Center for Integrated Computation and Analysis of Reconnection and Turbulence

Sun City Center, United States

Center for Integrated Computation and Analysis of Reconnection and Turbulence

Sun City Center, United States

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Bigot B.,University of New Hampshire | Bigot B.,Center for Integrated Computation and Analysis of Reconnection and Turbulence | Galtier S.,University Paris - Sud | Galtier S.,Institut Universitaire de France
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2011

The dynamics of the two-dimensional (2D) state in driven three-dimensional (3D) incompressible magnetohydrodynamic turbulence is investigated through high-resolution direct numerical simulations and in the presence of an external magnetic field at various intensities. For such a flow the 2D state (or slow mode) and the 3D modes correspond, respectively, to spectral fluctuations in the plane k∥=0 and in the area k∥>0. It is shown that if initially the 2D state is set to zero it becomes nonnegligible in few turnover times, particularly when the external magnetic field is strong. The maintenance of a large-scale driving leads to a break for the energy spectra of 3D modes; when the driving is stopped, the previous break is removed and a decay phase emerges with Alfvénic fluctuations. For a strong external magnetic field the energy at large perpendicular scales lies mainly in the 2D state, and in all situations a pinning effect is observed at small scales. © 2011 American Physical Society.


Huang Y.-M.,University of New Hampshire | Huang Y.-M.,Center for Integrated Computation and Analysis of Reconnection and Turbulence | Huang Y.-M.,Center for Magnetic Self Organization in Laboratory and Astrophysical Plasmas | Bhattacharjee A.,University of New Hampshire | And 3 more authors.
Astrophysical Journal Letters | Year: 2010

The recent demonstration of current singularity formation by Low etal. assumes that potential fields will remain potential under simple expansion or compression. An explicit counterexample to their key assumption is constructed. Our findings suggest that their results may need to be reconsidered. © 2009 The American Astronomical Society.


Huang Y.-M.,Princeton Plasma Physics Laboratory | Huang Y.-M.,Max Planck Princeton Center for Plasma Physics | Huang Y.-M.,Princeton Center for Heliospheric Physics | Huang Y.-M.,Center for Magnetic Self Organization in Laboratory and Astrophysical Plasmas | And 8 more authors.
Astrophysical Journal | Year: 2014

Magnetic fields without a direction of continuous symmetry have the generic feature that neighboring field lines exponentiate away from each other and become stochastic, and hence the ideal constraint of preserving magnetic field line connectivity becomes exponentially sensitive to small deviations from ideal Ohm's law. The idea of breaking field line connectivity by stochasticity as a mechanism for fast reconnection is tested with numerical simulations based on reduced magnetohydrodynamics equations with a strong guide field line-tied to two perfectly conducting end plates. Starting from an ideally stable force-free equilibrium, the system is allowed to undergo resistive relaxation. Two distinct phases are found in the process of resistive relaxation. During the quasi-static phase, rapid change of field line connectivity and strong induced flow are found in regions of high field line exponentiation. However, although the field line connectivity of individual field lines can change rapidly, the overall pattern of field line mapping appears to deform gradually. From this perspective, field line exponentiation appears to cause enhanced diffusion rather than reconnection. In some cases, resistive quasi-static evolution can cause the ideally stable initial equilibrium to cross a stability threshold, leading to formation of intense current filaments and rapid change of field line mapping into a qualitatively different pattern. It is in this onset phase that the change of field line connectivity is more appropriately designated as magnetic reconnection. Our results show that rapid change of field line connectivity appears to be a necessary, but not a sufficient condition for fast reconnection. © 2014. The American Astronomical Society. All rights reserved..


Lecoanet D.,University of Wisconsin - Madison | Zweibel E.G.,University of Wisconsin - Madison | Townsend R.H.D.,University of Wisconsin - Madison | Huang Y.-M.,University of New Hampshire | And 2 more authors.
Astrophysical Journal | Year: 2010

Shear flow instabilities can profoundly affect the diffusion of momentum in jets, stars, and disks. The Richardson criterion gives a sufficient condition for instability of a shear flow in a stratified medium. The velocity gradient V′ can only destabilize a stably stratified medium with squared Brunt-Väisälä frequency N 2 if V′2/ 4>N 2. We find this is no longer true when the medium is a magnetized plasma. We investigate the effect of stable stratification on the magnetic field and velocity profiles unstable to magneto-shear instabilities, i.e., instabilities which require the presence of both magnetic field and shear flow. We show that a family of profiles originally studied by Tatsuno & Dorland remains unstable even when V′2/4 < N 2, violating the Richardson criterion. However, not all magnetic fields can result in a violation of the Richardson criterion. We consider a class of flows originally considered by Kent, which are destabilized by a constant magnetic field, and show that they become stable when V′2/4 < N 2, as predicted by the Richardson criterion. This suggests that magnetic free energy is required to violate the Richardson criterion. This work implies that the Richardson criterion cannot be used when evaluating the ideal stability of a sheared, stably stratified, and magnetized plasma. We briefly discuss the implications for astrophysical systems. © 2010. The American Astronomical Society. All rights reserved..


Huang Y.-M.,Center for Integrated Computation and Analysis of Reconnection and Turbulence | Huang Y.-M.,University of Wisconsin - Madison | Huang Y.-M.,University of New Hampshire | Bhattacharjee A.,Center for Integrated Computation and Analysis of Reconnection and Turbulence | And 2 more authors.
Physics of Plasmas | Year: 2013

Our understanding of magnetic reconnection in resistive magnetohydrodynamics has gone through a fundamental change in recent years. The conventional wisdom is that magnetic reconnection mediated by resistivity is slow in laminar high Lundquist (S) plasmas, constrained by the scaling of the reconnection rate predicted by Sweet-Parker theory. However, recent studies have shown that when S exceeds a critical value ∼ 104, the Sweet-Parker current sheet is unstable to a super-Alfvénic plasmoid instability, with a linear growth rate that scales as S 1 / 4. In the fully developed statistical steady state of two-dimensional resistive magnetohydrodynamic simulations, the normalized average reconnection rate is approximately 0.01, nearly independent of S, and the distribution function f (ψ) of plasmoid magnetic flux ψ follows a power law f (ψ) ∼ ψ-1. When Hall effects are included, the plasmoid instability may trigger onset of Hall reconnection even when the conventional criterion for onset is not satisfied. The rich variety of possible reconnection dynamics is organized in the framework of a phase diagram. © 2013 AIP Publishing LLC.

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