| 1. |
- Brandenburg, Axel, et al.
(författare)
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Small-scale magnetic helicity losses from a mean-field dynamo
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 398:3, s. 1414-1422
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Tidskriftsartikel (refereegranskat)abstract
- Using mean-field models with a dynamical quenching formalism, we show that in finite domains magnetic helicity fluxes associated with small-scale magnetic fields are able to alleviate catastrophic quenching. We consider fluxes that result from advection by a mean flow, the turbulent mixing down the gradient of mean small-scale magnetic helicity density or the explicit removal which may be associated with the effects of coronal mass ejections in the Sun. In the absence of shear, all the small-scale magnetic helicity fluxes are found to be equally strong for both large- and small-scale fields. In the presence of shear, there is also an additional magnetic helicity flux associated with the mean field, but this flux does not alleviate catastrophic quenching. Outside the dynamo-active region, there are neither sources nor sinks of magnetic helicity, so in a steady state this flux must be constant. It is shown that unphysical behaviour emerges if the small-scale magnetic helicity flux is forced to vanish within the computational domain.
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| 2. |
- Brandenburg, Axel
(författare)
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The critical role of magnetic helicity in astrophysical large-scale dynamos
- 2009
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Ingår i: Plasma Physics and Controlled Fusion. - : IOP Publishing. - 0741-3335 .- 1361-6587. ; 51:12, s. 4043-
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Tidskriftsartikel (refereegranskat)abstract
- The role of magnetic helicity in astrophysical large-scale dynamos is reviewed and compared with cases where there is no energy supply and an initial magnetic field can only decay. In both cases magnetic energy tends to get redistributed to larger scales. Depending on the efficiency of magnetic helicity fluxes the decay of a helical field can speed up. Likewise, the saturation of a helical dynamo can speed up through magnetic helicity fluxes. The astrophysical importance of these processes is reviewed in the context of the solar dynamo and an estimated upper limit for the magnetic helicity flux of 1046 Mx2/cycle is given.
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| 3. |
- Brandenburg, Axel, et al.
(författare)
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Turbulent diffusion and galactic magnetism
- 2009
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Ingår i: Proceedings of the International Astronomical Union. - : Cambridge University Press. - 1743-9213 .- 1743-9221. ; 5, s. 432-433
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Using the test-field method for nearly irrotational turbulence driven by spherical expansion waves it is shown that the turbulent magnetic diffusivity increases with magnetic Reynolds numbers. Its value levels off at several times the rms velocity of the turbulence multiplied by the typical radius of the expansion waves. This result is discussed in the context of the galactic mean-field dynamo.
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| 4. |
- Hubbard, Alexander, et al.
(författare)
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Memory Effects in Turbulent Transport
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 706, s. 712-726
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Tidskriftsartikel (refereegranskat)abstract
- In the mean-field theory of magnetic fields, turbulent transport, i.e., the turbulent electromotive force is described by a combination of the α effect and turbulent magnetic diffusion, which are usually assumed to be proportional, respectively, to the mean field and its spatial derivatives. For a passive scalar, there is just turbulent diffusion, where the mean flux of concentration depends on the gradient of the mean concentration. However, these proportionalities are approximations that are valid only if the mean field or the mean concentration vary slowly in time. Examples are presented where turbulent transport possesses memory, i.e., where it depends crucially on the past history of the mean field. Such effects are captured by replacing turbulent transport coefficients with time integral kernels, resulting in transport coefficients that depend effectively on the frequency or the growth rate of the mean field itself. In this paper, we perform numerical experiments to find the characteristic timescale (or memory length) of this effect as well as simple analytical models of the integral kernels in the case of passive scalar concentrations and kinematic dynamos. The integral kernels can then be used to find self-consistent growth or decay rates of the mean fields. In mean-field dynamos, the growth rates and cycle periods based on steady state values of α effect, and turbulent diffusivity can be quite different from the actual values.
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| 5. |
- Hubbard, A., et al.
(författare)
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The α effect with imposed and dynamo-generated magnetic fields
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 398:4, s. 1891-1899
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Tidskriftsartikel (refereegranskat)abstract
- Estimates for the non-linear α effect in helical turbulence with an applied magnetic field are presented using two different approaches: the imposed-field method where the electromotive force owing to the applied field is used, and the test-field method where separate evolution equations are solved for a set of different test fields. Both approaches agree for stronger fields, but there are apparent discrepancies for weaker fields that can be explained by the influence of dynamo-generated magnetic fields on the scale of the domain that are referred to as meso-scale magnetic fields. Examples are discussed where these meso-scale fields can lead to both drastically overestimated and underestimated values of α compared with the kinematic case. It is demonstrated that the kinematic value can be recovered by resetting the fluctuating magnetic field to zero in regular time intervals. It is concluded that this is the preferred technique both for the imposed-field and the test-field methods.
