Bulletin of the American Physical Society
54th Annual Meeting of the APS Division of Plasma Physics
Volume 57, Number 12
Monday–Friday, October 29–November 2 2012; Providence, Rhode Island
Session PM9: Mini-Conference: Flux Ropes and 3D Dynamics II |
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Chair: Walter Gekelman, University of California, Los Angeles Room: 553AB |
Wednesday, October 31, 2012 2:00PM - 2:20PM |
PM9.00001: Plasmoid-like structures along the reversal surface in simulations of the MST RFP J.A. Reusch, J.K. Anderson, C.B. Forest, J.S. Sarff, D.D. Schnack Advances in computational power have now made possible nonlinear resistive MHD simulations of the RFP at experimentally relevant parameters. In particular, global magnetic reconnection events known as sawtooth crashes have been simulated at parameters matching those of 400kA discharges in MST (S$\sim $4x10$^{6})$ with the D\textsc{ebs} code. At these parameters, the simulated sawtooth event is not only similar in character, but also in duration to the events observed in MST. This implies that a single fluid MHD model is able to reproduce the dynamics leading to reconnection times that are significantly faster than Sweet-Parker. One possible mechanism for reducing the reconnection time is a plasmoid-like structure that creates multiple X-points along the reversal surface during a sawtooth crash. Such structures have been seen both experimentally [Tharp, et al., PoP (2010)] and in simulation. To explore this effect, several simulations with an artificially truncated m=0 mode spectrum were performed. As the number of allowed toroidal mode harmonics is reduced, the duration of the sawtooth crash increases and the magnitude of the dynamo electric field decreases. Interestingly, while it has long been known that the m=0, n=1 mode is critical to the sawtooth dynamics in MST, without the higher n, m=0 modes the simulations do not produce well defined sawteeth. The effects of limiting the number of m=0 modes on the sawtooth crash and the temporal behavior of the plasmoid like structures will be presented. This work supported by the US DOE and NSF. [Preview Abstract] |
Wednesday, October 31, 2012 2:20PM - 2:40PM |
PM9.00002: 3D Dynamics of Magnetic Flux Ropes Across Scales: Solar Eruptions and Sun-Earth Plasma Coupling James Chen Central to the understanding of the eruptive phenomena on the Sun and their impact on the terrestrial plasma environment is the dynamics of coronal mass ejections (CMEs)---a 3D magnetic flux rope configuration---and the evolution of their magnetic fields. I will discuss the basic physics of CME eruption and solar flare energy release in the context of the analytic erupting flux rope model of CMEs. In this ideal MHD model, a CME is treated as a 3D flux rope with its two stationary footpoints anchored in the Sun. The model structure is non-axisymmetric and embedded in a model corona/solar wind. The initial flux rope is driven out of equilibrium by ``injection'' of poloidal flux and propagates under the Lorentz hoop force from the Sun to 1 AU, across a wide range of spatial and temporal scales. Comparisons of the model results and recent STEREO observations show that the solutions that best fit the observed CME position-time data (to within 1-2\% of data) also correctly replicate the temporal profiles of associated flare X-ray emissions (GOES data) and the {\it in situ} magnetic field and plasma data of the CME ejecta at 1 AU where such data are available (e.g., ACE and STEREO/IMPAXCT/PLASTIC data), providing a unified basis of understanding CME dynamics and flare energetics. [Preview Abstract] |
Wednesday, October 31, 2012 2:40PM - 3:00PM |
PM9.00003: Flux ropes in 3D kinetic reconnection: psontaneous generation and evolution Giovanni Lapenta, Stefano Markidis, Anna Lisa Restante, Tom Intrator We present kinetic simulations with the full 3D electromagnetic code iPic3D on massively parallel computers. We report on state of the art simulations with the largest domains and resolutions afforded by the most advanced petascale computers (Curie in France, available via the PRACE European project and Pleiades in USA, available via the MMS misison). Our results cosndier a portion of the magnetosphere for realistic conditions and show the onset of reconnection forming multiple flux ropes. We follow then their evolution and characterise their behaviour, focusing on regions where the flux ropes kink and coalesce. The analysis tool incldue a novel implementation of the quasi singular layer analysis based on the squashing factor. [Preview Abstract] |
