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 CO6: Magnetic Reconnection and Turbulence |
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Chair: William Daughton, Los Alamos National Laboratory Room: Hanover FG |
Monday, November 2, 2009 2:00PM - 2:12PM |
CO6.00001: Instability of the Sweet Parker reconnection layer and onset of fast turbulent reconnection Marina Skender, Giovanni Lapenta Within purely resistive MHD without any anomolous effects reconnection sets into a steady state knox to center around an elongated current sheet, the Sweet-Parker (SP) layer, where dissipations allow reconnection. However, such layer is known to be possibly unstable to tearing-like modes. A recent discovery [1,2] is that following the destabilisation of the SP layer reconnection can develop into a fast trubulent regime. This new regime requires no anomlous processes and it is purely resistive MHD, progressing as fast as the fastests kinetic or MHD processes. We investigate such transition. \\[4pt] [1] Self-Feeding Turbulent Magnetic Reconnection on Macroscopic Scales Giovanni Lapenta, Phys. Rev. Lett. 100, 235001 (2008), DOI:10.1103/PhysRevLett.100.235001 \\[0pt] [2] Turbulent Magnetic Reconnection in Two Dimensions: Loureiro, N. F.; Uzdensky, D. A.; Schekochihin, A. A.; Cowley, S. C.; Yousef, T. A.: eprint arXiv:0904.0823 [Preview Abstract] |
Monday, November 2, 2009 2:12PM - 2:24PM |
CO6.00002: ABSTRACT WITHDRAWN |
Monday, November 2, 2009 2:24PM - 2:36PM |
CO6.00003: Electron scale structure of thin current sheets in collisionless magnetic reconnection Neeraj Jain, Surjalal Sharma Cluster observations have shown highly structured electron-scale current sheets (CS), viz. bifurcated, filamented and triple peak structures during magnetic reconnection. An electron-magnetohydrodynamic model shows that these structures develop at various stages of time dependent reconnection. The Lorentz force on electrons due to the interaction of electron outflow velocity and normal component of magnetic field bifurcates the CS in the outflow regions, with scale sizes of the individual peak $\sim 3d_e$ ($d_e$ being electron skin depth), similar to those observed by Cluster spacecraft. The bifurcation limits the length of the reconnecting CS which is further reduced by the secondary instabilities growing on the bifurcated CS. The secondary instabilities causes filamentation of the CS in the outflow region. Such filamentary structures have been observed by Cluster spacecraft in magnetopause. In the presence of many reconnection sites, triple peak structure of the CS forms at the secondary sites as a result of reconnection inside the bifurcated CS associated with the main reconnection site. [Preview Abstract] |
Monday, November 2, 2009 2:36PM - 2:48PM |
CO6.00004: Fast magnetic reconnection in fluid models of collisionless pair plasma Evan Johnson The original studies of the GEM magnetic reconnection challenge problem exhibited fast magnetic reconnection in models which included Hall term effects. Subsequent PIC simulations showed that fast reconnection occurs even in pair plasmas, where the Hall term is absent; these studies attributed fast reconnection to nongyrotropic pressure. So we ask, (1) \emph{Can a model of pair plasma with isotropic pressure exhibit fast reconnection?}, and (2) \emph{Can a fluid model replicate patterns of fast magnetic reconnection seen in PIC simulations}? We have simulated the GEM problem for a pair plasma using ten-moment gas-dynamics for each species coupled to Maxwell's equations. Surprisingly, we find that the adiabatic collisionless ten-moment model gives slow reconnection but that isotropization \emph{increases} the rate of reconnection to typical fast rates. We hope to demonstrate fast reconnection in anisotropic pair plasma without direct isotropization by using a nonadiabatic closure for the generalized heat flux; in particular, we expect to report results for David Levermore's 10-moment closure. [Preview Abstract] |
Monday, November 2, 2009 2:48PM - 3:00PM |