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| 6. |
- Käpylä, Petri J., et al.
(författare)
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Turbulent Dynamos with Shear and Fractional Helicity
- 2009
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 699, s. 1059-1066
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Tidskriftsartikel (refereegranskat)abstract
- Dynamo action owing to helically forced turbulence and large-scale shear is studied using direct numerical simulations. The resulting magnetic field displays propagating wave-like behavior. This behavior can be modeled in terms of an αΩ dynamo. In most cases super-equipartition fields are generated. By varying the fraction of helicity of the turbulence the regeneration of poloidal fields via the helicity effect (corresponding to the α-effect) is regulated. The saturation level of the magnetic field in the numerical models is consistent with a linear dependence on the ratio of the fractional helicities of the small and large-scale fields, as predicted by a simple nonlinear mean-field model. As the magnetic Reynolds number (Re M ) based on the wavenumber of the energy-carrying eddies is increased from 1 to 180, the cycle frequency of the large-scale field is found to decrease by a factor of about 6 in cases where the turbulence is fully helical. This is interpreted in terms of the turbulent magnetic diffusivity, which is found to be only weakly dependent on the Re M .
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| 7. |
- Mitra, Dhrubaditya, et al.
(författare)
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Oscillatory migratory large-scale fields in mean-field and direct simulations
- 2009
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Ingår i: Proceedings of the International Astronomical Union. - 1743-9213 .- 1743-9221. ; 5, s. 197-201
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Tidskriftsartikel (refereegranskat)abstract
- We summarise recent results form direct numerical simulations of both non-rotating helically forced and rotating convection driven MHD equations in spherical wedge-shape domains. In the former, using perfect-conductor boundary conditions along the latitudinal boundaries we observe oscillations, polarity reversals and equatorward migration of the large-scale magnetic fields. In the latter we obtain angular velocity with cylindrical contours and large-scale magnetic field which shows oscillations, polarity reversals but poleward migration. The occurrence of these behviours in direct numerical simulations is clearly of interest. However the present models as they stand are not directly applicable to the solar dynamo problem. Nevertheless, they provide general insights into the operation of turbulent dynamos.
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| 8. |
- Sur, Sharanya, et al.
(författare)
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The role of the Yoshizawa effect in the Archontis dynamo
- 2009
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 399:1, s. 273-280
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Tidskriftsartikel (refereegranskat)abstract
- The generation of mean magnetic fields is studied for a simple non-helical flow where a net cross-helicity of either sign can emerge. This flow, which is also known as the Archontis flow, is a generalization of the Arnold-Beltrami-Childress flow, but with the cosine terms omitted. The presence of cross-helicity leads to a mean-field dynamo effect that is known as the Yoshizawa effect. Direct numerical simulations of such flows demonstrate the presence of magnetic fields on scales larger than the scale of the flow. Contrary to earlier expectations, the Yoshizawa effect is found to be proportional to the mean magnetic field and can therefore lead to its exponential instead of just linear amplification for magnetic Reynolds numbers that exceed a certain critical value. Unlike α effect dynamos, it is found that the Yoshizawa effect is not notably constrained by the presence of a conservation law. It is argued that this is due to the presence of a forcing term in the momentum equation, which leads to a non-zero correlation with the magnetic field. Finally, the application to energy convergence in solar wind turbulence is discussed.
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| 9. |
- Vermersch, V., et al.
(författare)
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Shear-driven magnetic buoyancy oscillations
- 2009
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Ingår i: Astronomische Nachrichten. - : Wiley. - 0004-6337 .- 1521-3994. ; 330:8, s. 797-806
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Tidskriftsartikel (refereegranskat)abstract
- The effects of uniform horizontal shear on a stably stratified layer of gas is studied. The system is initially destabilized by a magnetically buoyant flux tube pointing in the cross-stream direction. The shear amplifies the initial field to Lundquist numbers of about 200-400, but then its value drops to about 100-300, depending on the value of the sub-adiabatic gradient. The larger values correspond to cases where the stratification is strongly stable and nearly isothermal. At the end of the runs the magnetic field is nearly axisymmetric, i.e. uniform in the streamwise direction. In view of Cowling's theorem the sustainment of the field remains a puzzle and may be due to subtle numerical effects that have not yet been identified in detail. In the final state the strength of the magnetic field decreases with height in such a way that the field is expected to be unstable. Low amplitude oscillations are seen in the vertical velocity even at late times, suggesting that they might be persistent.
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