Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session PM10: Mini-conference on Understanding Astrophysical Dynamos III |
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Sponsoring Units: GPAP Chair: Ben Brown, University of Wisconsin-Madison Room: 151 ABCG |
Wednesday, November 16, 2011 2:00PM - 2:25PM |
PM10.00001: Numerical Modelling of Planetary Dynamos Sabine Stanley Magnetic field observations by spacecraft missions have provided vital information on planetary dynamos. The four giant planets as well as Earth, Mercury and Ganymede have observable magnetic fields generated by active dynamos. In contrast, Moon and Mars only have remanent crustal fields from dynamo action in their early histories. The study of planetary dynamos has recently expanded to include new bodies. First, paleomagnetic records of meteorites hint that early solar system planetesimals had dynamo generated fields. Second, the diversity of recently discovered extrasolar planets has initiated a study of new planetary interior conditions and what these mean for the feasibility and morphology of dynamos in these bodies. Here we review numerical dynamo simulations of planetary bodies. We focus on planets other than Earth, however will appeal to Earth in a comparative sense. We will concentrate on models that have been tailored for specific planets or have addressed non-Earth-like magnetic field characteristics of planetary bodies. [Preview Abstract] |
Wednesday, November 16, 2011 2:25PM - 2:50PM |
PM10.00002: (M)HD: The surprisingly weak role of Lorentz forces in planetary dynamo models Eric King It is often thought that the fluid dynamics occurring within Earth's liquid metal outer core (and within the magnetic field generating regions of other planets) are governed by two dominant forces: the Coriolis force, resulting from rapid planetary rotation; and the Lorentz force, caused by the resistance of magnetic field lines to bending by flow. These two forces, acting together, are typically thought to reach an equilibrium state known as the magnetostrophic balance. We investigate the role of the Lorentz force in these systems using numerical models and laboratory analogs of core fluid dynamics. First, results from hydrodynamic rotating convection studies are applied to planetary dynamo models. Second, we investigate specifically the role of Lorentz forces by directly comparing planetary convection models with and without magnetic field generation. Both studies indicate, perhaps surprisingly, that the influence of magnetic fields is (in many ways) secondary, suggesting that convection motions in planetary dynamos may not be in magnetostrophic balance, as is typically assumed. We explain this result by reformulating the customary non-dimensional parameter used to define the force balance in a way that is more appropriate to dynamos. [Preview Abstract] |
Wednesday, November 16, 2011 2:50PM - 3:15PM |
PM10.00003: Recent and future liquid metal experiments on homogeneous dynamo action and magnetic instabilities Frank Stefani, Gunter Gerbeth, Andre Giesecke, Thomas Gundrum, Oleg Kirillov, Martin Seilmayer, Marcus Gellert, G\"unther R\"udiger, Agris Gailitis The present status of the Riga dynamo experiment is summarized and the prospects for its future exploitation are evaluated. We further discuss the plans for a large-scale precession driven dynamo experiment to be set-up in the framework of the new installation DRESDYN (DREsden Sodium facility for dynamo and thermohydraulic studies) at Helmholtz-Zentrum Dresden-Rossendorf. We report recent investigations of the magnetorotational instability and the Tayler instability and sketch the plans for another large-scale liquid sodium facility devoted to the combined study of both effects. [Preview Abstract] |
Wednesday, November 16, 2011 3:15PM - 3:40PM |
PM10.00004: Learning about dynamos in nature from experiments in Na, LN2, H2O and LHe Daniel Lathrop The geodynamo and solar dynamo occur at parameter values that cannot be captured entirely with theory, simulations or experiments. Experiments can offer realistic effects of rotation and turbulence on MHD and fluid flows. Our group has been exploring turbulent rotating flows in liquid sodium, liquid Nitrogen, superfluid helium, or water. After reviewing the summary observations, I'll argue three main points: (1) Systems with a very small Ekman number (very rapid rotation) are highly oscillatory. (2) The high magnetic Reynolds number states (state where resistivity is negligible) are determined by a competition of magnetic field generation and reconnection. (3) The magneto-rotational instability and shear instabilities are not exclusive. [Preview Abstract] |
Wednesday, November 16, 2011 3:40PM - 4:05PM |
