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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. Source


Kaplan E.J.,University of Wisconsin - Madison | Kaplan E.J.,Center for Magnetic Self Organization in Laboratory and Astrophysical Plasmas | Clark M.M.,University of Wisconsin - Madison | Clark M.M.,Center for Magnetic Self Organization in Laboratory and Astrophysical Plasmas | And 12 more authors.
Physical Review Letters | Year: 2011

Three-wave turbulent interactions and the role of eddy size on the turbulent electromotive force are studied in a spherical liquid-sodium dynamo experiment. A symmetric, equatorial baffle reduces the amplitude of the largest-scale turbulent eddies, which is inferred from the magnetic fluctuations spectrum (measured by a 2D array of surface probes). Differential rotation in the mean flow is >2 times more effective in generating mean toroidal magnetic fields from the applied poloidal field (via the Ω effect) when the largest-scale eddies are eliminated, thus demonstrating that the global turbulent resistivity (the β effect from the largest-scale eddies) is reduced by a similar amount. © 2011 American Physical Society. Source


Huang Y.-M.,Princeton Plasma Physics Laboratory | Huang Y.-M.,Princeton Center for Heliophysics | Huang Y.-M.,Max Planck Princeton Center for Plasma Physics | Huang Y.-M.,Center for Magnetic Self Organization in Laboratory and Astrophysical Plasmas | And 4 more authors.
Astrophysical Journal | Year: 2016

It has been established that the Sweet-Parker current layer in high Lundquist number reconnection is unstable to the super-Alfvénic plasmoid instability. Past two-dimensional magnetohydrodynamic simulations have demonstrated that the plasmoid instability leads to a new regime where the Sweet-Parker current layer changes into a chain of plasmoids connected by secondary current sheets, and the averaged reconnection rate becomes nearly independent of the Lundquist number. In this work, a three-dimensional simulation with a guide field shows that the additional degree of freedom allows plasmoid instabilities to grow at oblique angles, which interact and lead to self-generated turbulent reconnection. The averaged reconnection rate in the self-generated turbulent state is of the order of a hundredth of the characteristic Alfvén speed, which is similar to the two-dimensional result but is an order of magnitude lower than the fastest reconnection rate reported in recent studies of externally driven three-dimensional turbulent reconnection. Kinematic and magnetic energy fluctuations both form elongated eddies along the direction of the local magnetic field, which is a signature of anisotropic magnetohydrodynamic turbulence. Both energy fluctuations satisfy power-law spectra in the inertial range, where the magnetic energy spectral index is in the range from -2.3 to -2.1, while the kinetic energy spectral index is slightly steeper, in the range from -2.5 to -2.3. The anisotropy of turbulence eddies is found to be nearly scale-independent, in contrast with the prediction of the Goldreich-Sridhar theory for anisotropic turbulence in a homogeneous plasma permeated by a uniform magnetic field. © 2016. The American Astronomical Society. All rights reserved. Source


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.. Source


Magee R.M.,University of Wisconsin - Madison | Den Hartog D.J.,University of Wisconsin - Madison | Den Hartog D.J.,Center for Magnetic Self Organization in Laboratory and Astrophysical Plasmas | Kumar S.T.A.,University of Wisconsin - Madison | And 11 more authors.
Physical Review Letters | Year: 2011

Complementary measurements of ion energy distributions in a magnetically confined high-temperature plasma show that magnetic reconnection results in both anisotropic ion heating and the generation of suprathermal ions. The anisotropy, observed in the C+6 impurity ions, is such that the temperature perpendicular to the magnetic field is larger than the temperature parallel to the magnetic field. The suprathermal tail appears in the majority ion distribution and is well described by a power law to energies 10 times the thermal energy. These observations may offer insight into the energization process. © 2011 American Physical Society. Source

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