Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session PM9: Mini-Conference on Unsteady Reconnection in Laboratory and Nature IV: Turbulence and Multiple Reconnections |
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Chair: Masaaki Yamada, Princeton Plasma Physics Laboratory Room: Regency V |
Wednesday, November 4, 2009 2:00PM - 2:25PM |
PM9.00001: Three Dimensional Fast Reconnection in the Magnetohydrodynamic Limit: Theory Ethan Vishniac, Alex Lazarian, Grzegorz Kowal, Katarzyna Otmianowska-Mazur We examine the role of turbulence in mediating fast reconnection in three dimensions. We work in the single fluid MHD limit, appropriate to the interiors of stars and denser parts of accretion disks, although the model we sketch here should be broadly applicable. In the limit of infinitesimal resistivity, turbulence provides a roughness to the flow and to the magnetic field which renders the definition of magnetic field lines and fluid flow lines somewhat problematic. In this context, we generalize the Sweet-Parker current sheet to include turbulence. We note that in three dimensions this leads to a plasma outflow zone which is much wider than the current sheet, and whose width depends only on the properties of the turbulence. If this sets the speed of reconnection, then it is always of order the local turbulent velocity. We briefly discuss numerical tests of this conjecture and conclude that this conjecture is supported by the available evidence. [Preview Abstract] |
Wednesday, November 4, 2009 2:25PM - 2:45PM |
PM9.00002: Non-stationary Magnetic Reconnection: plasmoids, turbulence, and what it all means Dmitri Uzdensky, Nuno Loureiro, Alexander Schekochihin In this contribution the recent progress on understanding the role of secondary tearing instabilities and small-scale turbulence in resistive-MHD reconnection with very large Lundquist numbers $S\geq 10^4$ is reviewed. It is argued that the resulting formation of multiple plasmoids and their rapid nonlinear evolution greatly affect the basic structure of the reconnection layer and force upon us a total revision of the conventional picture of the reconnection processes. In particular, the very definition of the reconnection rate and its scaling with~$S$ are discussed and compared with the results of recent numerical simulations. [Preview Abstract] |
Wednesday, November 4, 2009 2:45PM - 3:10PM |
PM9.00003: Turbulent Reconnection in the Magnetic Reconnection Experiment (MRX) S. Dorfman, H. Ji, M. Yamada, E. Oz, J. Yoo, W. Daughton, V. Roytershteyn One of the key open questions in Magnetic Reconnection is the nature of the mechanism that governs the reconnection rate in real astrophysical and laboratory systems. For collisonless plasmas, the Hall effect removes an important bottleneck to fast reconnection as the heavier ions exit the reconnection layer over a broader region [1]. However, the Hall term cannot balance the reconnection electric field at the layer center, and the 2-D, collisionless expression for the electric field due to particle dynamics [2] has been shown to be insufficient in the Magnetic Reconnection Experiment (MRX) [1,3]. Turbulent 3-D effects such as lower hybrid frequency range fluctuations [4] may play an important role in fast reconnection in MRX. These electromagnetic fluctuations tend to be associated with high local currents and a rapid local reconnection rate. The precise relation of these fluctuations and associated 3-D asymmetries to fast reconnection is a topic of active investigations; the most up to date results will be discussed. This work was supported by NDSEG, DOE, NASA, and NSF.\\[4pt] [1] Y. Ren, et al., Phys. Plasmas 15, 082113 (2008). [2] M. Hesse, et al., Phys. Plasmas, 6:1781 (1999). [3] S. Dorfman, et al., Phys. Plasmas 15, 102107 (2008). [4] H. Ji, et al., Phys.Rev.Lett. 92 (2004) 115001. [Preview Abstract] |
Wednesday, November 4, 2009 3:10PM - 3:35PM |
PM9.00004: Statistical Properties of Magnetic Reconnection in MHD turbulence Sergio Servidio, William Matthaeus, Paul Cassak, Michael Shay, Pablo Dmitruk Magnetic reconnection is an integral part of MHD turbulence[1] in that the fragmentation of magnetic eddies into smaller structures necessarily involves change of magnetic topology. To better understand this relationship, recently the properties of thousands of magnetic reconnection events in moderate Reynolds number MHD turbulence have been studied [1] using 2D spectral method simulations of compressible and incompressible MHD. Reconnection between magnetic islands, different in size and energy, occurs locally and sporadically in time. The associated reconnection rates are distributed over a wide range of values and scale with the geometry of the diffusion region. Matching classical turbulence analysis with the Sweet-Parker theory, the main statistical features of these multi-scale reconnection events are identified. Magnetic reconnection in turbulence can be described through an asymmetric Sweet-Parker model, in which the parameters that control the reconnection rates are determined by turbulence itself. This new and general perspective on reconnection is relevant in space and astrophysical systems, where plasma is generally in a fully nonlinear regime. [1] W. Matthaeus and S. Lamkin, Phys. Fluids, 29, 2513 (1986). [2] S. Servidio et al, PRL, 102, 115003 (2009). [Preview Abstract] |
Wednesday, November 4, 2009 3:35PM - 4:00PM |
