Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session LZ2: Mini-conference on Dynamics of Magnetic Flux Tubes in Space and Laboratory Plasmas II |
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Chair: Mark Linton, Naval Research Laboratory Room: Adam's Mark Hotel Governor's Square 14 |
Wednesday, October 26, 2005 2:00PM - 2:30PM |
LZ2.00001: Flux Tubes in the Earth's Magnetotail Michael Hesse Magnetic flux ropes are a ubiquitous feature of the dynamics of the nightside current sheet in the Earth's magnetotail. Satellite observations indicate that magnetotail flux ropes can, depending on circumstances, propagate in either the sunward or anti-sunward direction. Furthermore, flux ropes often occur in multiples, indicating that their creation process may either operate quasi-periodically, or that it may create more than one flux rope at a time. The creation mechanism itself, however, remains a subject of current scientific debate. Proposed mechanisms range from flux rope formation by large-scale reconnection and subsequent interactions with the surrounding medium, to kinetic processes that are based either on Hall-electric field effects or multiple island formation by tearing of thin current sheets. This presentation will provide an overview of magnetotail flux rope observations with an emphasis on distilling common flux rope properties. In addition, discuss flux rope formation mechanisms will be evaluated in light of the observational evidence. [Preview Abstract] |
Wednesday, October 26, 2005 2:30PM - 2:45PM |
LZ2.00002: Current Sheet Formation and Parker Instability in Line-Tied Flux Tubes A. Bhattacharjee, K. Germaschewski, C.S. Ng, P. Zhu The dynamics of line-tied flux tubes is of great relevance to the Sun's convection zone and the corona, the Earth's magnetosphere, and the edge regions of tokamak plasmas. In this talk, we will focus on two aspects of line-tied flux tubes: the formation of current sheets and the stability of flux tubes with respect to the Parker instability (akin to the ballooning instability). In the first application, we demonstrate that for a fixed footpoint mapping between two perfectly conducting surfaces to which magnetic field lines are tied, there is at most one smooth equilibrium. This property implies that if the flux tube is driven by smooth footpoint motions to a smooth but unstable equilibrium, the tube must relax to an equilibrium with a current sheet (a tangential discontinuity). In the second application, we present direct numerical simulations of the line-tied Parker instability, based on the fully compressible ideal MHD equations. In the intermediate nonlinear phase, the instability continues to grow exponentially in time and the plasma tends to develop convection-induced discontinuities in the form of shock-like coherent structures. The system does not appear to ``detonate,'' predicted by recent analytical theories. Implications of these results for heating, and eruptive/disruptive behavior of space and laboratory plasmas will be discussed. [Preview Abstract] |
Wednesday, October 26, 2005 2:45PM - 3:15PM |
LZ2.00003: Toroidal flux tube reconnection experiments at SSX Michael Brown We report experimental results from toroidal flux tube reconnection measurements at the Swarthmore Spheromak Experiment (SSX). Coaxial magnetized plasma guns are used to generate toroidal flux tubes (ie spheromaks) of either sense of magnetic helicity (right or left- handed twist). Arrays of up to 600 magnetic probes are used map the dynamics of the flux tube merging. Several results will be presented. First, high spatial resolution three-dimensional arrays show that locally, flux tubes with opposite helicity (RL or LR) merge rapidly while flux tubes with the same helicity (RR or LL) merge less rapidly. Second, distributed arrays with coarser resolution show that opposite helicity flux tube merging (after a fast reconnection event) ultimately generates a single, large scale structure with opposite twist at the ends. Same helicity merging generates a single, large scale twisted spheromak without reconnection. Finally, reconnection events are correlated with bursts of nearly Alfv\'enic flow which we measure spectroscopically. [Preview Abstract] |
