Galactic Dynamos

Brandenburg, Axel; Ntormousi, Evangelia

doi:10.1146/annurev-astro-071221-052807

Cited by 14 publications

(4 citation statements)

References 230 publications

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“…This review summarises the current state-of-the-art of these numerical simulations and their results. See also the recent review by Brandenburg and Ntormousi ( 2023 ).…”

Section: Introductionmentioning

confidence: 99%

Computational approaches to modeling dynamos in galaxies

Korpi-Lagg,

Mac Low,

Gent

2024

Living Rev Comput Astrophys

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Galaxies are observed to host magnetic fields with a typical total strength of around 15 $$\upmu $$ μ G. A coherent large-scale field constitutes up to a few microgauss of the total, while the rest is built from strong magnetic fluctuations over a wide range of spatial scales. This represents sufficient magnetic energy for it to be dynamically significant. Several questions immediately arise: What is the physical mechanism that gives rise to such magnetic fields? How do these magnetic fields affect the formation and evolution of galaxies? In which physical processes do magnetic fields play a role, and how can that role be characterized? Numerical modelling of magnetized flows in galaxies is playing an ever-increasing role in finding those answers. We review major techniques used for these models. Current results strongly support the conclusion that field growth occurs during the formation of the first galaxies on timescales shorter than their accretion timescales due to small-scale turbulent dynamos. The saturated small-scale dynamo maintains field strengths at only a few percent of equipartition with turbulence. This is in contradiction with the observed magnitude of turbulent fields, but may be reconciled by the further contribution to the turbulent field of the large-scale dynamo. The subsequent action of large-scale dynamos in differentially rotating discs produces field strengths observed in low redshift galaxies, where it reaches equipartition with the turbulence and has substantial power at large scales. The field structure resulting appears consistent with observations including Faraday rotation and polarisation from synchrotron and dust thermal emission. Major remaining challenges include scaling numerical models toward realistic scale separations and Prandtl and Reynolds numbers.

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“…This review summarises the current state-of-the-art of these numerical simulations and their results. See also the recent review by Brandenburg and Ntormousi ( 2023 ).…”

Section: Introductionmentioning

confidence: 99%

Computational approaches to modeling dynamos in galaxies

Korpi-Lagg,

Mac Low,

Gent

2024

Living Rev Comput Astrophys

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“…Over the past decade it has become increasingly clear that magnetohydrodynamics (MHD) plays a significant role in a variety of astrophysical phenomena (e.g., Charbonneau 2014;Beck 2016;Han 2017;Kaspi & Beloborodov 2017;Naab & Ostriker 2017;Blandford et al 2019;Davis & Tchekhovskoy 2020;Nakariakov & Kolotkov 2020;Philippov & Kramer 2022;Trujillo Bueno & del Pino Alemán 2022;Brandenburg & Ntormousi 2023). Magnetic fields couple to gas both directly through plasma interactions with the magnetic field, and indirectly through cosmic ray transport (e.g., Pfrommer et al 2017;Chan et al 2019;Girichidis et al 2020Girichidis et al , 2022Buck et al 2020;Werhahn et al 2021Werhahn et al , 2023Yoshida et al 2021;Nuñez-Castiñeyra et al 2022), anisotropic conduction (e.g., Yang et al 2012;Hanasz et al 2013;Girichidis et al 2016;Pakmor et al 2016;Simpson et al 2016;Brüggen et al 2023), and other physical effects.…”

Section: Introductionmentioning

confidence: 99%

“…Different simulations employing different numerical methods show varying impacts of magnetic fields, ranging from magnetic fields being largely irrelevant on large scales to magnetic fields being critically important in determining the evolution and structure of the interstellar medium, modifying galactic feedback, and influencing the structure of the circumgalactic medium (CGM; Dobbs & Price 2008;Hanasz et al 2009;Pakmor & Springel 2013;Kim & Ostriker 2015;Banda-Barragán et al 2018;Martin-Alvarez et al 2020). One factor in this uncertainty is the effect of numerical resolution on MHD simulations-galactic magnetic fields are likely amplified by a turbulent dynamo, which operates over a large dynamic range (Beck et al 1996;Martin-Alvarez et al 2018;Gent et al 2021;Carteret et al 2023;Brandenburg & Ntormousi 2023). The accuracy with which the dynamo is captured thus depends on both the numerical method that is employed, as well as the resolution.…”

