TY - JOUR
T1 - Dynamo effect in decaying helical turbulence
AU - Brandenburg, Axel
AU - Kahniashvili, Tina
AU - Mandal, Sayan
AU - Pol, Alberto Roper
AU - Tevzadze, Alexander G.
AU - Vachaspati, Tanmay
N1 - Funding Information:
We thank Eric Blackman and two anonymous referees for useful comments. A.B. acknowledges the University of Colorado's support through the George Ellery Hale visiting faculty appointment. T.K. acknowledges the High Energy and Cosmology Division and Associate Membership Program at International Center for Theoretical Physics (Trieste, Italy) for hospitality and partial support. We also thank Nordita for hospitality during the programs on Cosmological Magnetic Fields in 2015 (A.B., T.K., A.G.T., T.V.) and on Chiral Magnetic Phenomena in 2018 (A.B., T.K., S.M., A.R.P., T.V.). Support through the NSF Astrophysics and Astronomy Grant (AAG) Program (Grants No. AST1615940 and No. AST1615100), the Research Council of Norway (FRINATEK Grant No. 231444), the Swiss NSF SCOPES (Grant No. IZ7370-152581), and the Georgian Shota Rustaveli NSF (Grant No. FR/264/6-350/14) are gratefully acknowledged. T.V. is supported by the U.S. Department of Energy, Office of High Energy Physics, under Award No. DE-SC0018330 at Arizona State University. We acknowledge the allocation of computing resources provided by the Swedish National Allocations Committee at the Center for Parallel Computers at the Royal Institute of Technology in Stockholm. This work utilized the Janus supercomputer, which is supported by the National Science Foundation (Award No. CNS-0821794), the University of Colorado Boulder, the University of Colorado Denver, and the National Center for Atmospheric Research. The Janus supercomputer is operated by the University of Colorado Boulder.
PY - 2019/2
Y1 - 2019/2
N2 - We show that in decaying hydromagnetic turbulence with initial kinetic helicity, a weak magnetic field eventually becomes fully helical. The sign of magnetic helicity is opposite to that of the kinetic helicity - regardless of whether the initial magnetic field was helical. The magnetic field undergoes inverse cascading with the magnetic energy decaying approximately like t-1/2. This is even slower than in the fully helical case, where it decays like t-2/3. In this parameter range, the product of magnetic energy and correlation length raised to a certain power slightly larger than unity is approximately constant. This scaling of magnetic energy persists over long timescales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a crossover to the t-2/3 decay law that is commonly observed for fully helical magnetic fields. Regardless of the presence or absence of initial kinetic helicity, the magnetic field experiences exponential growth during the first few turnover times, which is suggestive of small-scale dynamo action. Our results have applications to a wide range of experimental dynamos and astrophysical time-dependent plasmas, including primordial turbulence in the early universe.
AB - We show that in decaying hydromagnetic turbulence with initial kinetic helicity, a weak magnetic field eventually becomes fully helical. The sign of magnetic helicity is opposite to that of the kinetic helicity - regardless of whether the initial magnetic field was helical. The magnetic field undergoes inverse cascading with the magnetic energy decaying approximately like t-1/2. This is even slower than in the fully helical case, where it decays like t-2/3. In this parameter range, the product of magnetic energy and correlation length raised to a certain power slightly larger than unity is approximately constant. This scaling of magnetic energy persists over long timescales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a crossover to the t-2/3 decay law that is commonly observed for fully helical magnetic fields. Regardless of the presence or absence of initial kinetic helicity, the magnetic field experiences exponential growth during the first few turnover times, which is suggestive of small-scale dynamo action. Our results have applications to a wide range of experimental dynamos and astrophysical time-dependent plasmas, including primordial turbulence in the early universe.
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U2 - 10.1103/PhysRevFluids.4.024608
DO - 10.1103/PhysRevFluids.4.024608
M3 - Article
AN - SCOPUS:85062426577
VL - 4
JO - Physical Review Fluids
JF - Physical Review Fluids
SN - 2469-990X
IS - 2
ER -