Dynamo-generated Turbulence and Large-Scale Magnetic Fields in a Keplerian Shear Flow

Abstract

The nonlinear evolution of magnetized Keplerian shear ows is simulated in a local, three-dimensional model, including the e ects of compressibility and strati cation. Supersonic ows are initially generated by the Balbus-Hawley magnetic shear instability. The resulting ows regenerate a turbulent magnetic eld which, in turn, reinforces the turbulence. Thus, the system acts like a dynamo that generates its own turbulence. However, unlike usual dynamos, the magnetic energy exceeds the kinetic energy of the turbulence by a factor of 3{10. By assuming the eld to be vertical on the outer (upper and lower) surfaces we do not constrain the horizontal magnetic ux. Indeed, a large scale toroidal magnetic eld is generated, mostly in the form of toroidal ux tubes with lengths comparable to the toroidal extent of the box. This large scale eld is mainly of Present address: Nordita, Blegdamsvej 17, DK-2100 Copenhagen , Denmark y The National Center for Atmospheric Research is sponsored by the National Science Foundation 1 even (i.e. quadrupolar) parity with respect to the midplane and changes direction on a timescale of about 30 orbits, in a possibly cyclic manner. The e ective Shakura-Sunyaev alpha viscosity parameter is between 0.001 and 0.005, and the contribution from the Maxwell stress is about 3-7 times larger than the contribution from the Reynolds stress.

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

“…Essentially all MRI unstable simulations with large enough vertical domains, whether stratified, unstratified, local or global, show the generation of large scale toroidal fields of the same flux for cycle periods of ∼ 10 orbits (Brandenburg et al 1995, Lesur & Ogilvie 2008, Davis et al 2010Simon et al 2011;Guan & Gammie 2011;Sorathia et al 2012;Suzuki and Inusuka 2013;Ebrahimi & Bhattacharjee 2014). The patterns indicate a large scale dynamo operating contemporaneously with the small scale dynamo.…”

“…Moffatt 1978;Parker 1979;Krause & Rädler 1980;Pouquet et al 1976;Blackman & Brandenburg 2002). Alternatively, there is emerging agreement that a combination of shear and fluctuating kinetic helicity can conspire to produce an LSD (Vishniac & Brandenburg 1997;Brandenburg 1995;Brandenburg 2005;Yousef et al 2008;Heinemann et al 2011;Mitra & Brandenburg 2012;Sridhar & Singh 2013). The EMF can also be sustained by magnetic helicity fluxes, either local (within sectors of a closed volume) or global (fluxes thought the system boundary) as we will later discuss.…”

“…A plethora of numerical simulations now commonly reveal that the systems evolve to a nonlinear turbulent steady state whose Maxwell stress dominates the Reynolds stress and for which large scale ordered magnetic fields emerge with cycle periods ∼ 10 orbit periods (e.g. Brandenburg et al 1995;Davis et al 2010;Simon et al 2011;Guan & Gammie 2011;Sorathia et al 2012;Suzuki and Inutsuka 2013). With the caveat that angular momentum plays a comparatively subdominant role in structural support for stars, the coronae of stars and the emergence of large scale ordered fields that thread coronal holes of the sun (e.g.…”

Problems and Progress in Astrophysical Dynamos

“…These are the simulations of magnetized, ionized accretion disks (e.g. [36,39,40,73,14,15] see also [3,38] for a review) which are subject to the magneto-rotational instability. These simulations include extremely simplified physics and cover a limited set of spatial scales and geometries.…”

“…In accretion disks the typical magnetic field energy density is probably an order of magnitude below the ambient gas pressure (e.g. [39,40,73,14,15]) but they play a critical role in the outward transfer of angular momentum and the dissipation of orbital energy. Moreover, in optically thin environments the presence of a strong magnetic field can have a dramatic effect on the luminosity and spectrum of an object.…”