Wednesday, October 26, 2005 3:15PM - 3:45PM |
LZ2.00004: Experiments and simulations of flux rope dynamics in a plasma Thomas Intrator, Ivo Furno, Tsitsi Madziwa-Nussinov, Giovanni Lapenta, Adam Light, Sara Abbate, Dmitri Ryutov The behavior of flux ropes is a key issue in solar, space and astrophysics. For instance, magnetic fields and currents on the Sun are sheared and twisted as they store energy, experience an as yet unidentified instability, open into interplanetary space, eject the plasma trapped in them, and cause a flare. The Reconnection Scaling Experiment (RSX) provides a simple means to systematically characterize the linear and non-linear evolution of driven, dissipative, unstable plasma-current filaments. Topology evolves in three dimensions, supports multiple modes, and can bifurcate to quasi-helical equilibria. The ultimate saturation to a nonlinear force and energy balance is the link to a {\em spectrum of relaxation processes}. RSX has adjustable energy density $\beta \ll 1$ to $\beta \approx 1$, non-negligible equilibrium plasma flows, driven steady-state scenarios, and adjustable line tying at boundaries. We will show magnetic structure of a kinking, rotating single line tied column, magnetic reconnection between two flux ropes, and pictures of three braided flux ropes. We use computed simulation movies to bridge the gap between the solar physics scales and experimental data with computational modeling. In collaboration with Ivo Furno, Tsitsi Madziwa-Nussinovm Giovanni Lapenta, Adam Light, Los Alamos National Laboratory; Sara Abbate, Torino Polytecnico; and Dmitri Ryutov, Lawrence Livermore National Laboratory. [Preview Abstract] |
Wednesday, October 26, 2005 3:45PM - 4:00PM |
LZ2.00005: Simulation of Flux Ropes in Astrophysical Jets, Solar Corona and Laboratory Giovanni Lapenta, D. Knoll, I. Furno, T. Intrator, P. Kronberg We report on our simulation effort based on understanding the behaviour of flux ropes of interest to astrophysical and laboratory plasmas. We address three fundamental issues. First, what model of flux ropes is most appropriate in specific conditions? We focus here on the astrophysical jets created in active galactic nuclei and discuss models that can represent observed features [1]. Second, what leads to the formation of flux ropes? We consider specifically the creation of flux ropes in solar coronal processes involving the presence of bulk flows leading to localized compression and reconnection [2]. Third, how do flux ropes evolve? We focus here on comparing our simulations with observed solar coronal processes (namely the formation of blobs at the cusp of helmet streamers and their ejection into the solar wind) and with laboratory experiments conducted on RSX [3].\newline [1] G. Lapenta, P.P. Kronberg, ApJ, 625, 37, 2005.\newline [2] G. Lapenta, D.A. Knoll, Solar Phys., 214, 107, 2003; ApJ, 624, 1049, 2005.\newline [3] I. Furno, et al., Phys. Plasmas, 12, 055702, 2005. [Preview Abstract] |
Wednesday, October 26, 2005 4:00PM - 4:15PM |
LZ2.00006: Phenomenological theory of the kink instability in a slender plasma column D.D. Ryutov, I. Furno, T.P. Intrator, S. Abbate, T. Madziwa-Nussinov When one deals with a plasma column whose radius a is much smaller than its length L, one can think of it as of a thin filament whose kink instability can be adequately described simply by a 2D displacement vector, x=x(z,t); y=y(z,t). Details of the internal structure of the column such as the current, density, and axial flow velocity distribution would be lumped into some phenomenological parameters. This approach is particularly efficient in the problems with non-ideal (sheath) boundary conditions at the end electrodes, the finite plasma resistivity, and the case of a substantial axial flow. For the non-ideal situation, we find instability in the domain well below the classical Kruskal-Shafranov limit. The presence of an axial flow causes the onset of a rotation of the kink and strong axial ``skewness'' of the eigenfunction. We consider the limitations of the phenomenological approach and find that they are related to the steepness with which the plasma resistivity increases at the plasma boundary with vacuum. Work performed for US DOE by UC LLNL under contract {\#}W-7405-Eng-48. [Preview Abstract] |
