On the inverse transfer of (non-)helical magnetic energy in a decaying magnetohydrodynamic turbulence

Park, Kiwan

doi:10.1093/mnras/stx1981

Cited by 11 publications

(8 citation statements)

References 57 publications

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“…Previous decay simulations were always performed with strong initial magnetic fields. Only recently, the need for studying the evolution of hydromagnetic turbulence in kinetically dominated systems has been emphasized [31]. However, no detailed study has been presented as yet, except for our own work [32], which focused on the case without kinetic helicity.…”

Section: Introductionmentioning

confidence: 99%

Dynamo effect in decaying helical turbulence

et al. 2019

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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 or not 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 time scales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a cross-over 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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Section: Introductionmentioning

confidence: 99%

Dynamo effect in decaying helical turbulence

et al. 2019

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“…For a decaying MHD system, these classical results with Eq. (9), (10) reproduce the simulation results quite well (Park 2017b). But still more numerical test is necessary for a forced system.…”

Section: Derivation Of 'α' and 'β' Coefficients Using Semi-analytic Mmentioning

confidence: 66%

“…Here, we introduce an improved field structure model (Park 2017b) which can be measured in observation and simulation.…”

Section: Introductionmentioning

confidence: 99%

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Principle of the Helical and Nonhelical Dynamo and the α Effect in a Field Structure Model

Park

2019

ApJ

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We explain the (non)helical dynamo process using a field-structure model based on magnetic induction equation in an intuitive way. We show how nonhelical kinetic energy converts into magnetic energy and cascades toward smaller eddies in a mechanically forced plasma system. Also, we show how helical magnetic energy is inversely cascaded (α effect) toward large scale magnetic eddies in a mechanically or magnetically forced system. We, then, compare the simulation results with the model qualitatively for the verification of the model. In addition to these intuitive and numerical approaches, we show how to get α and β coefficient semi-analytically from the temporally evolving large scale magnetic energy and magnetic helicity.

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“…From now on, we will discuss the origin of α & β effect and their physical meaning with the evolving large scale magnetic field B in helical dynamo. In addition to the semi-analytic equations, we will use the field structure model based on the geometry of u & b that amplifies B (Park 2017b(Park , 2019.…”

Section: Analysis With Field Structurementioning

confidence: 99%

Negative Magnetic Diffusivity beta replacing alpha effect in Helical Dynamo

Park

2019

Preprint

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The α effect is known to be an indispensable energy source of the poloidal magnetic field B pol in the sun or planet. However, the α effect is quenched as the magnetic field grows due to the conservation of magnetic helicity. With these conventional understanding, what indeed generates and sustains the observed B pol remains a mystery. To solve this contradiction between theory and the real nature, we derived a semianalytic representation of α & β using large scale magnetic helicity H M and energy E M . Applying the simulation data to α & β, we found that the negative β effect is a promising substitution of the quenched α effect. However, since the negative β effect contradicts the conventional dynamo theory, we derived the new β expression referring to the field structure model. This analytic result with the field relation between velocity 'U' and magnetic field 'B' shows that the β effect in the helical system is not a fixed one. Rather, it plays a variable and dynamic role in dynamo depending on the interaction between the poloidal velocity field U pol and relative strength of large scale magnetic field B.

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On the inverse transfer of (non-)helical magnetic energy in a decaying magnetohydrodynamic turbulence

Cited by 11 publications

References 57 publications

Dynamo effect in decaying helical turbulence

Dynamo effect in decaying helical turbulence

Principle of the Helical and Nonhelical Dynamo and the α Effect in a Field Structure Model

Negative Magnetic Diffusivity beta replacing alpha effect in Helical Dynamo

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