Radiation Magneto-hydrodynamic Simulations of MRI with Zero-Net-Vertical-Flux: Necessity of Resolving the Thermal Scale For Strongly Magnetized Initial Conditions
Abstract
We perform 3D radiation magneto-hydrodynamic (RMHD) shearing-box simulations of local patches of optically thick accretion disks, applicable to sub-Eddington active galactic nuclei (AGN) at ∼ 1000 gravitational radii around a supermassive black hole (SMBH) of 107−108M⊙. In particular, we set up zero-net-vertical-flux (ZNVF) simulations with strong net azimuthal fields (By) characterized by initial gas-to-magnetic pressure ratio β0. We find that β0 ∼ 1 simulations relax to the classical MRI state with steady-state β∼ 10−20 and slow periodic reversals of the mean azimuthal field (dynamo cycles), losing memory of their initial By configuration. The outcome of simulations starting from β0=0.1 (superthermal magnetic pressures) depends upon the numerical resolution as quantified by the number of grid cells per thermal scale height Hth. Resolved simulations (Δz≤ Hth/5) settle into final states similar to the larger-β0 runs, exhibiting thermally dominated midplanes heated by MRI turbulence and undergoing dynamo cycles. In contrast, lower resolution β0 = 0.1 runs evolve towards states dominated by a coherent mean field, similar to what is observed in recent isothermal MHD simulations. However, those disks fail to sustain sufficient turbulent heating near the midplane and undergo runaway cooling and contraction. While such states might be sustained in global models with even lower initial β0 and/or continuous By injection, our results indicate that they could arise purely from under-resolving the midplane MRI dynamo that would otherwise be able to generate and emanate randomized fields. We highlight the need to resolve a fraction of the thermal scale height (the classical MRI wavelengths) for strongly magnetized initial conditions to obtain converged outcomes in RMHD simulations.
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BibTeX
@misc{chen2026radiation,
title = {{Radiation Magneto-hydrodynamic Simulations of MRI with Zero-Net-Vertical-Flux: Necessity of Resolving the Thermal Scale For Strongly Magnetized Initial Conditions}},
author = {Yi-Xian Chen and Yan-Fei Jiang and Jeremy Goodman},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.17813},
eprint = {2609.17813},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.17813},
}
RIS
TY - UNPB
TI - Radiation Magneto-hydrodynamic Simulations of MRI with Zero-Net-Vertical-Flux: Necessity of Resolving the Thermal Scale For Strongly Magnetized Initial Conditions
AU - Yi-Xian Chen
AU - Yan-Fei Jiang
AU - Jeremy Goodman
PY - 2026
DA - 2026/09/15
PB - arXiv
DO - 10.48550/arxiv.2609.17813
UR - https://arxiv.org/abs/2609.17813
AB - We perform 3D radiation magneto-hydrodynamic (RMHD) shearing-box simulations of local patches of optically thick accretion disks, applicable to sub-Eddington active galactic nuclei (AGN) at ∼ 1000 gravitational radii around a supermassive black hole (SMBH) of 10^7−10^8M_⊙. In particular, we set up zero-net-vertical-flux (ZNVF) simulations with strong net azimuthal fields (B_y) characterized by initial gas-to-magnetic pressure ratio β_0. We find that β_0 ∼ 1 simulations relax to the classical MRI state with steady-state β∼ 10−20 and slow periodic reversals of the mean azimuthal field (dynamo cycles), losing memory of their initial B_y configuration. The outcome of simulations starting from β_0=0.1 (superthermal magnetic pressures) depends upon the numerical resolution as quantified by the number of grid cells per thermal scale height H_th. Resolved simulations (Δz≤ H_th/5) settle into final states similar to the larger-β_0 runs, exhibiting thermally dominated midplanes heated by MRI turbulence and undergoing dynamo cycles. In contrast, lower resolution β_0 = 0.1 runs evolve towards states dominated by a coherent mean field, similar to what is observed in recent isothermal MHD simulations. However, those disks fail to sustain sufficient turbulent heating near the midplane and undergo runaway cooling and contraction. While such states might be sustained in global models with even lower initial β_0 and/or continuous B_y injection, our results indicate that they could arise purely from under-resolving the midplane MRI dynamo that would otherwise be able to generate and emanate randomized fields. We highlight the need to resolve a fraction of the thermal scale height (the classical MRI wavelengths) for strongly magnetized initial conditions to obtain converged outcomes in RMHD simulations.
ER -