CO6.00005: Phase-space signature of electron diffusion in collisionless magnetic reconnection: A standing electron phase-space hole Li-Jen Chen, William Daughton The structure and dynamics of the electron layer holds the ultimate mystery of how magnetic reconnection can occur in a collisionless plasma. Results from 2D fully kinetic simulations indicate that the electron crossing orbits within the electron layer is the main cause for the electron motion decoupling from the magnetic flux during collisionless magnetic reconnection with zero guide fields. The key evidence is revealed in a hole structure in the electron phase space that stands in the center of the electron layer. The hole structure starts to emerge at the beginning of the explosive growth phase of reconnection, becomes the most prominent when the reconnection rate peaks, and spatially extends, along the outflow direction, through the entire electron diffusion region. Corresponding to the electron hole are a layer of bipolar electric field embedded in the bipolar Hall electric field within the electron layer, and bifurcations of the electron density and the out-of-plane velocity. Flow diversion, energy conversion, as well as regulation of electric currents in the reconnection layer can be understood in terms of electron crossing orbit dynamics. [Preview Abstract] |
Monday, November 2, 2009 3:00PM - 3:12PM |
CO6.00006: Electron holes in PIC Simulations with Physical Mass Ratio Giovanni Lapenta, Martin Goldman, David Newman, Andrey Divin, S. Markidis Numerical simulation of reconnection with the kinetic approach is presented. Calculations are performed using the implicit particle-in-cell code PARSEK. The initial configuration is taken to be a conventional Harris current sheet with a guide field and an initial X-point perturbation is superposed to drive the reconnection process. The simulations reproduce such typical features as the Hall structure, the plasma exhaust jets and particle acceleration near X-point. Our attention is focused on the study of kinetic processes near the separatrix. Fast electron flows formed by Hall current system are favourable for development of electrostatic instabilities (namely, electron-ion Buneman instability) [Pritchett, 2005], [Goldman, 2008] as electrons stream much faster than ions there. The distribution functions are investigated for the evidence of electron holes formed by the Buneman instability and the corresponding bipolar spots of electric field. The relation between those reconnection signatures and development of separatrix instability in simulations is discussed. [Preview Abstract] |
Monday, November 2, 2009 3:12PM - 3:24PM |
CO6.00007: Dynamo action with flow shear and magnetic shear N. Leprovost, E. Kim Dynamo action is a fundamental mechanism that explains ubiquitous magnetic fields in a variety of systems, including astrophysical, geophysical and laboratory plasmas. In this contribution, we provide an analytical theory of dynamo ($\alpha$ and $\beta$ effects) in 3D forced helical MHD turbulence [1]. By non-perturbatively incorporating the effect of shear, we show that the $\alpha$ and $\beta$ effects are enhanced by a weak shear while strongly suppressed by strong shear. In particular, for strong shear, the $\beta$ effect is shown to be much more strongly suppressed than the $\alpha$ effect with the scalings $\alpha \propto A^{-5/3}$ and $\beta \propto A^{-7/3}$, respectively ($A$ is the strength of the shear). The quenching of the $\alpha$ and $\beta$ effect by shear has recently been confirmed in a numerical experiment [2]. One of the interesting implications of these results is that the dynamo efficiency, conventionally measured by the dynamo number $D$, depends more strongly on the shear than conventionally assumed. Specifically, $D$ scales as $A^{4}$ rather than $A$. Incorporating a shear in the magnetic field, we then discuss its effect on the stability. Magnetic shear is shown to destabilize when it is stronger than flow shear. On the other hand, a weak magnetic shear compared to flow shear weakens the stabilizing effect of flow shear, thereby leading to a stronger turbulence than in the case without magnetic shear. \\[0pt] [1] N. Leprovost and E. Kim, Astrophys J Lett. v696, L125 (2009); Phys. Rev. Lett., v100, 144502 (2008) \\[0pt] [2] D. Mitra et al, Astron \& Astrophys, v495, 1 (2009) [Preview Abstract] |
Monday, November 2, 2009 3:24PM - 3:36PM |
CO6.00008: The effect of two-dimensional turbulence on resistive-MHD reconnection Nuno Loureiro, Dmitri Uzdensky, Alexander Schekochihin, Stephen Cowley, Tarek Yousef Two-dimensional numerical simulations of the effect of background turbulence on 2D resistive magnetic reconnection are presented. For sufficiently small values of the resistivity ($\eta$) and moderate values of the turbulent power ($\epsilon$), the reconnection rate is found to have a much weaker dependence on~$\eta$ than the Sweet-Parker scaling of~$\eta^{1/2}$ and is even consistent with an $\eta-$independent value. For a given value of $\eta$, the dependence of the reconnection rate on the turbulent power exhibits a critical threshold in $\epsilon$ above which the reconnection rate is significantly enhanced. [Preview Abstract] |
