/inward/record.url?scp=85072342503&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85072342503&partnerID=8YFLogxK
U2 - 10.1093/mnras/stz1111
DO - 10.1093/mnras/stz1111
M3 - Article
C2 - 35136273
AN - SCOPUS:85072342503
VL - 486
SP - 4013
EP - 4029
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 3
ER -