Wednesday, October 26, 2005 4:15PM - 4:30PM |
LZ2.00007: Rotating kink modes in a non-line tied plasma column in the Reconnection Scaling experiment Tsitsi Madziwa-Nussinov, Ivo Furno, Thomas Intrator, Adam Light, Dmitri Ryutov, Sara Abbate The screw pinch is one of the simplest MHD equilibria, and is relevant to fusion physics, astro-physics, and basic plasma physics. It has been studied for many years, but usually in the context of a periodic toroidal plasma column. Reconnection Scaling Experiment (RSX)[1] is a cylindrical device built to study the linear and non-linear evolution of the current carrying screw pinch. A plasma column is injected into one end of the chamber from a plasma gun, and terminates at an anode that can be biased to draw current. This anode acts as an adjustable non-line tied end boundary for the column. Line-tying appears to give rise to several unexpected characteristics including finite rotation frequency, and a kink instability threshold less than the Kruskal Shafranov predictions. Experimental data is compared to a phenomenological theory of the kink instability developed for a slender plasma[2] column, including effects such as boundary conditions at the electrodes, finite plasma resistivity and axial flow. \newline \newline [1] I. Furno \textit{et al.}, Rev. Sci. Instrum. \textbf{74}, 2324 (2003).2] D. Ryutov et al., to be submitted to Phys. Plasmas. [Preview Abstract] |
Wednesday, October 26, 2005 4:30PM - 4:45PM |
LZ2.00008: Universal mechanism for collimation of magnetic flux tubes Paul Bellan Vacuum magnetic flux tubes are typically non-collimated. This is because a vacuum flux tube is produced by currents located outside the flux tube. The distance from these currents varies along the length of the flux tube causing $B$ to vary. Since the axial flux in the flux tube is $BA$, a vacuum flux tube will have an axially non-uniform cross-sectional area $ A \sim 1/B$, i.e., will not be collimated. Observations of flux tubes in laboratory, solar, and astrophysical situations show that bright flux tubes are always collimated and so cannot be vacuum flux tubes. Notable examples include solar coronal loops observed by the TRACE spacecraft, astrophysical jets, and Edge Localized Modes (ELMs) in tokamaks. Laboratory experiments at Caltech simulating astrophysical jets and solar coronal loops also show collimated profiles. A model has been developed showing that collimation results from axially directed MHD forces filling the flux tube with plasma which carries along frozen-in azimuthal flux that accumulates and pinches down the flux tube diameter until collimation results (P. M. Bellan, Phys. Plasmas {\bf10},1999(2003)). [Preview Abstract] |
Wednesday, October 26, 2005 4:45PM - 5:00PM |
LZ2.00009: Detailed Observations of Magnetic Flux Tube Collimation Setthivoine You, Gun Su Yun, Paul M. Bellan In the Caltech spheromak formation experiment, eight distinct magnetic flux tubes are observed to merge forming a plasma column that jets out into the vacuum vessel. The eight flux tubes are initially flared but quickly collimate while exhibiting strong plasma flows and high densities. The measurements show that in a few microseconds, MHD acceleration processes accelerate plasma from gas sources into the flux tubes. The acceleration is consistent with the gobble/collimation theory [1] and achieves flow velocities of the order of $\sim 100$~km/s and densities $\sim 10^{22}$~m$^{-3}$. Measurements show that the density of the initial neutral gas cloud required for breakdown cannot account for such a large jet density. The jet is a high-density, collimated, current-carrying magnetic flux tube located where, before breakdown, the magnetic field was negligible and the neutral density was negligible. The results [2] suggest that magnetically dominated astrophysical jets and current-carrying solar prominences may be collimated by the same process.\newline \newline [1] Bellan P. M., Phys. Plasmas, 10, 1999-2008 (2003). \newline [2] You S., Yun G. S., Bellan P. M., PRL, (2005) in press [Preview Abstract] |
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