Global Simulations of Accretion Disks. I. Convergence and Comparisons With Local Models

Sorathia, Kareem; Reynolds, C. S.; Stone, James M.; Beckwith, Kris

doi:10.1088/0004-637x/749/2/189

Cited by 136 publications

(232 citation statements)

References 34 publications

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“…The unstratified global simulations of Sorathia et al (2012) suggest that the magnetic tilt angle could be a reliable indicator of numerically resolved MRI. We measure the tilt angle for the magnetic field, which is defined by sin 2θ B = |B r B φ |/B 2 , at 4.5 AU and plot the time-averaged vertical profile of θ B in the bottom panel of Fig.…”

Section: Turbulent Accretion Diskmentioning

confidence: 99%

A conservative orbital advection scheme for simulations of magnetized shear flows with the PLUTO code

Mignone¹,

Flock

Stute

et al. 2012

A&A

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Context. Explicit numerical computations of hypersonic or super-fast differentially rotating disks are subject to the time-step constraint imposed by the Courant condition, according to which waves cannot travel more than a fraction of a cell during a single time-step update. When the bulk orbital velocity largely exceeds any other wave speed (e.g., sound or Alfvén), as computed in the rest frame, the time step is considerably reduced and an unusually large number of steps may be necessary to complete the computation. Aims. We present a robust numerical scheme to overcome the Courant limitation by improving and extending the algorithm previously known as FARGO (fast advection in rotating gaseous objects) to the equations of magnetohydrodynamics (MHD) using a more general formalism. The proposed scheme conserves total angular momentum and energy to machine precision and works in cartesian, cylindrical, or spherical coordinates. The algorithm has been implemented in the next release of the PLUTO code for astrophysical gasdynamics and is suitable for local or global simulations of accretion or proto-planetary disk models. Methods. By decomposing the total velocity into an average azimuthal contribution and a residual term, the algorithm approaches the solution of the MHD equations through two separate steps corresponding to a linear transport operator in the direction of orbital motion and a standard nonlinear solver applied to the MHD equations written in terms of the residual velocity. Since the former step is not subject to any stability restriction, the Courant condition is computed only in terms of the residual velocity, leading to substantially larger time steps. The magnetic field is advanced in time using the constrained transport method in order to fulfill the divergence-free condition. Furthermore, conservation of total energy and angular momentum is enforced at the discrete level by properly expressing the source terms in terms of upwind Godunov fluxes available during the standard solver. Results. Our results show that applications of the proposed orbital-advection scheme to problems of astrophysical relevance provides, at reduced numerical cost, equally accurate and less dissipative results than standard time-marching schemes.

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Section: Turbulent Accretion Diskmentioning

confidence: 99%

A conservative orbital advection scheme for simulations of magnetized shear flows with the PLUTO code

Mignone¹,

Flock

Stute

et al. 2012

A&A

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“…Interpreted in terms of the "tilt angle" θB = 1 2 arcsin(αβ), the estimated values yield θB ∼ 0.2÷5 • . Since comprehensive threedimensional MHD simulations argue for a universal value of θB 12÷13 • (Sorathia et al 2012), we conclude that in real discs the contribution of azimuthal fields should be smaller by a factor of several.…”

Section: Overviewmentioning

confidence: 72%

“…Recent more comprehensive estimates involving threedimensional (3D) MHD modeling argue for a somewhat smaller universal value of αβ 0.4 ÷ 0.5 (Sorathia et al 2012). This should be borne in mind when comparing the simulation results to the real astrophysical applications.…”

Section: Introductionmentioning

confidence: 99%

The thickness of a weakly magnetized accretion flow inside the last stable orbit of a Kerr black hole

Abolmasov¹

2014

Monthly Notices of the Royal Astronomical Society

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If accretion disc contains weak frozen-in entangled magnetic fields, their dynamical effect may be important inside the last stable orbit because of the decompression near the sonic point. Here, I consider the radial and vertical structure of a nearly free-falling flow inside the last stable orbit of a thin disc around a Kerr black hole. The thickness of such a flow is determined primarily by the vertical stress created by radial and azimuthal magnetic fields. The thickness is predicted to oscillate vertically around its equilibrium value determined by the magnetic field balance with gravity. For thin discs, this thickness is much larger than that of the accretion disc itself. Numerical simulations with HARM2d show the vertical structure is more complicated. In particular, magnetically supported disc seems to be unstable to segregation of matter into thinner streams with the vertical scale determined by thermal pressure or other processes.

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“…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.…”

Section: Helicity Fluxes In Accretion Disks and Shearing Boxesmentioning

confidence: 94%

“…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.…”

Section: Introductionmentioning

confidence: 99%

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

Blackman¹

2016

Space Sciences Series of ISSI

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Magnetic fields of laboratory, planetary, stellar, and galactic plasmas commonly exhibit significant order on large temporal or spatial scales compared to the otherwise random motions within the hosting system. Such ordered fields can be measured in the case of planets, stars, and galaxies, or inferred indirectly by the action of their dynamical influence, such as jets. Whether large scale fields are amplified in situ or a remnant from previous stages of an object's history is often debated for objects without a definitive magnetic activity cycle. Magnetic helicity, a measure of twist and linkage of magnetic field lines, is a unifying tool for understanding large scale field evolution for both mechanisms of origin. Its importance stems from its two basic properties: (1) magnetic helicity is typically better conserved than magnetic energy; and (2) the magnetic energy associated with a fixed amount of magnetic helicity is minimized when the system relaxes this helical structure to the largest scale available. Here I discuss how magnetic helicity has come to help us understand the saturation of and sustenance of large scale dynamos, the need for either local or global helicity fluxes to avoid dynamo quenching, and the associated observational consequences. I also discuss how magnetic helicity acts as a hindrance to turbulent diffusion of large scale fields, and thus a helper for fossil remnant large scale field origin models in some contexts. I briefly discuss the connection between large scale fields and accretion disk theory as well. The goal here is to provide a conceptual primer to help the reader efficiently penetrate the literature.

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Global Simulations of Accretion Disks. I. Convergence and Comparisons With Local Models

Cited by 136 publications

References 34 publications

A conservative orbital advection scheme for simulations of magnetized shear flows with the PLUTO code

A conservative orbital advection scheme for simulations of magnetized shear flows with the PLUTO code

The thickness of a weakly magnetized accretion flow inside the last stable orbit of a Kerr black hole

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

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