Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session TO7: Magnetic Reconnection |
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Chair: Seth Dorfman, University of California, Los Angeles Room: Galerie 6 |
Thursday, October 30, 2014 9:30AM - 9:42AM |
TO7.00001: Velocity-Shear Driving for Magnetic Reconnection in Kinetic Simulations Carrie Black, Spiro Antiochos, Kai Germaschewski, Naoki Bessho, C. Richard DeVore, Judith.T. Karpen In the standard model for coronal mass ejections (CME) and/or solar flares, the free energy for the event resides in the strongly sheared magnetic field of a filament channel. The pre-eruption force balance consists of an upward force due to the magnetic pressure of the sheared field countered by a downward tension due to overlying unsheared field. Magnetic reconnection is widely believed to be the mechanism that disrupts this force balance, leading to explosive eruption. For understanding CME/flare initiation, therefore, it is critical to model the onset of reconnection that is driven by the build-up of magnetic shear. More generally, shearing of magnetic fields is seen throughout the heliosphere. In MHD simulations, the application of a magnetic-field shear is a trivial matter. However, kinetic effects are important in the diffusion region and thus, it is important to examine this process with PIC simulations as well. The implementation of such a driver in PIC methods is nontrivial, however, and indicates the necessity of a true multiscale model for such processes in the solar environment. The field must be sheared self-consistently and indirectly to prevent the generation of waves that destroy the desired system. In the work presented here, we show results of velocity shear perpendicular to the plane of reconnection in a system with open boundary conditions. This material is based upon work supported by the National Science Foundation under Award No. AGS-1331356. [Preview Abstract] |
Thursday, October 30, 2014 9:42AM - 9:54AM |
TO7.00002: Strongly Driven Magnetic Reconnection in a Magnetized High-Energy-Density Plasma G. Fiksel, D.H. Barnak, P.-Y. Chang, D. Haberberger, S.X. Hu, S. Ivancic, P.M. Nilson, W. Fox, W. Deng, A. Bhattacharjee, K. Germaschewski Magnetic reconnection in a magnetized high-energy-density plasma is characterized by measuring the dynamics of the plasma density and magnetic field between two counter-propagating and colliding plasma flows. The density and magnetic field were profiled using the 4$\omega$ angular filter refractometry and fast proton deflectometry diagnostics, respectively. The plasma flows are created by irradiating oppositely placed plastic targets with 1.8-kJ, 2-ns laser beams on the OMEGA EP Laser System. The two plumes are magnetized by an externally controlled magnetic field with an $x$-type null point geometry with B $=$ 0 at the midplane and B $=$ 8 T at the targets. The interaction region is pre-filled with a low-density background plasma. The counterflowing super-Alfv\'{e}nic plasma plumes sweep up and compress the magnetic field and the background plasma into a pair of magnetized ribbons, which collide, stagnate, and reconnect at the midplane, allowing for the first detailed observation of a stretched current sheet in laser-driven reconnection experiments. The measurements are in good agreement with first-principles particle-in-cell simulations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944 and NLUF Grant DE-SC0008655. [Preview Abstract] |
Thursday, October 30, 2014 9:54AM - 10:06AM |
TO7.00003: PIC Simulations of the Omega-EP Magnetic Reconnection Experiment Wenda Liu, Eric Blackman, Rui Yan, Chuang Ren In an Omega EP experiment on magnetic reconnection, two laser beams with peak intensity of 7 $\times$ 10$^{18}$W/cm$^{2}$ are focused on a Cu-target. Here we report 2D PIC simulation results with parameters derived from the experiment including a realistic ion-electron mass ratio. We find that 1) toroidal and mega-gauss-scale magnetic fields are generated and a bubble of high-energy-density plasma is produced from single beam-target interactions and 2) the magnetic topology changes as two such bubbles expand and interact with each other indicating the occurrence of magnetic reconnection. The reconnection can occur even when the bubble expansion velocity is subsonic. Flux pileup is observed when the expansion velocity is supersonic. Energetic Cu-ions with energy up to 12 MeV are also observed in the outflow. [Preview Abstract] |
Thursday, October 30, 2014 10:06AM - 10:18AM |