PM9.00005: The time-dependent structure of the electron reconnection layer Michael Hesse, Seiji Zenitani, Masha Kuznetsova, Alex Klimas Collisionless magnetic reconnection is often associated with time-dependent behavior. Specifically, current layers in the diffusion region can become unstable to tearing-type instabilities on one hand, or to instabilities with current-aligned wave vectors on the other. In the former case, the growth of tearing instabilities typically leads to the production of magnetic islands, which potentially provide feedback on the reconnection process itself, as well as on the rate of reconnection. The second class on instabilities tend to modulate the current layer along the direction of the current flow, for instance generating kink-type perturbations, or smaller-scale turbulence with the potential to broaden the current layer. All of these processes contribute to rendering magnetic reconnection time-dependent. In this presentation, we will provide a summary of these effects, and a discussion of how much they contribute to the overall magnetic reconnection rate. [Preview Abstract] |
Wednesday, November 4, 2009 4:00PM - 4:25PM |
PM9.00006: Unsteady reconnection in MHD models Giovanni Lapenta Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible [1]. This regime has properties of immediate practical relevance: (i) it is much faster, developing on scales of the order of the Alfv\'en time, and (ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed-circulation patterns where the jets going out of the reconnection regions turn around and force their way back in, carrying along copious amounts of magnetic flux. We further investigate the presence of unsteady reconnection regimes in the RSX experiment in Los Alamos [2]. Work in collaboration with: Intrator TP, Sun X, Dorf L and Furno I. \\[4pt] [1] Lapenta, G., Self-Feeding Turbulent Magnetic Reconnection on Macroscopic Scales, Phys. Rev. Lett. 100, 235001 (2008)\\[0pt] [2] Intrator TP, Sun X, Dorf L, Lapenta G and Furno I, Experimental onset threshold and magnetic pressure pileup for 3D reconnection, Nature-Physics, 5, 521 - 526 (2009) [Preview Abstract] |
Wednesday, November 4, 2009 4:25PM - 4:45PM |
PM9.00007: Electron scale structures in collisionless magnetic reconnection with multiple reconnection sites Surjalal Sharma, Neeraj Jain The early time-dependent phase of collisionless reconnection, which is dominated by electron dynamics, is investigated using electron-magnetohydrodynamic simulations. In EMHD the frozen-in condition of magnetic field breaks down due to electron inertia, which is the dominant non-ideal term in generalized Ohm's law for very thin current sheets (CS) with thicknesses of the order of electron skin depth. This is in contrast with ion-scale current sheets for which divergence of pressure tensor is the dominant term. Simulations initialized with multi-wavelength perturbations lead to reconnection at multiple sites one of which is dominant and others are secondary. The current sheet bifurcation in the outflow regions limits the length of the reconnecting CS which is further reduced by the secondary instabilities growing on the bifurcated CS. The interaction of inflow to the secondary site and outflow from the central dominant site gives rise to the nested structure of quadrupoles. These structures have important implications for multi-spacecraft missions in Earth's magnetotail. [Preview Abstract] |
Wednesday, November 4, 2009 4:45PM - 5:10PM |
PM9.00008: A new mechanism for the generation of anomalous cosmic rays: dissipation of the sectored heliospheric magnetic field in the heliosheath J.F. Drake, M. Opher, M. Swisdak, K. Schoeffler The recent observations of the anomalous cosmic ray (ACR) energy spectrum as Voyagers 1 and 2 crossed the heliospheric termination shock have called into question the conventional shock source of these energetic particles. We suggest that the sectored heliospheric magnetic field, which results from the flapping of the heliospheric current sheet, piles up as it approaches the heliopause, narrowing the current sheets that separate the sectors and triggering the onset of collisionless magnetic reconnection. MHD simulations of the global heliosphere reveal the structure of sectored field and pileup. Particle-in-cell simulations reveal that the current layers break up into a turbulent bath of magnetic islands that merge to release a large fraction of the energy in the sectored magnetic field. Most of the magnetic energy goes into energetic ions with significant but smaller amounts of energy going into electrons. The ACR differential energy spectrum takes the form of a power law with a spectral index slightly above 1.5, which is consistent with the Voyager observations. The model has the potential to explain several other key observations, including the spectra of super-Alfvenic ions seen throughout the heliosphere. [Preview Abstract] |
Wednesday, November 4, 2009 5:10PM - 5:30PM |
PM9.00009: The Role of Turbulent Outflow Jets in Electron-Positron Plasmas Marc Swisdak, Yi-Hsin Liu, James Drake Numerical simulations of reconnection in electron-positron (pair) plasmas provide an interesting window into the role turbulence plays in current theories of whistler-mediated Hall reconnection. Because of the system's mass symmetry the Hall term vanishes from the generalized Ohm's law, suggesting that perhaps pair reconnection is slow, as in the classic Sweet-Parker picture. Large particle-in-cell simulations of pair reconnection confirm that the reconnection rate remains fast even as the system size changes by a factor of 8. For the largest systems a Weibel-like temperature anisotropy instability in the X-line outflow broadens the current layer and permits fast reconnection. This instability can be suppressed both artificially and for certain parameter choices, leading to non-steady behavior and a decrease in the reconnection rate. [Preview Abstract] |
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