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 TM9: Mini-Conference: Flux Ropes and 3D Dynamics III |
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Chair: Paul Bellan, California Institute of Technology Room: 553AB |
Thursday, November 1, 2012 9:30AM - 9:50AM |
TM9.00001: Experimental observation of three-dimensional, impulsive reconnection events and associated signatures of spontaneously generated flux ropes S. Dorfman, H. Ji, M. Yamada, J. Yoo, E. Lawrence, C. Myers, T.D. Tharp Fast, impulsive reconnection is commonly observed in laboratory, space, and astrophysical plasmas. Many existing models of reconnection attempt to explain this behaviour without including variation in the third direction. However, the impulsive reconnection events observed on the Magnetic Reconnection Experiment (MRX) are characterized by large local gradients in the third direction and cannot be explained by 2-D models. Signatures of flux rope dynamics in the two-fluid regime are identified and found to play a key role. These ``flux ropes'' are spontaneously generated in the layer and observed down to the smallest scales resolved by the diagnostics. The observed drop in the reconnection current and spike in the reconnection rate during the impulsive reconnection event are due to ejection of these structures from the layer. By contrast, even though electromagnetic fluctuations in the Lower Hybrid frequency range consistent with [1] are concurrently observed [2], they are not the key physics responsible. A qualitative, 3-D, two-fluid model consistent with [3] is proposed to explain the observations.\\[4pt] [1] H. Ji et al., Phys. Rev. Lett. 92, 115001 (2004).\\[0pt] [2] M. Yamada, Phys. Plasmas 18, 111212 (2011).\\[0pt] [3] J.D. Huba and L.I. Rudakov, Phys. Plasmas 10, 3139 (2003). [Preview Abstract] |
Thursday, November 1, 2012 9:50AM - 10:10AM |
TM9.00002: Emergence and Eruption of Magnetic Flux Ropes, a Mechanism for Coronal Mass Ejections Ward Manchester We present the results of simulations of flux ropes buoyantly emerging from the convection zone into the corona and examine shear flows that spontaneously occur during the emergence process. Shear flows have been prescribed in numerical models of coronal mass ejections and flares for decades as a way of energizing magnetic fields to erupt. While such shear flows have long been observed in the solar atmosphere, until recently, there was no compelling physical explanation for them. We will discuss the discovery that such shear flows are readily explained as a response to the Lorentz force that naturally occurs as bipolar magnetic fields emerge and expand in a gravitationally stratified atmosphere. It will be shown that shearing motions transport axial flux, and magnetic energy from the submerged portion of the field to the expanding portion, strongly coupling the solar interior to the corona. This physical process explains active region shear flows and why the magnetic field is found to be nearly parallel to photospheric polarity inversion lines where prominences form. Finally, shear flows driven by the Lorentz force are shown to produce a loss of equilibrium and eruption in magnetic arcades and flux ropes offering a convincing explanation for CMEs and flares. [Preview Abstract] |
Thursday, November 1, 2012 10:10AM - 10:30AM |
TM9.00003: Plasmoid dynamics in reconnection turbulence M.J. Pueschel, P.W. Terry, F. Jenko, E. Zweibel, V. Zhdankin, D. Told Circular structures of the parallel current, hereafter referred to as plasmoids, are found to form, merge, and dissipate recurringly in gyrokinetic simulations of driven reconnection turbulence in a strong guide field. A Krook term is used to force the system via a two-dimensional reconnecting current sheet, and the energy thus injected is partly transferred into the plasmoids. The plasmoids, which can be thought of as flux ropes in a guide field scenario, are attracting or repelling each other, depending on the signs of the associated currents; this mechanism, which highlights the importance of temporal effects and intermittency in reconnection setups, is described quantitatively. In this context, dependencies on physical parameters (such as the plasma pressure $\beta$ or the ion temperature $T_i$) are compared with linear reconnection physics in current sheets. Another important aspect of plasmoid dynamics is the occurrence of mergers. Such events underscore the need for (gyro)-kinetic descriptions, since it causes the parallel velocity space to undergo a significant departure from the usual Maxwellian distribution. [Preview Abstract] |
Thursday, November 1, 2012 10:30AM - 10:50AM |
TM9.00004: Magnetic field generation from shear flow in flux ropes T.P. Intrator, J. Sears, K. Gao, J. Klarenbeek, C. Yoo In the Reconnection Scaling Experiment (RSX) we have measured out of plane quadrupole magnetic field structure in situations where magnetic reconnection was minimal. This quadrupole out of plane magnetic signature has historically been presumed to be the smoking gun harbinger of reconnection. On the other hand, we showed that when flux ropes bounced instead of merging and reconnecting, this signature could evolve. This can follow from sheared fluid flows in the context of a generalized Ohms Law. We reconstruct a shear flow model from experimental data for flux ropes that have been experimentally well characterized in RSX as screw pinch equilibria, including plasma ion and electron flow, with self consistent profiles for magnetic field, pressure, and current density. The data can account for the quadrupole field structure. [Preview Abstract] |
Thursday, November 1, 2012 10:50AM - 11:10AM |