TO7.00004: Radiative Magnetic Reconnection in Astrophysics Dmitri Uzdensky Traditional magnetic reconnection has mostly focused on relatively tenuous solar-system environments, where radiation can be ignored. In contrast, in many astrophysical situations the energy density in the reconnection region is so high that radiation becomes important. I will give an overview of our recent progress in understanding radiative magnetic reconnection --- a new frontier in plasma astrophysics. I will describe how the key radiative effects, such as radiative cooling, radiation pressure, and Compton drag, affect reconnection dynamics and particle acceleration. I will illustrate these ideas with specific astrophysical examples, including magnetar flares; black-hole accretion-disk coronae; reconnection powering high-energy emission in pulsar magnetospheres; and GeV-range gamma-ray flares in the Crab Nebula. [Preview Abstract] |
Thursday, October 30, 2014 10:18AM - 10:30AM |
TO7.00005: Non-thermal particle acceleration in relativistic, collisionless, electron-positron magnetic reconnection Gregory Werner, Dmitri Uzdensky, Benoit Cerutti, Krzysztof Nalewajko, Mitchell Begelman Relativistic magnetic reconnection converts magnetic field energy to particle kinetic energy, and has been proposed to explain particle acceleration in pulsar wind nebulae (PWNe) and gamma ray bursts. Observable synchrotron and inverse Compton radiation from such sources depends on the resulting energy spectra of electrons and positrons. Our 2D reconnection simulations in relativistic, collisionless, electron-positron plasmas without guide field show that reconnection creates non-thermal particle distributions, with power-law energy spectra $dN/d\epsilon\sim\epsilon^{-\alpha}$ extending well beyond the mean dissipated magnetic energy per particle. For large system size $L$ and upstream magnetization $\sigma\gg 1$, the power-law index is independent of $L$ and $\sigma$: $\alpha\approx 1.2$. Decreasing $\sigma$, the power-law steepens to $\alpha\approx 2.3$ at $\sigma=3$. We find that for large $L$, the power law cuts off at high energies as $\exp(-\epsilon/\epsilon_1)$; the cutoff $\epsilon_1$ is proportional to $\sigma$ but nearly independent of $L$. Small system sizes exhibit a super-exponential cutoff, $\exp(-\epsilon^2/\epsilon_2^2)$, with $\epsilon_2$ proportional to $L$ but essentially independent of $\sigma$. Implications for interpreting PWN observations will be discussed. [Preview Abstract] |
Thursday, October 30, 2014 10:30AM - 10:42AM |
TO7.00006: The scaling of reconnection rate in relativistic collisionless magnetic reconnection Yi-Hsin Liu, William Daughton, Fan Guo, Hui Li, Michael Hesse Relativistic reconnection is suggested to play a crucial role in the energy release and non-thermal particle acceleration in pulsar winds, gamma-ray burst and astrophysical jets from active galactic nuclei or black holes. While there has been significant progress in understanding the particle spectrum generated during reconnection,\footnote{F. Guo, H. Li, W. Daughton and Y. -H. Liu, submitted to PRL} the scaling of the reconnection in relativistic regimes remains unclear. Several numerical studies suggest that the reconnection rate is only enhanced mildly from $\sim 0.1$ in the non-relativistic regime up to $\sim 0.3$ in the strongly relativistic regime, which appears to be consistent with the prediction of Lyubarsky.\footnote{Y. E. Lyubarsky, Mon. Not. R. Astron. Soc. {\bf 358}, 113, 2005} In this work, first-principle fully kinetic simulations are systematically conducted to explore this issue. In particular, scaling-studies of reconnection rate as function of various parameters, such as the magnetization parameter, upstream pressure and guide field, are performed. Relativistic Ohm's law is analyzed to identify the mechanism of flux-breaking. Theoretical models are derived and compared against the observed scaling. [Preview Abstract] |
Thursday, October 30, 2014 10:42AM - 10:54AM |
TO7.00007: How robust is the Fermi acceleration in magnetic reconnection? Fan Guo, Hui Li, William Daughton, Xiaocan Li, Yi-Hsin Liu Previous kinetic simulations (Guo et al. 2014) have found that magnetic reconnection in relativistic plasmas is highly efficient at accelerating particles through a first-order Fermi process resulting from the curvature drift of particles in the direction of the electric field induced by the relativistic flows. This abstract will focus on understanding how robust the mechanism is in magnetic reconnection. We will discuss this mechanism in large 2D systems with very long simulation time and 3D systems where flux tubes are not well defined. We will also discuss the influence of anisotropy and collision to the mechanism. \\[4pt] [1] Guo F., Li, H., Daughton, W., Liu, Y., Formation of Hard Power-laws in the Energetic Particle Spectra\\[0pt] [2] Resulting from Relativistic Magnetic Reconnection, Physical Review Letters, in review, arxiv: 1405.4040 [Preview Abstract] |
