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Title: UNIVERSALITY OF THE SMALL-SCALE DYNAMO MECHANISM

Abstract

We quantify possible differences between turbulent dynamo action in the Sun and the dynamo action studied in idealized simulations. For this purpose, we compare Fourier-space shell-to-shell energy transfer rates of three incrementally more complex dynamo simulations: an incompressible, periodic simulation driven by random flow, a simulation of Boussinesq convection, and a simulation of fully compressible convection that includes physics relevant to the near-surface layers of the Sun. For each of the simulations studied, we find that the dynamo mechanism is universal in the kinematic regime because energy is transferred from the turbulent flow to the magnetic field from wavenumbers in the inertial range of the energy spectrum. The addition of physical effects relevant to the solar near-surface layers, including stratification, compressibility, partial ionization, and radiative energy transport, does not appear to affect the nature of the dynamo mechanism. The role of inertial-range shear stresses in magnetic field amplification is independent from outer-scale circumstances, including forcing and stratification. Although the shell-to-shell energy transfer functions have similar properties to those seen in mean-flow driven dynamos in each simulation studied, the saturated states of these simulations are not universal because the flow at the driving wavenumbers is a significant source of energy formore » the magnetic field.« less

Authors:
; ;  [1];  [2]; ;  [3]
  1. Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau (Germany)
  2. Department of Applied Mathematics and Statistics, Johns Hopkins University, Baltimore, MD 21218 (United States)
  3. Max-Planck-Institut fuer Plasmaphysik, Boltzmannstrasse 2, 85748 Garching (Germany)
Publication Date:
OSTI Identifier:
21578336
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 736; Journal Issue: 1; Other Information: DOI: 10.1088/0004-637X/736/1/36
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; CONVECTION; INCOMPRESSIBLE FLOW; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; PHOTOSPHERE; SIMULATION; STRATIFICATION; SUN; TURBULENT FLOW; ATMOSPHERES; ENERGY TRANSFER; FLUID FLOW; FLUID MECHANICS; HEAT TRANSFER; HYDRODYNAMICS; MAIN SEQUENCE STARS; MASS TRANSFER; MECHANICS; SOLAR ATMOSPHERE; STARS; STELLAR ATMOSPHERES

Citation Formats

Moll, R., Cameron, R. H., Schuessler, M., Pietarila Graham, J., Pratt, J., and Mueller, W.-C. UNIVERSALITY OF THE SMALL-SCALE DYNAMO MECHANISM. United States: N. p., 2011. Web. doi:10.1088/0004-637X/736/1/36.
Moll, R., Cameron, R. H., Schuessler, M., Pietarila Graham, J., Pratt, J., & Mueller, W.-C. UNIVERSALITY OF THE SMALL-SCALE DYNAMO MECHANISM. United States. doi:10.1088/0004-637X/736/1/36.
Moll, R., Cameron, R. H., Schuessler, M., Pietarila Graham, J., Pratt, J., and Mueller, W.-C. 2011. "UNIVERSALITY OF THE SMALL-SCALE DYNAMO MECHANISM". United States. doi:10.1088/0004-637X/736/1/36.
@article{osti_21578336,
title = {UNIVERSALITY OF THE SMALL-SCALE DYNAMO MECHANISM},
author = {Moll, R. and Cameron, R. H. and Schuessler, M. and Pietarila Graham, J. and Pratt, J. and Mueller, W.-C.},
abstractNote = {We quantify possible differences between turbulent dynamo action in the Sun and the dynamo action studied in idealized simulations. For this purpose, we compare Fourier-space shell-to-shell energy transfer rates of three incrementally more complex dynamo simulations: an incompressible, periodic simulation driven by random flow, a simulation of Boussinesq convection, and a simulation of fully compressible convection that includes physics relevant to the near-surface layers of the Sun. For each of the simulations studied, we find that the dynamo mechanism is universal in the kinematic regime because energy is transferred from the turbulent flow to the magnetic field from wavenumbers in the inertial range of the energy spectrum. The addition of physical effects relevant to the solar near-surface layers, including stratification, compressibility, partial ionization, and radiative energy transport, does not appear to affect the nature of the dynamo mechanism. The role of inertial-range shear stresses in magnetic field amplification is independent from outer-scale circumstances, including forcing and stratification. Although the shell-to-shell energy transfer functions have similar properties to those seen in mean-flow driven dynamos in each simulation studied, the saturated states of these simulations are not universal because the flow at the driving wavenumbers is a significant source of energy for the magnetic field.},
doi = {10.1088/0004-637X/736/1/36},
journal = {Astrophysical Journal},
number = 1,
volume = 736,
place = {United States},
year = 2011,
month = 7
}
  • We propose a new mechanism for a turbulent mean-field dynamo in which the magnetic fluctuations resulting from a small-scale dynamo drive the generation of large-scale magnetic fields. This is in stark contrast to the common idea that small-scale magnetic fields should be harmful to large-scale dynamo action. These dynamos occur in the presence of a large-scale velocity shear and do not require net helicity, resulting from off-diagonal components of the turbulent resistivity tensor as the magnetic analogue of the "shear-current" effect. Furthermore, given the inevitable existence of nonhelical small-scale magnetic fields in turbulent plasmas, as well as the generic naturemore » of velocity shear, the suggested mechanism may help explain the generation of large-scale magnetic fields across a wide range of astrophysical objects.« less
  • A novel large-scale dynamo mechanism, the magnetic shear-current effect, is discussed and explored. Here, the effect relies on the interaction of magnetic fluctuations with a mean shear flow, meaning the saturated state of the small-scale dynamo can drive a large-scale dynamo – in some sense the inverse of dynamo quenching. The dynamo is non-helical, with the mean fieldmore » $${\it\alpha}$$coefficient zero, and is caused by the interaction between an off-diagonal component of the turbulent resistivity and the stretching of the large-scale field by shear flow. Following up on previous numerical and analytic work, this paper presents further details of the numerical evidence for the effect, as well as an heuristic description of how magnetic fluctuations can interact with shear flow to produce the required electromotive force. The pressure response of the fluid is fundamental to this mechanism, which helps explain why the magnetic effect is stronger than its kinematic cousin, and the basic idea is related to the well-known lack of turbulent resistivity quenching by magnetic fluctuations. As well as being interesting for its applications to general high Reynolds number astrophysical turbulence, where strong small-scale magnetic fluctuations are expected to be prevalent, the magnetic shear-current effect is a likely candidate for large-scale dynamo in the unstratified regions of ionized accretion disks. Evidence for this is discussed, as well as future research directions and the challenges involved with understanding details of the effect in astrophysically relevant regimes.« less
  • The dynamo equations are solved numerically with a helical forcing corresponding to the Roberts flow. In the fully turbulent regime the flow behaves as a Roberts flow on long time scales, plus turbulent fluctuations at short time scales. The dynamo onset is controlled by the long time scales of the flow, in agreement with the former Karlsruhe experimental results. The is governed by a generalized {alpha} effect, which includes both the usual {alpha} effect and turbulent diffusion, plus all higher order effects. Beyond the onset we find that this generalized {alpha} effect scales as O(Rm{sup -1}), suggesting the takeover ofmore » small-scale dynamo action. This is confirmed by simulations in which dynamo occurs even if the large-scale field is artificially suppressed.« less