Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session PM4: Mini-conference on Angular Momentum Transport in Laboratory and Nature IV |
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Chair: Philipp Kronberg, Los Alamos National Laboratory Room: Rosen Centre Hotel Salon 1/2 |
Wednesday, November 14, 2007 2:00PM - 2:30PM |
PM4.00001: Angular Momentum Transport in the Earth's Core Bruce Buffett Convection in the Earth's liquid iron core continually regenerates the geomagnetic field and sustains a differential rotation between the inner and outer boundaries. The differential rotation is thought to be a consequence of large-scale fluid circulation in a region defined by the tangent cylinder (a hypothetical cylinder that is tangent to the solid inner core at the equator). Magnetic stresses sweep the solid inner core in the direction of the overlying flow. An additional complication arises due to gravitational interactions between the solid inner core and the rocky outer region, known as the mantle. A heterogeneous distribution of mass in the inner core and mantle can yield a surprisingly large gravitational force when the inner core rotates away from its equilibrium alignment with the mantle. This restoring force is sufficient to oppose the magnetic stresses that drive inner-core rotation. However, the inner core can escape the lock of gravity by plastically deforming as it rotates. Numerical dynamo models that include the influences of gravitational coupling and plastic deformation suggest that the differential rotation is limited by the rate of deformation of the inner core. The calculations also predict large fluctuations in the rate of differential rotation. Any angular misalignment between the inner core and mantle transfers angular momentum to the mantle, which can be detected as a change in the length of day. The fluctuations in inner-core rotation also excite waves that transmit angular momentum through the liquid core. [Preview Abstract] |
Wednesday, November 14, 2007 2:30PM - 3:00PM |
PM4.00002: Angular momentum transport in cylindrical and spherical Couette flows Daniel Lathrop Cylindrical and spherical Couette flows (flows between differentially rotating cylinders and spheres) are simple geometries for understanding turbulent transport. I'll review experiments at high Reynolds number of turbulent flows in water, glycerin-water solutions, and liquid sodium. Instabilities can occur on top of relatively featureless turbulence due to restoring forces such as Lorentz forces, Coriolis forces or gravity waves on a free surface. The former shares some characteristics with the Magnetorotational instability. The latter cases speak to perhaps a general ability of turbulent shear flows to drive large scale oscillations when there exist near-neutral oscillatory modes. [Preview Abstract] |
Wednesday, November 14, 2007 3:00PM - 3:30PM |
PM4.00003: Understanding the Uniform Rotation of the Solar Radiative Interior Tamara Rogers Two models exist to explain the relatively uniform rotation of the solar radiative interior; one magnetic in nature and one purely hydrodynamic. In this talk I will discuss the strengths and weaknesses of both models. I will then focus on the currently favored magnetic model and discuss recent research which may be able to address previous weaknesses of this model. [Preview Abstract] |
Wednesday, November 14, 2007 3:30PM - 4:00PM |
PM4.00004: Angular Momentum Transport in the Solar Tachocline Steven Tobias The eleven year solar activity cycle is believed to be generated by a dynamo. The solar tachocline is a region of strong differential rotation located at the base of the solar convection zone, which is believed to play a crucial role in the operation of this dynamo. The dynamics of the shear layer is however poorly understood. Indeed there is no consistent theory available for the existence of such a thin tachocline. The problem of tachocline existence is inherently linked to that of instabilities and angular momentum transport in stably stratified magnetised collisional plasmas. In this talk I shall review the theory for this and discuss some ideas for future progress in this area. [Preview Abstract] |
Wednesday, November 14, 2007 4:00PM - 4:15PM |
PM4.00005: Magnetized Accretion Disk Corona Dmitri Uzdensky, Jeremy Goodman We present a statistical description of a force-free magnetic field in the corona above a turbulent accretion disk. The field is represented by a statistical ensemble of loops tied to the disk. Each loop evolves under several physical processes: Keplerian shear, turbulent random walk of the disk footpoints, and reconnection with other loops. To build a statistical description, we introduce the distribution function of loops over their sizes and construct a kinetic equation for this function. This Loop Kinetic Equation is similar to Boltzmann's kinetic equation, with reconnection described by a binary collision integral. We solve the equation numerically and obtain a statistical steady state. This allows us to calculate self-consistently the distribution of magnetic pressure with height, the equilibrium shapes of loops of different sizes, and the energy associated with a given loop. We then assess the effectiveness of the coronal magnetic field in transporting angular momentum across the disk and also calculate the energy- and height-distribution of coronal reconnection events. [Preview Abstract] |