Wednesday, October 31, 2012 3:00PM - 3:20PM |
PM9.00004: Reconnection experiments with flux ropes near 3D magnetic nulls A. Vrublevskis, J. Egedal, A. Le Depending on the topology and geometry of the magnetic field, a rich collection of magnetic reconnection scenarios is possible in 3D including reconnection at magnetic nulls. Nulls have been reported in the solar corona [1] and in Earth's magnetosphere [2], yet there are a limited number of laboratory observations. At the Versatile Toroidal Facility (VTF) we have implemented a new magnetic geometry with a pair of 3D null points in the background toroidal field. In the nominal symmetric configuration a field line connects the nulls. We form a flux rope along this field line and observe the rope rapidly restructuring and rewiring as the nulls develop. A suit of diagnostics will be deployed and results presented for the dynamics of the geometry. \\[1ex] [1] Fletcher et al., Astrophys. J. 554, 451(2001).\\[0ex] [2] Xiao et al., Nat. Phys. 2, 478 (2006). [Preview Abstract] |
Wednesday, October 31, 2012 3:20PM - 3:40PM |
PM9.00005: Flux-Tube Texture of the Solar Wind: Weakly Compressible MHD Theory and Direct Numerical Simulations A. Bhattacharjee, A. Sarkar, F. Ebrahimi Over the years, there has been a steady accumulation of observational evidence that the solar wind may be thought of as a network of individual magnetic flux tubes each with its own magnetic and plasma characteristics [Bartley et al. 1966, Marliani et al. 1973, Tu and Marsch 1990, Bruno et al. 2001, Borovsky 2008]. The weakly compressible MHD (WC-MHD) model [Bhattacharjee et al., 1998], which incorporates the effect of background spatial inhomogeneities, has been used recently to characterize the anisotropic magnetic fluctuation spectra (the so-called variance anisotropy) observed by ACE spacecraft. For a model of local pressure-driven interchange turbulence in a generic solar wind flux tube, the WC-MHD theory uses the Invariance Principle approach [Connor and Taylor 1997, Bhattacharjee and Hameiri 1988] to calculate explicitly the scaling of magnetic field fluctuations with plasma beta and other background plasma parameters. We test these theoretical predictions by direct numerical simulations of interchange turbulence in a flux tube using the DEBS MHD code. Synthetic variance anisotropy within a generic flux tube is computed in the high-Lundquist-number regime, and shows remarkable similarity with ACE observations. [Preview Abstract] |
Wednesday, October 31, 2012 3:40PM - 4:00PM |
PM9.00006: Electron acceleration during multi-island magnetic reconnection James Drake, Michael Swisdak Electron acceleration during the interaction with many magnetic islands or flux ropes is explored with large-scale PIC simulations and an analytic model. During reconnection with a guide field, reconnecting current layers spontaneously break up to form secondary magnetic islands or flux ropes so that reconnection becomes a multi-island phenomenon. Observations in the magnetosphere and in the solar corona support this picture. We recently developed a probabilistic model for the size distribution of magnetic islands which included island growth due to reconnection and island merging and loss. The action invariants of particles circulating in islands can be used to extend this model to include electron acceleration. We obtain an equation for the distribution of electron parallel and perpendicular velocities, which can be evolved simultaneously with the island distribution. The resulting pressure anisotropy feeds back on the island dynamics self-consistently. The solutions of the resultant equations are being compared with PIC simulations of large-scale current layers with many interacting flux ropes. Implications for particle acceleration in flares will be discussed. [Preview Abstract] |