Monday, November 2, 2009 3:36PM - 3:48PM |
CO6.00009: Fast 3D Reconnection of Weakly Stochastic Magnetic Field: Prospects of the Research Alex Lazarian, Ethan Vishniac, Grzegorz Kowal, Hoang Thiem, Reinaldo Lima If the necessary requirement for the fast reconnection is the fluid being collisionless, then most of MHD simulations, including those of interstellar medium, do not represent astrophysical reality, as high numerical diffusivity makes reconnection fast for present-day simulations. Fortunately, the model of fast reconnection proposed in Lazarian \& Vishniac (1999) does not have these restrictive constraints as it allows for fast reconnection in MHD limit provided that magnetic field is weakly stochastic. As turbulence is ubiquitous in astrophysical environments, weak stochasticity of magnetic field lines is a default state for most of astrophysical magnetic fields. This still leaves a question of what is happening when the magnetic fields are rather laminar. Our research shows that the reconnection itself may increase the level of turbulence resulting in reconnection instability or bursts of reconnection. The most evident application of the model is related to explaining of Solar flares. The predictions of the model above include First Order Fermi acceleration of energetic particles within the extended reconnection layers predicted in the model as well as the removal of magnetic flux during star formation. More astrophysical applications of the process are to follow. [Preview Abstract] |
Monday, November 2, 2009 3:48PM - 4:00PM |
CO6.00010: Spectrum of weak MHD turbulence Stanislav Boldyrev, Jean Carlos Perez Turbulence of magnetohydrodynamic waves in nature and in the laboratory is generally cross-helical or non-balanced, in that the energies of Afv\'en waves moving in opposite directions along the guide magnetic field are unequal. We propose that such turbulence spontaneously generates a condensate of the residual energy $E_v-E_b$ at small field-parallel wave numbers. As a result, the energy spectra of counter-propagating Alfv\'en waves are generally not scale-invariant. In the limit of infinite Reynolds number, the universality is asymptotically restored at large wave numbers, and both spectra attain the scaling $E(k)\propto k_{\perp}^{-2}$. [Preview Abstract] |
Monday, November 2, 2009 4:00PM - 4:12PM |
CO6.00011: Numerical studies of weak MHD turbulence Jean C. Perez, Stanislav Boldyrev Results from numerical simulations of weak magnetohydrodynamic (MHD) turbulence in steady-state are presented, with resolutions as high as $1024^2\times256$ grid points. Weak turbulence refers to the limit of MHD turbulence in which the energy transfer toward smaller scales results from the weak interaction between Alfv\'en waves moving along of against a strong guide magnetic field. The energies of the Alfven waves moving in the opposite directions can be either equal, in which case the turbulence is called balanced, or unequal, in which case it is unbalanced. The numerical set up is optimized as to drive either balanced or unbalanced turbulent cascades. We obtain the spectra of Alfv\'en waves for various degrees of imbalance and Reynolds numbers. We discuss our results and compare with recent theories of weak MHD turbulence. [Preview Abstract] |
Monday, November 2, 2009 4:12PM - 4:24PM |
CO6.00012: Theory of incompressible MHD turbulence with scale dependent alignment and cross-helicity J.J. Podesta, A. Bhattacharjee Boldyrev's theory of incompressible MHD turbulence is different from that of Goldreich \& Sridhar (1995) and others in that it contains a scale dependent alignment of velocity and magnetic field fluctuations in the inertial range. Evidence for this alignment comes from direct numerical simulations and from solar wind data. An extension of Boldyrev's theory to imbalanced turbulence has been proposed by Perez \& Boldyrev. We propose a different theoretical approach which generalizes the results of Perez \& Boldyrev and is based on two new solar wind observations. The first is the observation that the normalized cross-helicity is approximately constant in the inertial range. The second is the observation that the probabilities $p$ and $q$ are approximately constant in the inertial range, where $p$ and $q$ are the probabilities that the velocity and magnetic field fluctuations at a randomly chosen point $(x,t)$ are either aligned or anti-aligned, respectively. Aligned means that the angle between the velocity and magnetic field fluctuations is between 0 and $\pi/2$; anti-aligned means that the angle is between $\pi/2$ and $\pi$. Using these two observational constraints, a generalization of Boldyrev's theory is constructed in which the cascades of the two Elsasser species are each in a state of critical balance and the eddy geometries are required to be scale-invariant. In the new theory, $E(k_{\perp})\propto k_{\perp}^{-3/2}$, $k_{\parallel} \propto k_{\perp}^{ 1/2}$, and the normalized cross-helicity is scale-invariant (a constant). [Preview Abstract] |
