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 JM4: Mini-conference on Angular Momentum Transport in Laboratory and Nature II |
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Chair: Dmitri Uzdensky, Princeton University Room: Rosen Centre Hotel Salon 1/2 |
Tuesday, November 13, 2007 2:00PM - 2:30PM |
JM4.00001: Angular momentum transport at early times Ellen Zweibel It is well known that angular momentum must be efficiently removed from interstellar clouds as they contract and eventually collapse to form stars. At the present epoch, angular momentum is transported primarily by magnetic fields. At the time the first stars formed, galactic magnetic fields were probably either absent or were much weaker than they are now. I will discuss the growth of magnetic fields in star forming regions and their role in angular momentum transport under primordial conditions. [Preview Abstract] |
Tuesday, November 13, 2007 2:30PM - 3:00PM |
JM4.00002: Heating and Angular Momentum Transport in Hot Accretion Flows Prateek Sharma, Eliot Quataert, Gregory W. Hammett, James M. Stone The magnetorotational instability (MRI), an instability of magnetized differentially rotating plasmas, has been studied extensively with MHD. However, MHD is not a good description when the plasma mean free path is larger than the scales of interest. Sgr A*, the accretion flow around the supermassive black hole in the center of our Galaxy, is the prime example of a collisionless accretion flow. We use the kinetic MHD formalism (valid if Larmor radius $\ll$ length scales), closed with parallel heat fluxes, for local simulations of the collisionless MRI. Kinetic MHD differs from MHD in that the pressure is anisotropic with respect to the magnetic field lines. Pressure anisotropy ($p_\perp>p_\parallel$) results because of adiabatic invariance ($\mu \propto p_\perp/B$ = constant) as magnetic field is amplified by the MRI. Pressure anisotropy cannot become arbitrarily large; we use models of pressure isotropization by different microinstabilities so that $\Delta p/p \leq S/\beta^\alpha$. Anisotropic viscous stress, due to momentum transport by parallel free streaming particles, is comparable to the Maxwell stress. Moreover, electrons can be significantly heated due to anisotropic viscous stress. Large electron heating results in a significant radiative efficiency, ruling out the models which ascribe the low luminosity of Sgr A* to only a low efficiency; a supression of net mass accretion rate is required for the low luminosity. [Preview Abstract] |
Tuesday, November 13, 2007 3:00PM - 3:30PM |
JM4.00003: Evidence for the energetics, efficiency, and magnetic fields from BH angular momentum transport Philipp Kronberg I will review what observations can tell us about the energetics and magnetic field structures in jets that are produced by black hole angular momentum transport. These show that this must be a very efficient process, which must put tight constraints on the ``machine'' that converts gravitational to magnetic energy. I also discuss limitations in the current observational diagnostics, which are mostly in the radio, and prospects for overcoming some of these in the near future. [Preview Abstract] |
Tuesday, November 13, 2007 3:30PM - 3:45PM |
JM4.00004: Angular momentum transport and state transitions in magnetized black hole accretion disks Edison Liang, Guy Hilburn, Siming Liu, Hui Li, Charles Gammie Black hole accretion disks exhibit multiple spectral states with distinct spectra and dynamical behaviors. While the origin of such spectral states remains a mystery, one likely cause is variation in the effective viscosity that transports angular momentum and drives the accretion flow. MHD turbulence driven by the magneto-rotational instability (MRI) is currently the favored candidate of viscosity. Saturation of the MRI instability regulates the accretion flow, but its effect on global spectral states remains an open question. Using MHD simulations and semi-analytic accretion disk models, we have studied the role of magnetic fields in accretion disk emissions and dynamics. In this paper we first review observational data of black hole spectral states and their implications for accretion disk structure, evolution and viscosity. We then report new results of the spectral-temporal manifestations of MRI-driven accretion flows, including the effects of radiative cooling (cyclo-synchrotron, bremsstrahlung, Compton), electron heating by MHD turbulence and strong ordered seed fields. [Preview Abstract] |
Tuesday, November 13, 2007 3:45PM - 4:00PM |