Section: Introductionmentioning

confidence: 99%

Cholla-MHD: An Exascale-capable Magnetohydrodynamic Extension to the Cholla Astrophysical Simulation Code

Caddy,

Schneider

2024

ApJ

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We present an extension of the massively parallel, GPU native, astrophysical hydrodynamics code Cholla to magnetohydrodynamics (MHD). Cholla solves the ideal MHD equations in their Eulerian form on a static Cartesian mesh utilizing the Van Leer + constrained transport integrator, the HLLD Riemann solver, and reconstruction methods at second and third order. Cholla’s MHD module can perform ≈260 million cell updates per GPU-second on an NVIDIA A100 while using the HLLD Riemann solver and second order reconstruction. The inherently parallel nature of GPUs combined with increased memory in new hardware allows Cholla’s MHD module to perform simulations with resolutions ∼5003 cells on a single high-end GPU (e.g., an NVIDIA A100 with 80 GB of memory). We employ GPU direct Message Passing Interface to attain excellent weak scaling on the exascale supercomputer Frontier, while using 74,088 GPUs and simulating a total grid size of over 7.2 trillion cells. A suite of test problems highlights the accuracy of Cholla’s MHD module and demonstrates that zero magnetic divergence in solutions is maintained to round off error. We also present new testing and CI tools using GoogleTest, GitHub Actions, and Jenkins that have made development more robust and accurate and ensure reliability in the future.

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“…The origin and evolution of cosmic magnetic fields is one of the most important long-standing problems in astrophysics and cosmology (Kulsrud & Zweibel 2008;Brandenburg & Ntormousi 2023). In galaxies and clusters of galaxies, large-scale magnetic fields with up to μG strengths are found to be ubiquitous through observations of Faraday rotation, synchrotron emission, and Zeeman splitting (e.g., Beck et al 1996;Carilli & Taylor 2002;Bonafede et al 2010).…”

Section: Introductionmentioning

confidence: 99%

Magnetogenesis in a Collisionless Plasma: From Weibel Instability to Turbulent Dynamo

Zhou,

Zhdankin,

Kunz

et al. 2023

ApJ

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We report on a first-principles numerical and theoretical study of plasma dynamo in a fully kinetic framework. By applying an external mechanical force to an initially unmagnetized plasma, we develop a self-consistent treatment of the generation of “seed” magnetic fields, the formation of turbulence, and the inductive amplification of fields by the fluctuation dynamo. Driven large-scale motions in an unmagnetized, weakly collisional plasma are subject to strong phase mixing, which leads to the development of thermal pressure anisotropy. This anisotropy triggers the Weibel instability, which produces filamentary “seed” magnetic fields on plasma-kinetic scales. The plasma is thereby magnetized, enabling efficient stretching and folding of the fields by the plasma motions and the development of Larmor-scale kinetic instabilities such as the firehose and mirror. The scattering of particles off the associated microscale magnetic fluctuations provides an effective viscosity, regulating the field morphology and turbulence. During this process, the seed field is further amplified by the fluctuation dynamo until energy equipartition with the turbulent flow is reached. By demonstrating that equipartition magnetic fields can be generated from an initially unmagnetized plasma through large-scale turbulent flows, this work has important implications for the origin and amplification of magnetic fields in the intracluster and intergalactic mediums.

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Galactic Dynamos

Cited by 14 publications

References 230 publications

Computational approaches to modeling dynamos in galaxies

Computational approaches to modeling dynamos in galaxies

Cholla-MHD: An Exascale-capable Magnetohydrodynamic Extension to the Cholla Astrophysical Simulation Code

Magnetogenesis in a Collisionless Plasma: From Weibel Instability to Turbulent Dynamo

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