PM10.00005: The role of system-scale turbulence on MHD activity in the Madison Dynamo Experiment Kian Rahbarnia, Elliot Kaplan, Zane Taylor, Mark Nornberg, Mike Clark, John Wallace, Alex Rasmus, Cary Forest, Eric Spence The Madison Dynamo Experiment studies the onset conditions for magnetic field growth in a two-vortex flow of liquid sodium. Very high Reynolds numbers of the experiment lead to strong turbulence and an increase of the effective resistivity, which hinders self-excitation of magnetic field. An equatorial baffle has been installed to reduce the largest scale turbulent eddies in the flow and a set of rotatable baffles optimizes the flow ratio of poloidal and toroidal components. With the equatorial baffle only a spherical harmonic decomposition of the measured magnetic field shows a reduction of the largest scale magnetic fluctuations, consistent with a reduction of the large-scale velocity fluctuations. A decrease of the $\alpha$-effect induced dipole moment together with an increase of the effective magnetic Reynolds number is observed. First time measurements of the local turbulent electromotive force confirm these observations. A strong flux compression in the center and increasing decay times of the expected dynamo mode are found. Present experiments investigate how the adjustment of the pitch of the rotatable baffles optimizes the flow and minimizes the critical velocity at which the dynamo onset occurs. This work is supported by the CMSO and NSF/DOE. [Preview Abstract] |
Wednesday, November 16, 2011 4:05PM - 4:17PM |
PM10.00006: Modeling the effects of large scale turbulence in the Madison Dynamo Experiment Elliot Kaplan, Mike Clark, Kian Rahbarnia, Mark Nornberg, Zane Taylor, Alex Rasmus, Cary Forest, Erik Spence Early experiments in the Madison Dynamo Experiment (MDE) demonstrated the existence of electric curresnt which correspond to the $\alpha$ and $\beta$ effects of mean field MHD, {\i.e.} currents driven parallel to B, and turbulent resistivity respectively. A magnetic dipole moment was measured parallel to the symmetry axis of the flow ($\alpha$) and the induced toroidal field was less than half what would be expected from the mean flow ($\beta$). Traditionally, mean field theory requires a large separation in scale between the mean magnetic field and turbulent eddies in the conductive medium. However, the recent campaign on the MDE eliminated these effects when a baffle was added to eliminate the {\it largest} scale turbulent eddies. A model is presented that builds $\alpha$- and $\beta$- like effects from these large scale eddies without any assumption of scale separation. [Preview Abstract] |
Wednesday, November 16, 2011 4:17PM - 4:29PM |
PM10.00007: Efficiency of magnetic dynamo action at low magnetic Prandtl numbers Stanislav Boldyrev, Leonid Malyshkin Amplification of magnetic field due to kinematic turbulent dynamo action is studied in the regime of small magnetic Prandtl numbers. An analysis based on the Kazantsev-Kraichnan model is used to establish the dynamo threshold and the dynamo growth rates for varying kinetic helicity of turbulent fluctuations. It is found that in contrast with the case of large magnetic Prandtl numbers, the kinematic dynamo action at small magnetic Prandtl numbers is significantly affected by kinetic helicity, and it can be made quite efficient with an appropriate choice of the helicity spectrum. [Preview Abstract] |
Wednesday, November 16, 2011 4:29PM - 4:41PM |
PM10.00008: Influence of large-scale zonal flows on the evolution of stellar and planetary magnetic fields Ludovic Petitdemange, Martin Schrinner, Emmanuel Dormy Zonal flows and magnetic field are present in various objects as accretion discs, stars and planets. Observations show a huge variety of stellar and planetary magnetic fields. Of particular interest is the understanding of cyclic field variations, as known from the sun. They are often explained by an important $\Omega$-effect, i.e., by the stretching of field lines because of strong differential rotation. We computed the dynamo coefficients for an oscillatory dynamo model with the help of the test-field method. We argue that this model is of $\alpha^2\Omega$-type and here the $\Omega$-effect alone is not responsible for its cyclic time variation. More general conditions which lead to dynamo waves in global direct numerical simulations are presented. Zonal flows driven by convection in planetary interiors may lead to secondary instabilities. We showed that a simple, modified version of the MagnetoRotational Instability, i.e., the MS-MRI can develop in planteray interiors. The weak shear yields an instability by its constructive interaction with the much larger rotation rate of planets. We present results from 3D simulations and show that 3D MS-MRI modes can generate wave pattern at the surface of the spherical numerical domain. [Preview Abstract] |
Wednesday, November 16, 2011 4:41PM - 4:53PM |
PM10.00009: Role of Electrostatic Fields in Reconnection and Dynamo Processes in LAboratory and Astrophysical Plasmas Giovanni Lapenta, Stefano Markidis, Tom Intrator Several of the key electric fields measured in laboratory, observed in space and computed in simulations of processes of energy exchange are electrostatic. In a project at LANL based on the RSX experiment back some years ago, we started investigating this point based on previous simulation and experimental work. Since then we have conducted a number of investigations with the new massively parallel iPic3D code and with MHD codes for conditions typical of the RSX device. We present here our recent conclusions: the electric field plays, as well know, a key role. But the new important conclusion is that this field is by the large part electrostatic. We will present our evidence for discussion. [Preview Abstract] |
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