Athena: A New Code for Astrophysical MHD

Stone, James M.; Gardiner, Thomas A.; Teuben, Peter; Hawley, John F.; Simon, Jacob B.

doi:10.1086/588755

Cited by 936 publications

(1,099 citation statements)

References 73 publications

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“…It was quantitatively tested by Latter et al (2010) and the results obtained in 3D simulations of MRI-driven turbulence were shown to compare successfully with those obtained with the code Athena (Stone et al 2008) by Fromang & Stone (2009). In the present paper, several simulations are performed in a radially extended box (larger than the disk scale height H).…”

Section: Numerical Setupmentioning

confidence: 84%

Local outflows from turbulent accretion disks

Fromang

Latter

Lesur

et al. 2013

A&A

142

168

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Aims. The aim of this paper is to investigate the properties of accretion disks threaded by a weak vertical magnetic field, with a particular focus on the interplay between magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI) and outflows that might be launched from the disk. Methods. For that purpose, we use a set of numerical simulations performed with the MHD code RAMSES in the framework of the shearing box model. We concentrate on the case of a rather weak vertical magnetic field such that the initial ratio β 0 of the thermal and magnetic pressures in the disk midplane equals 10 4 . Results. As reported recently, we find that MHD turbulence drives an efficient outflow out of the computational box. We demonstrate a strong sensitivity of that result to the box size: enlargements in the radial and vertical directions lead to a reduction of up to an order of magnitude in the mass-loss rate. Such a dependence prevents any realistic estimates of disk mass-loss rates being derived using shearing-box simulations. We find however that the flow morphology is robust and independent of the numerical details of the simulations. Its properties display some features and approximate invariants that are reminiscent of the Blandford & Payne launching mechanism, but differences exist. For the magnetic field strength considered in this paper, we also find that angular momentum transport is most likely dominated by MHD turbulence, the saturation of which scales with the magnetic Prandtl number, the ratio of viscosity and resistivity, in a way that is in good agreement with expectations based on unstratified simulations. Conclusions. This paper thus demonstrates for the first time that accretion disks can simultaneously exhibit MRI-driven MHD turbulence along with magneto-centrifugally accelerated outflows. However, in contradiction with previously published results, such outflows probably have little impact on the disk dynamics.

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Section: Numerical Setupmentioning

confidence: 84%

Local outflows from turbulent accretion disks

Fromang

Latter

Lesur

et al. 2013

A&A

142

168

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“…For our simulation we use the MHD code ATHENA [22,23], a cartesian, time explicit, unsplit, Godunov, code parallelized by MPI for compressible MHD. The mass conservation, momentum, and magnetic induction equation to be used are given by ∂ρ ∂t + ∇· (ρv) = 0…”

Section: Methodsmentioning

confidence: 99%

Simulations reveal fast mode shocks in magnetic reconnection outflows

2011

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Magnetic reconnection is commonly perceived to drive flow and particle acceleration in flares of solar, stellar, and astrophysical disk coronae but the relative roles of different acceleration mechanisms in a given reconnection environment are not well understood. While outflow fast mode shocks have been predicted analytically, we show for the first time via direct numerical simulations that such shocks do indeed occur in the outflows of fast reconnection when an obstacle is present. These shocks are are distinct from the slow mode Petschek inflow shocks. If Fermi acceleration of electrons operates in the weak fast shocks the associated compression ratios will induce a Fermi acceleration particle spectrum that is significantly steeper than strong fast shocks commonly studied, but consistent with the demands of solar flares. While this is not the only acceleration mechanism operating in a reconnection environment, it is plausibly a ubiquitous one.PACS codes: 96.60. Iv, 52.35.Vd, 95.30.Qd, 52.35.Tc, 96.60.qe, 98.54.Cm

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“…Then the ideal MHD equations with radiation energy and momentum source terms are (Stone et al 2008;JSD12) …”

Section: Equationsmentioning

confidence: 99%

An Algorithm for Radiation Magnetohydrodynamics Based on Solving the Time-Dependent Transfer Equation

Jiang¹,

Stone²,

Davis³

2014

ApJS

Self Cite

131

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We describe a new algorithm for solving the coupled frequency-integrated transfer equation and the equations of magnetohydrodynamics in the regime that light-crossing time is only marginally shorter than dynamical timescales. The transfer equation is solved in the mixed frame, including velocity-dependent source terms accurate to O(v/c). An operator split approach is used to compute the specific intensity along discrete rays, with upwind monotonic interpolation used along each ray to update the transport terms, and implicit methods used to compute the scattering and absorption source terms. Conservative differencing is used for the transport terms, which ensures the specific intensity (as well as energy and momentum) are conserved along each ray to round-off error. The use of implicit methods for the source terms ensures the method is stable even if the source terms are very stiff. To couple the solution of the transfer equation to the MHD algorithms in the Athena code, we perform direct quadrature of the specific intensity over angles to compute the energy and momentum source terms. We present the results of a variety of tests of the method, such as calculating the structure of a non-LTE atmosphere, an advective diffusion test, linear wave convergence tests, and the well-known shadow test. We use new semi-analytic solutions for radiation modified shocks to demonstrate the ability of our algorithm to capture the effects of an anisotropic radiation field accurately. Since the method uses explicit differencing of the spatial operators, it shows excellent weak scaling on parallel computers.

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Athena: A New Code for Astrophysical MHD

Cited by 936 publications

References 73 publications

Local outflows from turbulent accretion disks

Local outflows from turbulent accretion disks

Simulations reveal fast mode shocks in magnetic reconnection outflows

An Algorithm for Radiation Magnetohydrodynamics Based on Solving the Time-Dependent Transfer Equation

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