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Magnetic Field Amplification during a Turbulent Collapse
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Carnegie Mellon University, USA; Ilia State University, Georgia.ORCID iD: 0000-0002-7304-021X
Number of Authors: 22025 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 990, no 2, article id 223Article in journal (Refereed) Published
Abstract [en]

The question of whether a dynamo can be triggered by gravitational collapse is of great interest, especially for the early Universe. Here, we employ supercomoving coordinates to study the magnetic field amplification from decaying turbulence during gravitational collapse. We perform 3D simulations and show that for large magnetic Reynolds numbers, there can be exponential growth of the comoving magnetic field with conformal time before the decay of turbulence impedes further amplification. The collapse dynamics only affect the nonlinear feedback from the Lorentz force, which diminishes more rapidly for shorter collapse times, allowing nearly kinematic continued growth. We confirm that helical turbulence is more efficient in driving dynamo action than nonhelical turbulence, but this difference decreases for larger collapse times. We also show that for nearly irrotational flows, dynamo amplification is still possible, but it is always associated with a growth of vorticity—even if it still remains very small. In nonmagnetic runs, the growth of vorticity is associated with viscosity and grows with the Mach number. In the presence of magnetic fields, vorticity emerges from the curl of the Lorentz force. During a limited time interval, an exponential growth of the comoving magnetic field with conformal time is interpreted as clear evidence of dynamo action.

Place, publisher, year, edition, pages
2025. Vol. 990, no 2, article id 223
Keywords [en]
Magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-247298DOI: 10.3847/1538-4357/adf725ISI: 001568029600001Scopus ID: 2-s2.0-105015866670OAI: oai:DiVA.org:su-247298DiVA, id: diva2:2000041
Available from: 2025-09-23 Created: 2025-09-23 Last updated: 2025-09-23Bibliographically approved

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Brandenburg, Axel

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Nordic Institute for Theoretical Physics (Nordita)Department of AstronomyThe Oskar Klein Centre for Cosmo Particle Physics (OKC)
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