### Abstract

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.

Original language | English (US) |
---|---|

Journal | Physical Review Fluids |

Volume | 4 |

Issue number | 2 |

DOIs | |

State | Published - Feb 1 2019 |

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### ASJC Scopus subject areas

- Computational Mechanics
- Modeling and Simulation
- Fluid Flow and Transfer Processes

### Cite this

*Physical Review Fluids*,

*4*(2). https://doi.org/10.1103/PhysRevFluids.4.024608

**Dynamo effect in decaying helical turbulence.** / Brandenburg, Axel; Kahniashvili, Tina; Mandal, Sayan; Pol, Alberto Roper; Tevzadze, Alexander G.; Vachaspati, Tanmay.

Research output: Contribution to journal › Article

*Physical Review Fluids*, vol. 4, no. 2. https://doi.org/10.1103/PhysRevFluids.4.024608

}

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

PY - 2019/2/1

Y1 - 2019/2/1

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.

UR - http://www.scopus.com/inward/record.url?scp=85062426577&partnerID=8YFLogxK

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U2 - 10.1103/PhysRevFluids.4.024608

DO - 10.1103/PhysRevFluids.4.024608

M3 - Article

VL - 4

JO - Physical Review Fluids

JF - Physical Review Fluids

SN - 2469-990X

IS - 2

ER -