Wednesday, October 31, 2012 4:00PM - 4:20PM |
PM9.00007: Eruption of an arched magnetic flux rope in an ambient magnetoplasma Shreekrishna Tripathi, Walter Gekelman Arched magnetic flux ropes (AMFRs) are arch-shaped, current-carrying, magnetized plasma structures that ubiquitously exist in the solar atmosphere. A laboratory plasma experiment [{\it Tripathi and Gekelman, PRL 105, 075005 (2010)}] has been built to study the eruption of AMFRs in two essential steps: (i) production of an AMFR (n$\sim$ 10$^{19}$ m$^{-3}$, T$_e \sim$14 eV, B$\sim$1 kG, L$\sim$0.5 m) with a persistent appearance lasting several Alfven transit times using a Lanthnum Hexaboride (LaB$_6$) plasma source, and (ii) generation of controlled plasma flows from the foot-points of the AMFR using two laser beams (1064 nm, 1 J/pulse). An additional LaB$_6$ source produces a large magnetoplasma in the background. The laser generated flows drive the eruption by injecting plasma and magnetic flux in the AMFR. The experiment is highly reproducible and runs continuously with a 0.5 Hz repetition rate, hence evolution of the AMFR is recorded using computer-controlled movable probes in 3D. High-speed imaging, Langmuir and 3-axis magnetic-loop probes are the main diagnostic tools. New results from this experiment on global kink-mode oscillations of the AMFR, excitation of fast waves, and ejection of a large magnetic flux rope from the apex of the AMFR will be presented. [Preview Abstract] |
Wednesday, October 31, 2012 4:20PM - 4:40PM |
PM9.00008: Flux Rope Formation from Magnetic and Velocity Shear William Daughton, Yi-Hsin Liu, Takuma Nakamura, Homa Karimabadi, Vadim Roytershteyn Spacecraft observations and simulations both suggest that magnetic islands are commonly associated with the onset and nonlinear development of reconnection. While most theoretical efforts have focused on 2D models, in real 3D systems the islands correspond to flux ropes which can form and interact in a variety of complex ways. The most common explanation is the tearing instability driven by the magnetic shear. In large 3D systems, the spectrum of unstable modes can be much richer due to multiple resonance surfaces, but the details depend strongly on the parameter regime. A distinctly different explanation for generating flux ropes is the Kelvin-Helmholtz instability driven by Alfv\'enic flow shear. For layers above the ion-scale, the vortex leads to wrapping of the field lines and generation of flux ropes comparable to the size of the vortex. This is in sharp contrast to flux ropes formed from the tearing instability which start on small scales and grow in time to reach larger sizes. Here, we compare and contrast these two mechanisms using 3D fully kinetic simulations for configurations involving various combinations of magnetic and velocity shear. Characteristic properties of the flux ropes, fluctuation spectra and influence on particle acceleration will be discussed. [Preview Abstract] |
Wednesday, October 31, 2012 4:40PM - 5:00PM |
PM9.00009: Alfvenic turbulence in the solar wind and plasma experiments Andrey Beresnyak If perturbations are perpendicular to the strong mean magnetic field and are anisotropic, they could be fairly well described by so-called reduced MHD -- a fluid-like description of plasma which, nevertheless, works very well for collisionless plasmas. A physical interpretation of this is that Alfvenic perturbations rely on magnetic tension as a restoring force and it is sufficient that charged particles be tied to magnetic field lines to provide inertia. RMHD is applicable to the solar wind and laboratory experiments with strong mean field. In particular, experiments that show potential motions when amplitude is small and turbulent or reconnection-like behavior when amplitude is larger, could be very well described as weak and strong Alfvenic turbulence correspondingly. Recently, there has been a big progress in understanding Alfvenic turbulence, both weak and strong, balanced and imbalanced. I will point out some consequences of these theories for the solar wind and plasma experiments. [Preview Abstract] |
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