Thursday, October 30, 2014 10:54AM - 11:06AM |
TO7.00008: Dynamics of the current sheet during driven guide-field reconnection Olaf Grulke, Hannes Bohlin, Dusan Milojevic, Kian Rahbarnia, Ilya Shesterikov, Adrian von Stechow A key issue in the spatiotemporal evolution of magnetic reconnection is the geometry and dynamics of the current sheet, which forms in response to the inductive electric field around the X-point. This paper presents experimental investigations of the current sheet during driven reconnection in the cylindricalexperiment VINETA.II. Due to the superimposed homgoenous axial guide magnetic field it is observed that the current sheet thickness varies along the X-line due to magnetic mapping effects. Consequently the reconnection rate is a function of axial position. Within the current sheet electromagnetic fluctuations are observed. The fluctuation amplitude correlates with the local current density and has a similar spatial profile as the current sheet. The fluctuations are incoherent with nearly isotropic correlation lengths on the order of the electron skin depth. Power spectra display a power law decrease with a breaking slope at the lower hybrid frequency. [Preview Abstract] |
Thursday, October 30, 2014 11:06AM - 11:18AM |
TO7.00009: Enhanced Magnetic Reconnection By Drift-Wave Instabilities M.J. Pueschel, P.W. Terry, D. Told A physical process is presented by which magnetic reconnection may be accelerated far beyond the rates usually achievable by tearing modes. The growth rates of reconnection processes are normally regulated by current gradients, whereas micro-scale instabilities in fusion devices tend to be driven by pressure gradients. Through gyrokinetic simulations, the interplay of these mechanisms and its potential consequences for reconnection physics are studied. As in [M.J.~Pueschel et al., Phys.~Plasmas \textbf{18}, 112102 (2011)], current sheets are used to drive reconnection, with background density or temperature gradients added independently. If sufficiently large, these gradients may excite a novel, drift-wave-type instability -- described here in some detail -- that relies fundamentally on parallel magnetic fluctuations. In particular, it is able to couple to the tearing mode via the Vlasov nonlinearity. In this case, the tearing mode starts to grow at the rate of the drift wave, independently of its own drive. Applying this mechanism to the solar corona, it is found that for pressure gradient length scales below 200 km, such gradient-enhanced tearing may be expected to exceed the usual reconnection rate. [Preview Abstract] |
Thursday, October 30, 2014 11:18AM - 11:30AM |
TO7.00010: Re-assessing how much parallel and perpendicular electric fields accelerate electrons during magnetic reconnection Naoki Bessho, Li-Jen Chen, Kai Germaschewski, Amitava Bhattacharjee By means of 2-D PIC simulations applicable to reconnection in the Earth's magnetotail, we show that the parallel electric field accelerates electrons only up to 40 keV, and further acceleration above that energy in fact comes from the perpendicular electric field, which can explain observations of energetic electrons with energies greater than 100 keV. We show that the parallel potential, which is the integral of the parallel electric field along the field line, is proportional to $(\omega_{pe}/\Omega_e)^{-2}$, and also to $(n_b/n_0)^{-1/2}$, where $\omega_{pe}/\Omega_e$ is the ratio of the plasma frequency to the electron cyclotron frequency, and $n_b/n_0$ is the ratio of the lobe density to the density of the current sheet. Applying the parameters in the Earth's magnetotail to the above relations, we demonstrate that the parallel potential is not more than 40 keV. In addition to pitch angle scattering from the parallel to the perpendicular velocity for electron beams along magnetic field, which was suggested in previous studies, energetic electrons accelerated by the perpendicular electric field experience pitch angle scattering from the perpendicular to the parallel velocity, which can isotropize plasma in the exhaust. [Preview Abstract] |
Thursday, October 30, 2014 11:30AM - 11:42AM |
