Physics Highlights, 2010
Synopsis: Learning the ropes
December 22, 2010
Credit: Courtesy Tom Intrator, Los Alamos National Laboratory
posted by Tom Intrator

In ionized environments such as the surface of the sun, magnetic field lines can become trapped in moving columns of plasma. These flux ropes can exhibit complex dynamics, and may attract or repel each other depending on the direction of current flow in the rope. In simulations and calculations, it is often assumed that if two flux ropes are attracted to one another by the Lorentz force, they will merge and annihilate. Writing in Physical Review Letters, Xuan Sun and colleagues at Los Alamos National Laboratory, US, report experimental evidence that mutually attracting flux ropes can actually bounce off each other instead of merging.

The authors studied this phenomenon at the Los Alamos Reconnection Scaling Experiment. The group created a pair of flux ropes inside the device with plasma guns and allowed the pair to evolve in three dimensions. They used a magnetic probe to map out the field strength and plasma parameters throughout the volume where the ropes were interacting. For some conditions the ropes merged as expected, but in other cases, Sun et al. observed ropes that began to merge but then separated. The Los Alamos team reports that these observations agree with a theoretical model of flux rope interaction and may have implications for understanding turbulence and energy production in astrophysical events like solar flares.

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Experimental detection of relaxation to a helical state
July 21, 2010
Left: Theoretical calculation of the helical equilibrium state predicted for two merged spheromaks with large total helicity. Right: The top row shows experimentally measured values of the azimuthal field at three axial positions; the bottom row shows the field for a Taylor state. Solid contours are positive, negative are dashed. From Cothran et al. 2010.
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One of the earliest predictions of a magnetic self-organization phenomenon was J.B. Taylor's hypothesis that magnetically confined plasmas would relax to the lowest energy state subject to the constraint that their magnetic helicity be conserved. For systems with low helicity, the relaxed state is axisymmetric, while at large helicity, the relaxed state is helically symmetric, or kinked. Taylor's relaxation hypothesis has been confirmed in the laboratory for axisymmetric states; the spontaneous toroidal field reversal central to MST and other Reversed Field Pinch experiments is the most prominent example. On the strength of this evidence, Taylor's hypothesis has been used to explain solar flares, coronal heating, and even the morphologies of extragalactic radio jets. However, it remains a hypothesis, with substantial theoretical support, but no rigorous proof. The long predicted helical state, which is expected for relaxation at large helicity, has now been observed in SSX. In the experiment, two compact, toroidal, axisymmetric spheromak plasmas with tunable magnetic helicity are injected at either end of a cylindrical volume. After a brief dynamical phase, the plasmas merge and achieve an equilibrium state. When the combined helicity exceeds the predicted threshold, this equilibrium state is observed to be helical, as shown below.

This work was published as a PRL and selected by AIP in its electronic Physics: Spotlighting Exceptional Research: http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.103.215002 It is the first experimental description of non-axisymmetric magnetic self-organization in a compact toroidal plasma. It demonstrates the explanatory power of self-organization, may inspire further theoretical development, and should enhance confidence in the application of Taylor's hypothesis to space and astrophysical plasmas.

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Theory and simulation of imbalanced turbulence
July 21, 2010
Numerical simulation of globally balanced MHD turbulence in a large aspect ratio domain with long axis parallel to the background magnetic field. Red and blue regions correspond to streams of nonlinear Alfven waves propagating along the background field in opposite directions. The top panel shows large regions in which one sign of propagation dominates. In the bottom panel the large scales have been filtered out, revealing fine scale streams. From S. Boldyrev & J. Perez.
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In many space and astrophysical environments, a single source of turbulent energy predominates. This leads to so-called imbalanced MHD turbulence, in which the constituent disturbances move in a prevailing direction.; the solar wind is a particularly well-studied example. Moreover, even globally balanced turbulence spontaneously generates regions where the energies of Alfven waves propagating in opposite directions along the background magnetic field are unequal. Since the development of an MHD turbulent cascade is based on the interaction of waves propagating in opposite directions, it is not obvious that the theory developed for balanced turbulence is applicable to the imbalanced case. This problem is of great interest, and was the subject of a CMSO focused workshop in April 2010.

