Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session PO5: Magnetic Reconnection: Theory, Experiment, and Astrophysical Applications |
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Chair: Michael Brown, Swarthmore College Room: Ballroom F |
Wednesday, November 16, 2011 2:00PM - 2:12PM |
PO5.00001: Flux ropes, current sheets, islands and turbulence T.P. Intrator, J. Sears, T. Weber, D.T. Liu, D.R. Pulliam, A. Lazarian, G. Lapenta We describe earth bound laboratory experiment investigations of patchy, unsteady, bursty, magnetic field structures that are unifying features of magnetic reconnection and turbulence in helio, space and astro physics. Flux ropes are ubiquitous structures on the sun and the rest of the heliosphere. We use experimental probes inside the the flux ropes to macroscopic magnetic field lines, unsteady wandering characteristics, and dynamic objects with structure down to the dissipation scale length. We also show some theta pinch data that appear to be in the plasmoid formation regime for magnetic reconnection. Computational approaches are finally able to tackle simple 3D systems and we sketch some intriguing simulation results that are consistent with experimental data for magnetic reconnection and turbulence. [Preview Abstract] |
Wednesday, November 16, 2011 2:12PM - 2:24PM |
PO5.00002: Instabilities of the separatrix regions in 3D kinetic reconnection Giovanni Lapenta, Stefano Markidis, Andrey Divin, Martin Goldman, David Newman The separatrix and its immediate vicinity is a region of strongly focused and highly accelerated electron flows. We have conducted several massively parallel kinetic simulations with thenew code iPic3D. We report: 1) the nature of the electron flow and of the electric fields in the separatrix region 2) a new instability, different in nature from that recently reported by Daughton et al, Nature Phys. 2011. 3) we investigate the properties of the phase space and the spectral properties of the new instability 4) Different diagnostics will be deployed to gather all the needed information 4) the nature is identified and will be disclosed at the meeting and its relation to know linear modes will be presented. [Preview Abstract] |
Wednesday, November 16, 2011 2:24PM - 2:36PM |
PO5.00003: Internal structure of plasmoids in collisionless magnetic reconnection Li-Jen Chen, Yi-Min Huang, Amitava Bhattacharjee, Brian Sullivan, William Daughton, Naoki Bessho Strong unipolar core magnetic fields and density compression are observed in the plasmoids which produce suprathermal electrons during magnetotail reconnection with a weak guide field ($< 3\% B_0$, where $B_0$ is the reconnecting field strength)[1]. The in-plane electric fields in these plasmoids are localized near the plasmoid core and point toward the core. Hall MHD and PIC simulations show that these features are consistent with plasmoids generated in the reconnection electron current sheet. In particular, the strong density compression and unipolar core field can be generated under sufficiently low upstream beta ($\le 0.4$) with a weak ambient guide field. The beta-dependence of the core-field generation allows us to infer the upstream condition of the observed magnetotail reconnection based on the observed plasmoid internal structures. \\[4pt] [1] Phys. Plasmas, 16, 056501 (2009) [Preview Abstract] |
Wednesday, November 16, 2011 2:36PM - 2:48PM |
PO5.00004: Onset and stagnation of reconnection in 3D geometry J. Sears, T.P. Intrator, T.E. Weber, D. Liu, D. Pulliam, G. Lapenta, A. Lazarian The bursty onset of reconnection is partly determined by a balance of macroscopic MHD forces. In a setting of multiple interacting flux ropes, there exist many individual reconnection sites, each X-line being finite in axial extent and thus intrinsically three-dimensional (3D) in structure. The balance between MHD forces and flux pile-up continuously shifts as mutually tangled flux ropes merge or bounce. The spatial scale and thus the rate of reconnection are therefore intimately related to the unsteady dynamics that may become turbulent. In the Reconnection Scaling Experiment (RSX) we study intermittent 3D reconnection along spatially localized x-lines between two or more flux ropes. The threshold of MHD instability which in this case is the kink threshold is varied by modifying the line-tying boundary conditions. For fast inflow speed of approaching ropes, there is merging and magnetic reconnection which is a well known and expected consequence of the 2D coalescence instability. On the other hand, for slower inflow speed the flux ropes bounce. The threshold appears to be the Sweet Parker speed $v_A/S^{1/2}$, where $v_A$ is the Alfven speed and $S$ is the Lundquist number. The flux rope boundary conditions also influence the propagation of the merging interface and the reconnection site along the flux rope axes. (LA-UR 11-03936) [Preview Abstract] |
Wednesday, November 16, 2011 2:48PM - 3:00PM |
