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Plasmoid ejections driven by dynamo action underneath a spherical surface  [PDF]
J?rn Warnecke,Axel Brandenburg,Dhrubaditya Mitra
Physics , 2010, DOI: 10.1017/S1743921311007186
Abstract: We present a unified three-dimensional model of the convection zone and upper atmosphere of the Sun in spherical geometry. In this model, magnetic fields, generated by a helically forced dynamo in the convection zone, emerge without the assistance of magnetic buoyancy. We use an isothermal equation of state with gravity and density stratification. Recurrent plasmoid ejections, which rise through the outer atmosphere, is observed. In addition, the current helicity of the small--scale field is transported outwards and form large structures like magnetic clouds.
Direct numerical simulations of helical dynamo action: MHD and beyond
D. O. Gómez,P. D. Mininni
Nonlinear Processes in Geophysics (NPG) , 2004,
Abstract: Magnetohydrodynamic dynamo action is often invoked to explain the existence of magnetic fields in several astronomical objects. In this work, we present direct numerical simulations of MHD helical dynamos, to study the exponential growth and saturation of magnetic fields. Simulations are made within the framework of incompressible flows and using periodic boundary conditions. The statistical properties of the flow are studied, and it is found that its helicity displays strong spatial fluctuations. Regions with large kinetic helicity are also strongly concentrated in space, forming elongated structures. In dynamo simulations using these flows, we found that the growth rate and the saturation level of magnetic energy and magnetic helicity reach an asymptotic value as the Reynolds number is increased. Finally, extensions of the MHD theory to include kinetic effects relevant in astrophysical environments are discussed.
Cosmic-ray driven dynamo in galactic disks  [PDF]
M. Hanasz,K. Otmianowska-Mazur,H. Lesch,G. Kowal,M. Soida,D. Wóltański,K. Kowalik,R. K. Paw?aszek,B. Kulesza-?ydzik
Physics , 2008, DOI: 10.1017/S1743921309031147
Abstract: We present new developments on the Cosmic--Ray driven, galactic dynamo, modeled by means of direct, resistive CR--MHD simulations, performed with ZEUS and PIERNIK codes. The dynamo action, leading to the amplification of large--scale galactic magnetic fields on galactic rotation timescales, appears as a result of galactic differential rotation, buoyancy of the cosmic ray component and resistive dissipation of small--scale turbulent magnetic fields. Our new results include demonstration of the global--galactic dynamo action driven by Cosmic Rays supplied in supernova remnants. An essential outcome of the new series of global galactic dynamo models is the equipartition of the gas turbulent energy with magnetic field energy and cosmic ray energy, in saturated states of the dynamo on large galactic scales.
Kinematic dynamo in spherical Couette flow  [PDF]
Xing Wei,Andrew Jackson,Rainer Hollerbach
Physics , 2010, DOI: 10.1080/03091929.2011.620569
Abstract: We investigate numerically kinematic dynamos driven by flow of electrically conducting fluid in the shell between two concentric differentially rotating spheres, a configuration normally referred to as spherical Couette flow. We compare between axisymmetric (2D) and fully three dimensional flows, between low and high global rotation rates, between prograde and retrograde differential rotations, between weak and strong nonlinear inertial forces, between insulating and conducting boundaries, and between two aspect ratios. The main results are as follows. Azimuthally drifting Rossby waves arising from the destabilisation of the Stewartson shear layer are crucial to dynamo action. Differential rotation and helical Rossby waves combine to contribute to the spherical Couette dynamo. At a slow global rotation rate, the direction of differential rotation plays an important role in the dynamo because of different patterns of Rossby waves in prograde and retrograde flows. At a rapid global rotation rate, stronger flow supercriticality (namely the difference between the differential rotation rate of the flow and its critical value for the onset of nonaxisymmetric instability) facilitates the onset of dynamo action. A conducting magnetic boundary condition and a larger aspect ratio both favour dynamo action.
A simplified model of collision-driven dynamo action in small bodies  [PDF]
Xing Wei,Rainer Arlt,Andreas Tilgner
Physics , 2013, DOI: 10.1016/j.pepi.2014.03.004
Abstract: We investigate numerically the self-sustained dynamo action in a spinning sphere whose sense of rotation reverses periodically. This system serves as a simple model of a dynamo in small bodies powered by frequent collisions. It is found that dynamo action is possible in some intervals of collision rates. At high Ekman numbers the laminar spin-up flow is helical in the boundary layers and the Ekman circulation together with the azimuthal shear powers the dynamo action. At low Ekman number a non-axisymmetric instability helps the dynamo action. The intermittency of magnetic field occurs at low Ekman number.