TM9.00005: MHD Simulations of the Eruption of Coronal Flux Ropes Yuhong Fan I present MHD simulations of the eruption of coronal flux ropes and the initiation of coronal mass ejections (CMEs). It is found that the eruption of the flux rope in the corona is triggered when the flux rope rises to a critical height where the corresponding potential field declines with height at a sufficiently steep rate, a mechanism consistent with the onset of the torus instability. The simulations show that S (or inverse S) shaped current sheets develop along topological structures identified as Quasi Separatrix Layers (QSLs), during the quasi-static phase before the eruption. Reconnections in the current sheets effectively add twisted flux to the flux rope, allowing it to rise quasi-statically to the critical height and then the dynamic eruption ensues. It is found that the resulting coronal magnetic field evolution can explain some of the commonly observed features associated CMEs and their pre-cursor structures. Finally I also show an MHD simulation that models qualitatively the magnetic field evolution of the eruptive flare occurred on December 13, 2006 in the emerging $\delta$ sunspot region NOAA 10930 observed by the Hinode satellite. [Preview Abstract] |
Thursday, November 1, 2012 11:10AM - 11:30AM |
TM9.00006: Steady-State Merger of Flux Ropes and the Role of Counter-Current Ropes on Stability Brookhart Matthew, Carlos Paz-Soldan, Andrew Eckhart, Cary Forest The Line-tied Reconnection Experiment, a linear screw pinch with line-tied axial boundaries, creates plasma via a hexagonally-packed array of nineteen electrostatic current injectors (washer guns). These guns inject individually controlled current and density filaments, called flux ropes. When firing the central seven guns, the individual flux ropes merge without time dependence. This merger, consistent with particle diffusion and magnetic shear, does not necessarily invoke magnetic reconnection. Hollow current profiles are explored and shown to be unstable at high safety factor ($>$10). The injection of opposing current at the center of the device stabilizes the hollow current plasmas and may provide more rigorous line tying. Plasmas with no net current are explored. Finally, recent improvements to the device, along with plans for simultaneous detailed measurements of flux rope merger and stability, are presented. [Preview Abstract] |
Thursday, November 1, 2012 11:30AM - 11:50AM |
TM9.00007: Flux Rope Formation in Stratified Convection Robert Stein, Aake Nordlund Numerical simulations of solar surface convection have been performed for a domain 48 Mm wide by 20 Mm deep. This depth is 10\% of the depth of the convection zone, but 2/3 its scale heights. Weak, uniform, untwisted, horizontal magnetic field was advected into the computational domain by inflows at the bottom. The magneto-convection produces a hierarchy of magnetic loops. In general, magnetic field is pumped down and the Poynting flux is downward. However, some flux ropes emerge through the surface and form small active regions. The field initially appears at the surface with mixed polarities in a small subregion of the entire domain. The opposite polarities separate into unipolar intense, nearly vertical ``flux tubes'' which produce large pores. This active region has dimensions comparable to the scale of the supergranulation near the bottom of the domain and the orientation of the incoming advected field. Penumbra do not form because of the nearby upper potential field boundary condition on the field. [Preview Abstract] |
Thursday, November 1, 2012 11:50AM - 12:10PM |
TM9.00008: Morphology and dynamics of three interacting flux ropes Bart Van Compernolle, Walter Gekelman The interaction of three flux ropes is studied in a laboratory magnetoplasma (n$_{background}$ = 2 10$^{12}$ cm$^{-3}$, n$_{flux rope}$ = 5 10$^{12}$ cm$^{-3}$, Helium, B$_{0z}$ = 330 G, plasma diameter = 60 cm). The ropes are made using a 8 cm diameter Lanthanum Hexaboride (LaB$_6$) cathode and remote anode, 11 m away. Each rope carries 30 A of current and produces magnetic fields on the order of a few \% of $B_{0z}$. Volumetric magnetic field data was acquired and the magnetic field structure and dynamics of the flux ropes can thus be reconstructed. The flux ropes are found to propagate at the Alfv\'en speed. Merging and bouncing of the flux ropes has been observed. The ropes twist and writhe as they propagate through the plasma. They are line tied and clearly separate at the cathode end but further away they merge into one extended rope. The steady state of the flux ropes is characterized by a rotation of the three flux ropes as a whole. At the same time the flux ropes are twisting around each other. Time resolved density and temperature measurements from Langmuir probe data show increases by a factor of two in local electron temperature and electron density due to the presence of the ropes. These local increases in density and temperature are associated with the location [Preview Abstract] |
Thursday, November 1, 2012 12:10PM - 12:22PM |
TM9.00009: Computer Simulations and Observations of Solar Magnetic Flux Ropes and Eruptions Alexander Kosovichev, Irina Kitiashvili, Nagi Mansour, Alan Wray Solar observations reveal a great variety of plasma eruptions of different scales and energetics, from small-scale jets and spicules to giant coronal mass ejections. The magnetic flux ropes observed in the solar atmosphere and corona are formed in the turbulent convection zone by a dynamo process, which is poorly understood. We present 3D radiative MHD simulations of the upper convective boundary layer and the chromosphere, which capture the basic physics of magnetic self-organization and dynamics of the turbulent solar plasma, formation of magnetic flux ropes and spontaneous small-scale spicule-like eruptions. The simulations show that the key mechanism of the flux-rope formation and eruptions is in the generation of compact vortex tubes and their interaction with magnetic field. The simulation results are compared with observations from the Solar Dynamics Observatory and large ground-based telescopes. [Preview Abstract] |
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