PO5.00005: Onset of collisionless magnetic reconnection in two-dimensional current sheets and formation of dipolarization fronts Mikhail Sitnov, Marc Swisdak The onset of reconnection caused by the thinning of 2D current sheet equilibria with an X-line separating tail-like regions with magnetized electrons is simulated using a PIC code. For the case of tearing-stable tails, the electric field penetrates into the sheet near the X-line and forms there the electron diffusion region. In contrast, in multiscale current sheets, where the X-line is framed by local areas of enhanced magnetic flux, the electric field avoids the X-line, penetrates directly into those areas and ejects their plasma and magnetic flux, forming dipolarization fronts (DFs), sharp magnetic pileup structures, moving with plasma outflows in the direction opposite to the initial magnetic field stretching. New X-lines with their electron diffusion regions form behind DFs and away from the original X-line, which transforms eventually into the O-line. Simulations with a reduced driving electric field suggest, that the DF formation has properties of the ion tearing instability, and it is consistent with its predicted potential destabilization in multiscale current sheets. Weak driving of equilibria with tearing-stable tails first forms flux accumulation regions, similar to the ones adopted in multiscale equilibria, which then undergo rapid transformation to DFs, making 2D equilibria inherently metastable. [Preview Abstract] |
Wednesday, November 16, 2011 3:00PM - 3:12PM |
PO5.00006: Fast Shocks in Magnetic Reconnection Outflows Eric Blackman, Jared Workman, Chuang Ren Magnetic reconnection is commonly perceived to drive flow and particle acceleration in flares of solar, stellar, and astrophysical disk coronae but the relative roles of different acceleration mechanisms in a given reconnection environment are not well understood. Analytic theory predicts the existence of weak fast shocks in reconnection outflows. We show via direct numerical simulations that such weak fast mode shocks do indeed occur in the outflows of fast reconnection when an obstacle is present. These shocks are distinct from slow mode Petschek inflow shocks. If Fermi acceleration of electrons operates in the weak fast outflow shocks, the associated compression ratios will induce a Fermi acceleration particle spectrum that is significantly steeper than strong fast shocks commonly studied, but consistent with the demands of solar flares. While this is not the only particle acceleration mechanism operating in a reconnection environment, it is plausibly a ubiquitous one. [Preview Abstract] |
Wednesday, November 16, 2011 3:12PM - 3:24PM |
PO5.00007: Electron heating in collisionless magnetic reconnection Nuno Loureiro, Alexander Schekochihin, Alessandro Zocco A reduced gyrokinetic model [1] is used to numerically investigate magnetic reconnection in the strong guide field, weakly collisional regime. The model retains fully gyrokinetic ions (finite $T_i$), and electrons are described by a reduced drift-kinetic equation (i.e., are {\it not} assumed isothermal). The reconnection rate is found to depend on the system size, asymptoting to the usual value of $E \sim 0.1V_A B_0$ when the system is large enough. Small scales in velocity space are shown to form during the nonlinear evolution of the reconnection process. Though our simulations are conducted in the so-called collisionless regime, it is found that weak collisions are sufficient to convert energy into electron heating via the formation of small scales in velocity space. As theoretically predicted in [1], we demonstrate that electron heating during reconnection is independent of the collision frequency, and represents a substantial fraction of the available energy. \\[4pt] [1] A. Zocco \& A. Schekochihin, ArXiv:1104.4622 [Preview Abstract] |
Wednesday, November 16, 2011 3:24PM - 3:36PM |
PO5.00008: Gyrokinetic Studies of Magnetic Reconnection Moritz J. Pueschel, Frank Jenko, Daniel Told, Joerg Buechner Collisionless magnetic reconnection constitutes an effective mechanism for particle acceleration in astrophysical plasmas, in particular the solar corona. In addition, it is also of relevance to fusion experiments. Gyrokinetic simulations with the \textsc{Gene} code are performed to explore the temporal evolution of current sheets in two-dimensional slab geometry with a strong guide field. After successful code-code benchmarking, Extensive parameter studies are performed, covering a wide range of physical scenarios. In particular, differing findings regarding the influence of the ion temperature are explained. In its nonlinear phase, the characteristics of the reconnection process depend on whether the system is driven or decaying. Decaying turbulence sees an inverse cascade, and all energy is ultimately transferred to the largest radial scale. If driven by a Krook-type term, the system develops into a turbulent, quasi-stationary state. An important quantity to investigate in nonlinear simulations is the parallel electric field which is able to accelerate particles along the background magnetic field. The spatial structure of this field is studied for the different nonlinear cases, and its amplitude reported as a function of the drive frequency. [Preview Abstract] |
Wednesday, November 16, 2011 3:36PM - 3:48PM |
