Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L6: Earth and Space Magneto-Fluid Dynamics |
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Sponsoring Units: DFD Chair: John Hawley, University of Virginia Room: 406 |
Tuesday, March 17, 2009 2:30PM - 3:06PM |
L6.00001: Simulating Astrophysical Flows in Laboratory Experiments Invited Speaker: A laboratory plasma configuration which simulates astrophysical jets has been developed. The experimental geometry is arranged so that the jet is unaffected by walls and the experimental time scale is such that ideal magnetohydrodynamics is reasonably approximated. The jet evolves through a reproducible sequence consisting of formation, collimation, kink instability, and at sufficiently drive high currents, detachment. Diagnostics include high speed imaging, magnetic probing, spectroscopy, and interferometry. The collimated nature of the jet and of a related experiment simulating solar corona loops suggest that collimation is a ubiquitous feature of flux tubes having axial electric currents. This observation has motivated a model for the collimation mechanism. According to this model, pile-up of convected, frozen-in toroidal magnetic flux near the jet tip increases the toroidal magnetic flux density near the tip. This flux accumulation corresponds to an increase of the toroidal field near the tip so that the pinch force is increased, thereby collimating the jet. The model shows that plasma-filled coronal loops can be considered as resulting from two counter-propagating jets colliding head-on. Color-coded images of two colliding jets confirm this. The experiments have also motivated development of a dusty-plasma dynamo mechanism suitable for driving an actual astrophysical jet. This mechanism involves dust grains having a charge to mass ratio so small that their cyclotron frequency becomes comparable to the Kepler frequency. The resulting collisionless orbits spiral across magnetic field lines towards the central object and the accumulation of charged dust grains creates a radial electromotive force appropriate for driving an astrophysical jet. The spiral orbits are not described by magnetohydrodynamics but instead result from detailed considerations of canonical angular momentum in an axisymmetric Hamiltonian system. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:42PM |
L6.00002: Flows and jets around compact astrophysical objects Invited Speaker: |
Tuesday, March 17, 2009 3:42PM - 4:18PM |
L6.00003: Fluid Mechanics of the Geodynamo Invited Speaker: Fluid dynamical processes in the molten, iron-rich, electrically conducting core sustain Earth's magnetic field. Convection driven by secular cooling and chemical differentiation is the primary energy source for the geodynamo. Earth's rotation imparts helicity to the convection, which amplifies the geomagnetic field, balancing losses from Ohmic dissipation. Both the Ekman and Rossby numbers are very small in the outer core, so the convection is partly aligned with the planetary spin axis, which tends to orient the geomagnetic dipole axis in the north-south direction. The magnetic Reynolds number in the outer core is about 20 times the critical value for sustained dynamo action and the Reynolds number is about $10^7$, implying turbulent conditions. Fluctuations in the turbulence induce continuous changes in the geomagnetic field, including occasional polarity reversals. Geomagnetic polarity reversals have occurred about once every 250 kyr on average over the past 5 Myr, the last reversal occurred around 780 ka and there have been several long constant- polarity superchrons. The axial dipole collapses before a reversal, exposing the complex non-dipolar transition field, then the axial dipole is regenerated in the opposite polarity, the entire process lasting 10-20 kyr. Spontaneous polarity reversals have been observed in at least one liquid sodium dynamo experiment. Downward-extrapolated measurements from Earth-orbiting satellites reveal the axial dipole comes mostly from a few high-latitude concentrated flux spots on the core boundary. About 15\% of the core boundary has reversed-direction magnetic field, mostly in the southern hemisphere. Proliferation and growth of reversed flux regions are major reasons why the axial dipole is in decline, decreasing at 10 times its free decay rate and suggesting (to some) that the geomagnetic field may be in early stage of a polarity reversal. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:54PM |
L6.00004: Statistics and scaling in magnetohydrodynamic turbulence Invited Speaker: The nonlinear cascade of energy is one of the most prominent processes in turbulent systems. The associated self-similarity of two- point statistics leads to the appearance of inertial-range scaling laws, e.g. in the energy spectrum of turbulence. The scaling exponents that are observed in experiments or direct numerical simulations allow to verify the validity of cascade phenomenologies. Currently, controversial findings have led to a confusing situation in the phenomenological understanding of nonlinear inertial-range dynamics of magnetohydrodynamic turbulence which is discussed using recent results of direct numerical simulations. A new approach for investigating turbulent nonlinear dynamics which is based on the Lagrangian description of turbulence is also presented. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:30PM |
L6.00005: Turbulence in the interstellar and interplanetary medium Invited Speaker: |
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