JM4.00005: Magnetic Field Configurations Associated With Angular Momentum Transport in Astrophysics and the Accretion Theory of Spontaneous Rotation in the Laboratory$^*$ B. Coppi Differentially rotating structures in the prevalent field of a central object have been shown to develop a ``crystal'' magnetic structure resulting from toroidal internal currents and leading to the formation of density ring sequences\footnote{B. Coppi and F. Rousseau, \textit{Ap. J.} \textbf{641}, 458 (2006)} rather than disks. Poloidal current densities with appropriate symmetries are found to be connected with angular momentum transport processes represented by an effective viscosity. Jets are suggested to consist of a series of stable ``smoke- rings'' ejected vertically in opposite directions from the central region of the considered ring sequence. A small inward flow velocity is shown to induce a spiral pattern in the magnetic field lines on a selected family of magnetic surfaces. The accretion theory\footnote{B. Coppi, \textit{Nuc. Fus.} \textbf{42}, 1 (2002)} of the spontaneous rotation phenomenon in toroidal laboratory plasmas relies on the ejection of angular momentum toward the surrounding material wall, by collisional ballooning modes excited at the edge, whose phase velocity depends on collisionality. The resulting recoil gives rise to the rotation of the main body of the plasma column as other plasma modes (called VTG) provide the needed inward transport of angular momentum. *Sponsored in part by the US D.O.E. [Preview Abstract] |
Tuesday, November 13, 2007 4:00PM - 4:30PM |
JM4.00006: Studying the Physics of AGN Jets Near Their Origin Daniel Homan I will discuss the use of Very Long Baseline Array (VLBA) observations to probe the Physics of jets from Active Galactic Nuclei (AGN) on parsec scales, near their origin from the accretion disk/super massive black hole system. These high resolution observations can study not only the kinematics, acceleration, and collimation of these relativistic jets, but also their spectral and polarization properties. Polarization, whether intrinsic to the emitted synchrotron radiation or the result of birefringence effects within the jet, serves as a probe of the particle population and the 3-D magnetic field structure of jets, and I will discuss results from recent and ongoing work to constrain these properties. [Preview Abstract] |
Tuesday, November 13, 2007 4:30PM - 5:00PM |
JM4.00007: Momentum Transport by Current-Driven Reconnection S.C. Prager, A. Almagri, D.J. Den Hartog, F. Ebrahimi, G. Fiksel, A. Kuritsyn, M. Miller, V. Mirnov, J. Sarff, D. Brower, W. Ding, D. Craig Radial transport of poloidal and toroidal angular momentum is rapid during a global reconnection event in the MST reversed field pinch experiment. Theoretical explanation has evolved for transport from Maxwell and Reynolds stresses from multiple nonlinearly coupled tearing modes. Comparing stresses from multimode computation with those for a single mode shows that nonlinear coupling (multiple reconnections) greatly enhances transport. Measurement of stresses in MST (edge and core) supports, but does not yet completely confirm, this explanation. In the edge, Reynolds and Maxwell stresses are very large and oppositely directed, with the difference of order of the measured inertial (acceleration) term. These results raise the possibility that current-driven instability (reconnection) could be active in astrophysical accretion disks, for which flow-driven instability is the leading explanation. Thus, we have begun computation of transport from current-driven instability in disks. [Preview Abstract] |
Tuesday, November 13, 2007 5:00PM - 5:15PM |
JM4.00008: An Unmagnetized, Plasma Couette flow for investigating the Magnetorotational Instability Cary Forest A new plasma experiment to investigate the magnetorotational instability, dynamos, and other fundamental plasma processes for astrophysics is proposed. The experiment consists of a vacuum chamber with a series of permanent magnetics, with electrically insulated pole faces, in a ring cusp geometry (poles facing inward with alternating polarity along the vessel wall). The resulting field is axisymmetric and decays quickly away from the walls providing a large, magnetic field free region in the center of the device. To stir the plasma, cathodes positioned between the magnet rings are biased such that the resulting electric field induces plasma rotation through the ExB drift. The flow drive principle is quite general and simulations indicate that the high magnetic Reynolds number plasmas flows can in principle be generated that are unstable to the magnetorotational instability. Use of a plasma for such an experiment may allow the magnetic Reynolds number (the dimensionless parameter governing self-excitation of magnetic fields) to be approximately a factor of 10 larger than in liquid metal experiments and will be the first experiment to investigate the MRI in a plasma, the state of matter that makes up naturally occurring accretion disks. [Preview Abstract] |
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