Energy transfer in Hall-MHD turbulence: cascades, backscatter, and dynamo action  [PDF]
P. D. Mininni,A. Alexakis,A. Pouquet
Physics , 2005, DOI: 10.1017/S0022377806004624
Abstract: Scale interactions in Hall MHD are studied using both the mean field theory derivation of transport coefficients, and direct numerical simulations in three space dimensions. In the magnetically dominated regime, the eddy resistivity is found to be negative definite, leading to large scale instabilities. A direct cascade of the total energy is observed, although as the amplitude of the Hall effect is increased, backscatter of magnetic energy to large scales is found, a feature not present in MHD flows. The coupling between the magnetic and velocity fields is different than in the MHD case, and backscatter of energy from small scale magnetic fields to large scale flows is also observed. For the magnetic helicity, a strong quenching of its transfer is found. We also discuss non-helical magnetically forced Hall-MHD simulations where growth of a large scale magnetic field is observed.
Isospectrality of spherical MHD dynamo operators: pseudo-Hermiticity and a no-go theorem  [PDF]
U. Guenther,F. Stefani
Mathematics , 2002, DOI: 10.1063/1.1573741
Abstract: The isospectrality problem is studied for the operator of the spherical hydromagnetic alpha^2-dynamo. It is shown that this operator is formally pseudo-Hermitian (J-symmetric) and lives in a Krein space. Based on the J-symmetry, an operator intertwining Ansatz with first-order differential intertwining operators is tested for its compatibility with the structure of the alpha^2-dynamo operator matrix. An intrinsic structural inconsistency is obtained in the set of associated matrix Riccati equations. This inconsistency is interpreted as a no-go theorem which forbids the construction of isospectral alpha^2-dynamo operator classes with the help of first-order differential intertwining operators.
Remarks on some typical assumptions in dynamo theory  [PDF]
R. D. Simitev,F. H. Busse
Physics , 2010, DOI: 10.1080/03091929.2010.519891
Abstract: Some concepts used in the theory of convection-driven dynamos in rotating spherical fluid shells are discussed. The analogy between imposed magnetic fields and those generated by dynamo action is evaluated and the role of the Elsasser number is considered. Eddy diffusivities are essential ingredients in numerical dynamo simulations, but their effects could be misleading. New aspects of the simultaneous existence of different dynamo states are described.
Ejections of magnetic structures above a spherical wedge driven by a convective dynamo with differential rotation  [PDF]
J?rn Warnecke,Petri J. K?pyl?,Maarit. J. Mantere,Axel Brandenburg
Physics , 2011, DOI: 10.1007/s11207-012-0108-4
Abstract: We combine a convectively driven dynamo in a spherical shell with a nearly isothermal density-stratified cooling layer that mimics some aspects of a stellar corona to study the emergence and ejections of magnetic field structures. This approach is an extension of earlier models, where forced turbulence simulations were employed to generate magnetic fields. A spherical wedge is used which consists of a convection zone and an extended coronal region to $\approx1.5$ times the radius of the sphere. The wedge contains a quarter of the azimuthal extent of the sphere and $150\degr$ in latitude. The magnetic field is self-consistently generated by the turbulent motions due to convection beneath the surface. Magnetic fields are found to emerge at the surface and are ejected to the coronal part of the domain. These ejections occur at irregular intervals and are weaker than in earlier work. We tentatively associate these events with coronal mass ejections on the Sun, even though our model of the solar atmosphere is rather simplistic.
Turbulent 3D MHD dynamo model in spherical shells: Regular oscillations of the dipolar field  [PDF]
R. D. Simitev,F. H. Busse,A. G. Kosovichev
Physics , 2011,
Abstract: We report the results of three-dimensional numerical simulations of convection-driven dynamos in relatively thin rotating spherical shells that show a transition from an strong non-oscillatory dipolar magnetic field to a weaker regularly oscillating dipolar field. The transition is induced primarily by the effects a stress-free boundary condition. The variation of the inner to outer radius ratio is found to have a less important effect.
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