Direct numerical simulations of MHD turbulence supported by analytical theory, reveal that the scaling of the spectra of the counter-propagating Alfven modes do not differ from the balanced case. However, their amplitudes and the corresponding rates of energy cascades are significantly affected by the imbalance. It was also found that a large scale, strongly interacting magnetic field - dubbed a condensate - can form in the presence of imbalanced turbulence, and fundamentally changes its dynamics.

Experimental Observation of Anisotropic Magnetic Turbulence in a Reversed Field Pinch
July 21, 2010
Power spectra for the magnetic turbulence measured at several radii in the edge of MST reversed field pinch plasmas. The background magnetic field is strongly sheared, and the turbulence maintains anisotropy locally as the field line orientation changes. From Y. Ren, A.F. Almagri, G. Fiksel, S.C. Prager, J.S. Sarff, and P.W. Terry.
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In contrast to hydrodynamic turbulence, which is generally expected to be spatially isotropic, MHD turbulent structures are expected to be highly elongated along the background magnetic field. Anisotropy has been measured in solar wind turbulence, and is inferred from observations of pulsar scintillation.
Fully developed magnetic turbulence with this strong anisotropy has now been measured in the reversed field pinch laboratory plasma device MST. The observed magnetic turbulence has much larger kperp than kpar with respect to the background magnetic field, a feature that is consistent with Alfvenic turbulence. Furthermore, a linkage between large-scale tearing modes that act as a stirring mechanism and the small-scale magnetic fluctuations has been demonstrated through the variation of the tearing mode amplitudes. This nonlinear cascade has both power-law and exponential-law spectral features, which are consistent with inertial and dissipation ranges, respectively, in MHD turbulence theories. It is possible that the dissipation associated with the observed exponential decrease in the small-scale fluctuations is related to the strong collisionless ion heating observed in these plasmas.

These results may help improve our understanding of how anisotropic magnetic turbulent cascades form and evolve in astrophysical plasmas, and how they can heat background particles.

Structure of the electron dissipation region in magnetic reconnection
July 21, 2010
Top 3 panels (a-c): Experimental example taken from a hydrogen plasma with a fill pressure of 2 mTorr. Bottom panels (d-f). Results from a corresponding simulation shown in the same format as the experiment.
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Magnetic reconnection, the topological rearrangement of magnetic field lines in highly conducting plasma, comes about through microscopic processes which break the field lines on the electron dissipation scale and macroscopic dynamical processes which couple the local and global scales. Understanding the physics of the microscopic electron dissipation layer is critical to understanding magnetic reconnection and how it energizes the ambient plasma.
Detailed comparisons have now been made between laboratory observations of electron-scale dissipation layers near a reconnecting X-line in the Magnetic Reconnection Experiment (MRX) and direct two-dimensional full-particle simulations using realistic Coulomb collisions and boundary conditions relevant to the MRX. Many experimental features of the electron layers, such as insensitivity to the ion mass, are reproduced by the simulations.
All ion scale features are successfully reproduced. However, the electron layer thickness, is 3-5 times larger than the predictions. In view of the excellent agreement found for other quantities, this is a serious discrepancy.
No known 2D mechanism, including the one based on collisionless electron nongyrotropic pressure, is sufficient to explain the observed reconnection rates. These results suggest that 3D effects play an important role in electron-scale dissipation during fast reconnection. Currently, this subject is under further investigation in MRX.

Characterization of interstellar turbulence
July 21, 2010
The ''Big Power Law in the Sky'' compiled originally by Armstrong, Rickett, & Spangler, with the WHAM measurements shown in the upper left. From A. Chepurnov and A. Lazarian
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Observations of the density and velocity structure of the interstellar medium have long suggested that it is turbulent. Characterizing this turbulence presents some unique challenges, as the observations are often averaged along the line of sight and in the plane of the sky, and the gas may be optically thick. Because turbulence has important effects on the structure and dynamics of the interstellar medium, on the dynamo, which sustains the galactic magnetic field, and on the acceleration and propagation of cosmic rays, characterization of interstellar turbulence is of great importance.
Some time ago, a variety of different measurements were combined to argue that interstellar turbulence has a Kolmogorov spectrum. Recent research in CMSO applying novel analysis tools to Ha observations obtained at Wisconsin with the NSF supported WHAM instrument supports this picture.