Wednesday, November 14, 2007 4:15PM - 4:30PM |
PM4.00006: Analytical theory of the PDFs of momentum transport and the formation of shear flow in plasmas Johan Anderson, Eun-jin Kim There has been overwhelming evidence that coherent structures play a critical role in determining the overall transport in a variety of systems. A crucial question in momentum transport theory is thus the prediction of the PDFs of the momentum transport due to these structures and of the formation of shear flow. Here, we report on a first analytical result on these two closely related problems by using a novel non-perturbative method. We first compute the PDF tails of global momentum flux in the ion-temperature-gradient turbulence, by assuming that a short-lived modon is a coherent structure responsible for bursty and intermittent events, contributing to the PDF tails. The tail of PDF of global momentum flux $R = \langle v_x v_y \rangle$ is shown to be exponential with the form $\exp{\{-c R^ {3/2}\}}$, which is broader than a Gaussian, similarly to what was found in the previous local studies [1-2]. The non-Gaussian tail is a manifestation of intermittency due to rare events. The overall amplitude of the PDFs crucially depends on the temperature and density scale lengths through the constant c. We then present a consistent theory of the PDFs of the formation of a shear flow by incorporating its generation via the computed momentum flux. Implications for momentum transport in astrophysical plasmas will be addressed. 1 E. Kim and P. H. Diamond, Phys. Rev. Lett. 88 225002 (2002) 2 E. Kim et al, Nucl. Fusion 43 961 (2003) [Preview Abstract] |
Wednesday, November 14, 2007 4:30PM - 4:45PM |
PM4.00007: Experimental Verification of Braginskii's Viscosity in MHD Plasma Jet of Reconnection Scaling Experiment. L. Dorf, X. Sun, T. Intrator, J. Hendryx, G. Wurden, I. Furno, G. Lapenta Braginskii's theory gives a simple and useful expression for ion-ion viscosity in magnetohydrodynamic (MHD) plasmas. While this formula has been used extensively, no experimental verification of it can be found in the literature. A few direct measurements of Fokker-Plank diffusion coefficients have been reported, but ion viscosity has not been recognized in those works. This talk will describe the first experimental evaluation of ion viscosity, performed by measuring and comparing the terms of the MHD momentum balance equation. Namely, time-dependent, 2D profiles of the axial flow velocity, density, electron temperature, and magnetic field components were measured at two axial locations in a plasma column of the Reconnection Scaling Experiment. Significant plasma flow with the velocity on the order of the ion acoustic speed was detected, with the velocity decreasing downstream. The results show that the ion momentum flux is dissipated by the ion-ion viscosity due to a significant radial shear of the axial velocity. Chord-integrated ion temperature measurements performed at several radial locations using Doppler broadening spectroscopy show temperature of about 1eV. Comparison of the measured viscosity with Braginskii's predictions demonstrates a good agreement. Supported by OFES, and DOE/LANL contract DE-AC52-06NA25396. [Preview Abstract] |
Wednesday, November 14, 2007 4:45PM - 5:00PM |
PM4.00008: Supersonic radiatively cooled rotating flows and jets produced by wire array z-pinches D.J. Ampleford, A. Ciardi, S.V. Lebedev, S.N. Bland, S.C. Bott A modification of the wire array z-pinch is used to produce a rotating shock and jet in the laboratory. The wires in a wire array z-pinch undergo a steady ablation as the Lorentz force acts to accelerate plasma from the wire cores, which remain stationary, towards the array axis. In a twisted conical array both a radial component to the current and an axial component to the magnetic field are present, which provides an azimuthal component to the Lorentz force (in addition to axial and radial components). These streams collide on axis, however both the angular and axial momentum are conserved in the standing conical shock which is formed. The presence of angular momentum leads to a shock with large diameter and hollow mass profile. A jet is accelerated out of the top of the conical shock. This jet has a high degree of radiative cooling, retains angular momentum and also has a hollow density profile. [Preview Abstract] |
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