PO5.00009: 3D-MHD Simulation of the Dynamics of the Plasma Flow through a Magnetic Nozzle Alfonso Tarditi The present study focuses on the characterization of the plasma flow as it transitions through a diverging, axisymmetric, dipolar magnetic field that, performing the function of a `magnetic nozzle'. For sufficiently large plasma densities, the nozzle magnetic field (externally imposed) is perturbed as the plasma transitions along the axial direction. This scenario was modeled with the 3D-MHD NIMROD code [1] for the purpose of analyzing the details of the resulting nonlinear interaction of the plasma with the magnetic field. The simulations show the formation of regions with reconnecting closed field lines: in the plasma parameter range that has been considered, these patterns occur on a faster time scale than the one characterizing the plasma convective motion, but on the same time scale as the thermal energy confinement time. The 3D results are analyzed to show quantitatively the role of the diamagnetic current that is generated in the plasma along the azimuthal direction. The relevance of this analysis for the establishment of flow conditions that lead to an effective detachment of the plasma from the magnetic field is discussed. Further developments are also considered in relation to the application of Rotating Magnetic Fields to FRC plasmas, as described in [2-3]. References: [1] C. R. Sovinec et al., J. Comput. Phys. 195, 355 (2004) [2] R. D. Milroy et al., Phys. Plasmas 17, 062502 (2010) [3] Y. Petrov et al., Phys. Plasmas 17, 012506 (2010) [Preview Abstract] |
Wednesday, November 16, 2011 3:48PM - 4:00PM |
PO5.00010: The study of mode instability in rotamak plasmas driven by rotating magnetic field Xiaokang Yang, Jermain Goss, Dharah Kalaria, Saeid Houshmandyar, Tian-Sen Huang The instability modes, which include the n = 1 tilt, radial shift and kink-like mode, have been observed in rotamak plasma through the measurements of Mirnov coil array and the images of a high speed CCD camera. The effect of three instability modes on plasma discharge is completely different: plasma current can be decreased, terminated and enhanced respectively by tilt, radial shift and kink-like mode. Experiments clearly demonstrate that the appearance and suppression of instability modes strongly depends on the configuration and the strength of magnetic field (X. Yang, et al, Phys. Rev. Lett. \textbf{102}, 255004~(2009)). Mode switching also has been observed in disruptive discharges. [Preview Abstract] |
Wednesday, November 16, 2011 4:00PM - 4:12PM |
PO5.00011: Satellite observations of plasma waves in the reconnection regions Chijie Xiao, Xiaogang Wang, Haoming Liang, Zuyin Pu The waves and related anomalous resistivities for fast magnetic reconnection are long-standing problems for decades. Some kinds of plasma waves are suggested to response to via wave-particles interactions are suggested. We here report several reconnection events observed by Cluster spacecraft in the plasma sheet. First the inflow regions, null-null lines and outflow regions are clear identified via Poincare index calculating, as well as the magnetic fields and plasma properties. Then using the SVD and k-filtering methods, the Alfven waves, whistler waves, and lower-hybrid (LH) waves are identified in the different parts of reconnection regions: such as the exhaust region, the ion diffusion region, and the electron diffusion region near the x-point. The wave vectors, ellipticities, and polarizations, and the power spectrum of these modes are also quantitatively analyzed. Furthermore, the anomalous resistivities due to wave-particle interactions of those modes are also calculated, and in comparison with the effective resistivity calculated from electrical field and current data. It is found that, the anomalous resistivity induced by LH waves near the x-point may be sufficient to trigger fast reconnection. [Preview Abstract] |
Wednesday, November 16, 2011 4:12PM - 4:24PM |
PO5.00012: Reconnection-Powered Extreme Particle Acceleration and Gamma-Ray Flares in Crab Nebula Dmitri Uzdensky, Benoit Cerutti, Mitchell Begelman Recent discovery of gamma-ray flares in the Crab Nebula challenges traditional relativistic particle acceleration models. These flares are presumably produced by PeV electrons radiating $>$100 MeV synchrotron photons in a milli-gauss magnetic field. In traditional models, where the accelerating electric field is smaller than the magnetic field, synchrotron radiation cannot exceed 100 MeV because radiative losses balance the acceleration rate. We propose that linear electric acceleration in a magnetic reconnection layer can resolve this difficulty. The gyroradii of PeV electrons are so large that their motion is insensitive to small-scale turbulent structures and is controlled only by large-scale fields. As these particles are accelerated by the reconnection electric field, their relativistic Speiser-like orbits collapse deep into the layer and get focused into a tight beam. Furthermore, since perpendicular magnetic field is small inside the layer, the radiation reaction there is suppressed, so the particles can reach higher energies and emit synchrotron radiation in excess of the 100 MeV limit, resolving the Crab gamma-ray flare paradox. [Preview Abstract] |
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