8 results on '"*DYNAMO theory (Physics)"'
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2. Optimization of the Magnetic Dynamo.
- Author
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Willis, Ashley P.
- Subjects
- *
DYNAMO theory (Physics) , *MAGNETIC fields , *SPEED , *DIFFUSION , *STANDARD deviations - Abstract
In stars and planets, magnetic fields are believed to originate from the motion of electrically conducting fluids in their interior, through a process known as the dynamo mechanism. In this Letter, an optimization procedure is used to simultaneously address two fundamental questions of dynamo theory: "Which velocity field leads to the most magnetic energy growth?" and "How large does the velocity need to be relative to magnetic diffusion?" In general, this requires optimization over the full space of continuous solenoidal velocity fields possible within the geometry. Here the case of a periodic box is considered. Measuring the strength of the flow with the root-mean-square amplitude, an optimal velocity field is shown to exist, but without limitation on the strain rate, optimization is prone to divergence. Measuring the flow in terms of its associated dissipation leads to the identification of a single optimal at the critical magnetic Reynolds number necessary for a dynamo. This magnetic Reynolds number is found to be only 15% higher than that necessary for transient growth of the magnetic field. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
3. Long-Range Correlations and Coherent Structures in Magnetohydrodynamic Equilibria.
- Author
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Weichman, Peter B.
- Subjects
- *
GIBBS' equation , *MAGNETOHYDRODYNAMICS , *DYNAMO theory (Physics) , *EDDIES , *FLUID dynamics - Abstract
The equilibrium theory of the 2D magnetohydrodynamic equations is derived, accounting for the full infinite hierarchies of conserved integrals. An exact description in terms of two coupled elastic membranes emerges, producing long-ranged correlations between the magnetic and velocity fields. This is quite different from the results of previous variational treatments, which relied on a local product ansatz for the thermodynamic Gibbs distribution. The equilibria display the same type of coherent structures, such as compact eddies and zonal jets, previously found in pure fluid equilibria. Possible consequences of this for recent simulations of the solar tachocline are discussed. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
4. Not much helicity is needed to drive large-scale dynamos.
- Author
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Graham, Jonathan Pietarila, Blackman, Eric G., Mininni, Pablo D., and Pouquet, Annick
- Subjects
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HELICITY of nuclear particles , *MOLECULAR dynamics , *MAGNETIC fields , *TURBULENCE , *DYNAMO theory (Physics) , *SEPARATION (Technology) - Abstract
Understanding the in situ amplification of large-scale magnetic fields in turbulent astrophysical rotators has been a core subject of dynamo theory. When turbulent velocities are helical, large-scale dynamos that substantially amplify fields on scales that exceed the turbulent forcing scale arise, but the minimum sufficient fractional kinetic helicity fh,c has not been previously well quantified. Using direct numerical simulations for a simple helical dynamo, we show that fh,c decreases as the ratio of forcing to large-scale wave numbers kF/kmin increases. From the condition that a large-scale helical dynamo must overcome the back reaction from any nonhelical field on the large scales, we develop a theory that can explain the simulations. For kF/kmin ⪰ 8 we find fh,c ≲ 3%, implying that very small helicity fractions strongly influence magnetic spectra for even moderate-scale separation. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
5. Universal Nonlinear Small-Scale Dynamo.
- Author
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Beresnyak, A.
- Subjects
- *
MAGNETOHYDRODYNAMICS , *REYNOLDS number , *MATHEMATICAL constants , *ENERGY dissipation , *DYNAMO theory (Physics) - Abstract
The article argues that nonlinear magnetohydrodynamics (MHD) dynamo at large Reynolds numbers (Re) is universal in a sense that magnetic energy grows at a rate which is a constant fraction of the total turbulent dissipation rate. Particular focus is given to the issue of constant fraction being small, compared to Kolmogorov constant which is of order unity.
