Bulletin of the American Physical Society
APS April Meeting 2011
Volume 56, Number 4
Saturday–Tuesday, April 30–May 3 2011; Anaheim, California
Session R4: So Many Dynamos: Flow Generated Magnetic Fields in Nature, in the Computer, and in the Lab II |
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Sponsoring Units: DPP DCOMP Chair: Cary Forest, University of Wisconsin-Madison Room: Garden 4 |
Monday, May 2, 2011 1:30PM - 2:06PM |
R4.00001: Aneesur Rahman Prize for Computational Physics Talk: Numerical Modeling of Accretion Disk Dynamos driven by the MRI Invited Speaker: Numerical methods have proved crucial for the study of the nonlinear regime of the magnetorotational instability (MRI) and resulting dynamo action. After a brief introduction to the methods, a variety of results from new simulations of the MRI in both local (shearing box approximation) and global domains will be presented. Previous work on the saturation level and numerical convergence in both stratified and unstratified domains with no net flux (both with and without explicit dissipation) will be described, and the connection to dynamo theory will be mentioned. Results from several groups in which the size of the computational domain, and the vertical boundary conditions, are varied will be discussed. Finally, new work on the direct comparison between high-resolution global and shearing box simulations will be presented, and new studies of stratified disks with radiative transfer will be introduced. [Preview Abstract] |
Monday, May 2, 2011 2:06PM - 2:42PM |
R4.00002: Magnetogenesis and magnetothermal equilibria in turbulent galaxy-cluster plasmas Invited Speaker: We do not know the exact mechanism of magnetic field generation in magnetised weakly collisional (or collisionless) turbulent plasma. We do know that large-scale MHD motions in such plasmas are subject to fast small-scale kinetic instabilities (mirror and firehose) triggered (at high beta) by pressure anisotropies and that these anisotropies will always arise in a turbulent plasma. Therefore, standard MHD equations cannot be used to describe the turbulent dynamo. I will argue that the likely scenario in such plasmas is explosively fast growth of magnetic fluctuations to dynamical levels. I will further argue that if an efficient turbulent dynamo is assumed, radiative cooling in such plasmas can be balanced in a thermally stable way by turbulent heating, whose rate is set by the condition that plasma locally remains in a marginal state with respect to the mirror and firehose instabilities. This thermal stability suggests that a cooling catastrophe is not inevitable, although whether this old problem is thus resolved depends on whether a number of assumptions about the nonlinear behaviour of the instabilities, strength of turbulence and efficiency of the dynamo are borne out by first- principles microphysical theory, simulations or plasma experiments.\\ References:\\ A. A. Schekochihin, M. Brueggen, L. Feretti, M. W. Kunz, and L. Rudnick, Space Sci. Rev., in preparation (2011)\\ M. W. Kunz, A. A. Schekochihin, S. C. Cowley, J. J. Binney, and J. S. Sanders, Mon. Not. R. Astron. Soc., in press (2011) [e-print arXiv:1003.2719]\\ M. S. Rosin, A. A. Schekochihin, F. Rincon, and S. C. Cowley, Mon. Not. R. Astron. Soc., in press (2011) [e-print arXiv:1002.4017]\\ A. A. Schekochihin, S. C. Cowley, F. Rincon, and M. S. Rosin, Mon. Not. R. Astron. Soc. 405, 291 (2010) [e-print arXiv:0912.1359]\\ A. A. Schekochihin, S. C. Cowley, R. M. Kulsrud, M. S. Rosin, and T. Heinemann, Phys. Rev. Lett. 100, 081301 (2008) [e-print arXiv:0709.3828]\\ A. A. Schekochihin and S. C. Cowley, Phys. Plasmas 13, 056501 (2006) [e-print astro-ph/0601246]\\ A. A. Schekochihin, S. C. Cowley, R. M. Kulsrud, G. W. Hammett, and P. Sharma, Astrophys. J. 629, 139 (2005) [e-print astro-ph/0501362] [Preview Abstract] |
Monday, May 2, 2011 2:42PM - 3:18PM |
R4.00003: Simulations of global-scale dynamo action in the Sun and other stars Invited Speaker: Our Sun is a magnetic star, and its eleven-year cycles of magnetic activity profoundly affect our modern technological society. The magnetic fields we see at the solar surface are built by dynamo processes in the Sun's sub-surface convection zone. There, global-scale plasma motions couple with rotation to build and rebuild the global-scale magnetic fields and drive cycles of magnetic activity, though the exact processes at work in solar and stellar dynamos remain elusive. The Sun is not the only magnetic star: indeed magnetism is a ubiquitous feature of stars that have convection zones near their surfaces. Observations of younger suns indicate that they rotate quite rapidly, have strong magnetic fields at their surfaces, and show signs of cyclic activity. Here we explore recent 3-D MHD simulations of the solar dynamo and of stellar dynamos in younger, more rapidly rotating solar-type stars. These are conducted with the anelastic spherical harmonic (ASH) code on modern supercomputers. These simulations of global-scale convection and dynamo action produce strikingly organized magnetic structures in the bulk of their convection zones. Wreaths of magnetic field fill the convection zone and can undergo regular cycles of polarity reversal. Indeed, we find that cyclic behavior is a common feature throughout the parameter space we have explored. Simulations like these are providing new views on the phenomena of solar and stellar dynamo action. [Preview Abstract] |
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