TO7.00011: Fundamental limitation of a two-dimensional description of magnetic reconnection Marie-Christine Firpo For magnetic reconnection to be possible, the electrons have at some point to ``get free from magnetic slavery,'' according to von Steiger's formulation [1]. Stochasticity may be considered as one possible ingredient through which this may be realized in the magnetic reconnection process. It will be argued that non-ideal effects may be considered as a ``hidden'' way to introduce stochasticity. Then it will be shown that there exists a generic intrinsic stochasticity of magnetic field lines that does not require the invocation of non-ideal effects but cannot show up in effective two-dimensional models of magnetic reconnection. Possible implications will be discussed in the frame of tokamak sawteeth that form a laboratory prototype of magnetic reconnection. \\[4pt] [1] R. von Steiger, Space physics{-}grand challenges for the 21st century. Front. Physics 1:6 (2013). [Preview Abstract] |
Thursday, October 30, 2014 11:42AM - 11:54AM |
TO7.00012: Non-linear Tearing of 3D Null Point Current Sheets Peter Wyper, David Pontin The manner in which the rate of magnetic reconnection scales with the Lundquist number in realistic three dimensional (3D) geometries is still an unsolved problem. It has been demonstrated that in 2D rapid non-linear tearing allows the reconnection rate to become almost independent of the Lundquist number (the ``plasmoid instability''). Here we present the first study of an analogous instability in a fully 3D geometry, defined by a magnetic null point. The 3D null current layer is found to be susceptible to an analogous instability, but is marginally more stable than an equivalent 2D Sweet-Parker-like layer. Tearing of the sheet creates a thin boundary layer around the separatrix surface where efficient mixing of flux between the two topological domains occurs as the flux rope structures created during the tearing process evolve. This leads to a substantial increase in the rate of reconnection between the two domains. [Preview Abstract] |
Thursday, October 30, 2014 11:54AM - 12:06PM |
TO7.00013: Global stability versus diffusion region micro-physics in flare reconnection during CME formation Vyacheslav S. Lukin, Ed Lee, Mark G. Linton A model of coronal mass ejection (CME) initiation and flare magnetic reconnection is implemented within the HiFi framework with different combinations of initial conditions governing the magnetic equilibrium and gravitational stratification of the atmosphere. The model's sensitivity to the initial conditions and reconnection micro-physics is then investigated by a systematic parameter study. We find that the initial equilibrium, which includes a pre-existing magnetic flux rope located above a magnetic X-point, can be unstable to the X-point collapse due to wave accumulation there. However, introduction of an overall guide field allows small-amplitude waves to pass through the X-point and stabilize the equilibrium. Simulations of magnetic flux emergence via photospheric boundary driving demonstrate the impact of the guide field on the dynamics of resulting eruptions. In particular, we find that the rate of flare reconnection and the speed of the CME can be determined by the magnitude of the guide field irrespective of the micro-physics of magnetic reconnection. In a stratified atmosphere, we also identify a novel mechanism for producing quasi-periodic behavior at the flare reconnection site as a possible explanation of similar phenomena observed in solar and stellar flares. [Preview Abstract] |
Thursday, October 30, 2014 12:06PM - 12:18PM |
TO7.00014: The Onset of Ion Heating During Magnetic Reconnection with a Strong Guide Field James Drake, Michael Swisdak The onset of the acceleration of ions during magnetic reconnection in the limit of a strong guide field is explored via PIC simulations that self-consistently follow the motions of protons and $\alpha$ particles. Heating parallel to the local field is strongly reduced compared with the anti-parallel reconnection. The dominant heating of thermal ions results from pickup behavior during entry into reconnection exhausts and produces heating perpendicular rather than parallel to the local field. Pickup behavior requires that the ion transit time across the boundary (with a transverse scale of order $\rho_s$) be short compared with the cyclotron period. This translates into a threshold in the strength of reconnecting field that favors the heating of ions with high mass-to-charge. A simulation with a broad initial current layer causes the amplitude of the reconnecting field upstream of the dissipation region to increase with time and a sharp onset of perpendicular heating when the pickup threshold is crossed. A comparison of the time variation of the parallel and perpendicular heating with that predicted establishes the scaling of ion heating with ambient parameters both below and above the pickup threshold. The relevance to observations in the solar corona is discussed. [Preview Abstract] |
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