- Published
- 2012
- Full Text
- View/download PDF
6. Dynamo Effect and Turbulence in Hydrodynamic Weyl Metals.
- Author
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Galitski, Victor, Kargarian, Mehdi, and Syzranov, Sergey
- Subjects
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DYNAMO theory (Physics) , *WEYL groups , *HYDRODYNAMICS - Abstract
The dynamo effect is a class of macroscopic phenomena responsible for generating and maintaining magnetic fields in astrophysical bodies. It hinges on the hydrodynamic three-dimensional motion of conducting gases and plasmas that achieve high hydrodynamic and/or magnetic Reynolds numbers due to the large length scales involved. The existing laboratory experiments modeling dynamos are challenging and involve large apparatuses containing conducting fluids subject to fast helical flows. Here we propose that electronic solid-state materials--in particular, hydrodynamic metals--may serve as an alternative platform to observe some aspects of the dynamo effect. Motivated by recent experimental developments, this Letter focuses on hydrodynamic Weyl semimetals, where the dominant scattering mechanism is due to interactions. We derive Navier-Stokes equations along with equations of magnetohydrodynamics that describe the transport of a Weyl electron-hole plasma appropriate in this regime. We estimate the hydrodynamic and magnetic Reynolds numbers for this system. The latter is a key figure of merit of the dynamo mechanism. We show that it can be relatively large to enable observation of the dynamo-induced magnetic field bootstrap in an experiment. Finally, we generalize the simplest dynamo instability model--the Ponomarenko dynamo--to the case of a hydrodynamic Weyl semimetal and show that the chiral anomaly term reduces the threshold magnetic Reynolds number for the dynamo instability. [ABSTRACT FROM AUTHOR]
- Published
- 2018
- Full Text
- View/download PDF
7. Fate of Alpha Dynamos at Large Rm.
- Author
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Cameron, Alexandre and Alexakis, Alexandros
- Subjects
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DYNAMO theory (Physics) , *SOLAR magnetic fields , *REYNOLDS number - Abstract
At the heart of today's solar magnetic field evolution models lies the alpha dynamo description. In this work, we investigate the fate of alpha dynamos as the magnetic Reynolds number Rm is increased. Using Floquet theory, we are able to precisely quantify mean-field effects like the alpha and beta effect (i) by rigorously distinguishing dynamo modes that involve large-scale components from the ones that only involve small scales, and by (ii) providing a way to investigate arbitrary large-scale separations with minimal computational cost. We apply this framework to helical and nonhelical flows as well as to random flows with short correlation time. Our results determine that the alpha description is valid for Rm smaller than a critical value Rmc at which small-scale dynamo instability starts. When Rm is above Rmc, the dynamo ceases to follow the mean-field description and the growth rate of the large-scale modes becomes independent of the scale separation, while the energy in the large-scale modes is inversely proportional to the square of the scale separation. The results in this second regime do not depend on the presence of helicity. Thus, alpha-type modeling for solar and stellar models needs to be reevaluated and new directions for mean-field modeling are proposed. [ABSTRACT FROM AUTHOR]
- Published
- 2016
- Full Text
- View/download PDF
8. Loaded magnetohydrodynamic flows in Kerr spacetime.
- Author
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Globus, Noemie and Levinson, Amir
- Subjects
- *
MAGNETOHYDRODYNAMIC waves , *DYNAMO theory (Physics) , *PLASMA injection - Abstract
The effect of mass and energy loading on the efficiency at which energy can be extracted magnetically from a Kerr black hole is explored, using a semianalytic, ideal magnetohydrodynamics model that incorporates plasma injection on magnetic field lines. We find a critical load below which the specific energy of the plasma inflowing into the black hole is negative, and above which it is positive, and identify two types of flows with distinct properties; at subcritical loads a magnetic outflow is launched from the ergosphere, owing to extraction of the black hole spin energy, as originally proposed by Blandford and Znajek. At supercritical loads the structure of the flow depends on the details of the injection process. In cases where the injected plasma is relativistically hot, a pressure-driven, double transmagnetosonic flow is launched from a stagnation point located outside the ergosphere, between the inner and outer light cylinders. Some fraction of the energy deposited in the magnetosphere is then absorbed by the black hole and the rest emerges at infinity in the form of a relativistic outflow. When the injected plasma is cold an outflow may not form at all. We discuss the implications of our results to gamma ray bursts and active galactic nuclei. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
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