Monday, November 2, 2009 4:24PM - 4:36PM |
CO6.00013: Scaling criteria for high Reynolds number MHD turbulence Ye Zhou Magnetohydrodynamic (MHD) turbulence has been employed as a physical model for a wide range of applications in astrophysical and space plasma physics. This paper addresses the following questions. At what MHD flow condition can investigators be sure that their numerical simulations have reproduced all of the most influential physics of the flows and fields of practical interest? Another question, perhaps more specific, is can one define a metric to indicate whether the necessary physics of the flows of interest have been captured and suitably resolved using the tools available to the researcher? This issue was previously addressed in the context of high energy density physics where the Reynolds number of the minimum state was determined to be 1.6$\times $10$^{5}$ [Zhou, Phys. Plasma, 14, 082701 (2007)]. The current paper focuses on extending the threshold minimum state criteria to include the correspondingly high Magnetic Reynolds number influences in MHD applications. [Preview Abstract] |
Monday, November 2, 2009 4:36PM - 4:48PM |
CO6.00014: Thermodynamics, Vertical Structure, and Coronal Power of Optically-Thick MRI-Turbulent Accretion Disks Dmitri Uzdensky Determining the thermal structure of an accretion disk heated by the dissipation of MRI turbulence, and the fraction of accretion power released in the disk corona are two critical problems in plasma astrophysics. In this contribution, these two intertwined problems are considered in the case of a disk threaded by a weak vertical magnetic field. The vertical disk structure is calculated by balancing the local turbulent heating due to the disruption of MRI channel flows by parasitic instabilities, and the cooling by radiative diffusion. It is argued that, neglecting the effects of large-scale MHD disk dynamo, the MRI dissipation rate should be uniform across the disk, almost up to its photosphere. This enables one to obtain a self-consistent solution for the disk's thermal structure. Next, the efficiency of Parker instability, viewed as a secondary parasitic instability feeding off MRI channel flows, is assessed by comparing its growth rate with that of other parasitic instabilities. It is shown that Parker instability becomes important near the disk surface, leading to a certain minimal coronal power fraction. [Preview Abstract] |
Monday, November 2, 2009 4:48PM - 5:00PM |
CO6.00015: Magnetosphere-Ionosphere Coupling through Plasma Turbulence in Electrojet Y.S. Dimant, M.M. Oppenheim Field-aligned currents enter the high latitude E-region ionosphere from the magnetosphere causing cross-field electric fields and currents. During periods of intense geomagnetic activity, these fields induce the formation of strong electrojets, plasma instabilities, and turbulence. This turbulence gives rise to intense anomalous electron heating and nonlinear transport which significantly affects the E-region conductivity. Electrojet conductivities play an important role in the Magnetosphere-Ionosphere system. These conductivities determine the polar-cap potential saturation level and the evolution of field-aligned currents. Quantitative understanding of turbulent conductivities and energy conversion issues is important to accurately model magnetic storms and substorms essential for Space Weather predictions. We will present results of recent theoretical efforts of global energy flow, along with results of 2D and 3D fully kinetic, particle-in-cell, simulations. These simulations reproduce many of the observational characteristics of radar signals and provide information useful in accurately modeling plasma turbulence. They demonstrate the significant progress we have made simulating physical processes in E-region electrojets. [Preview Abstract] |
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