Bulletin of the American Physical Society
51st Annual Meeting of the APS Division of Plasma Physics
Volume 54, Number 15
Monday–Friday, November 2–6, 2009; Atlanta, Georgia
Session GM9: Mini-Conference on Unsteady Reconnection in Laboratory and Nature I: Reconnection Rate and Dynamics |
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Chair: Giovanni Lapenta, Centre for Space Weather, Leuven, Belgium Room: Regency V |
Tuesday, November 3, 2009 9:30AM - 9:55AM |
GM9.00001: Reconnection in the Solar Wind: An Update John Gosling Magnetic reconnection plays a central role in a wide variety of observed solar and space phenomena. In the solar wind magnetic reconnection commonly occurs in a quasi-stationary mode at extended X-lines. It produces Petschek-like exhausts of roughly Alfv\'{e}nic jetting plasma typically bounded by back-to-back rotational discontinuities that bifurcate a reconnecting current sheet. It occurs at thin current sheets most frequently associated with relatively small (less than 90 deg) magnetic field rotations in low beta plasma. Reconnection exhausts are observed most frequently (typically 1-3 events/day at 1 AU) in the low-speed wind and within interplanetary coronal mass ejections, and less frequently (0.6 events/day) in the Alfv\'{e}nic turbulence characteristic of the high-speed wind from coronal holes. Reconnection occurs relatively infrequently at the heliospheric current sheet, HCS, but observations of exhausts at the HCS are particularly revealing of the magnetic field topology changes associated with the reconnection process. Reconnection in the solar wind is usually fast, but not explosive -- the magnetic energy release occurs over a long time interval following reconnection as the Alfv\'{e}nic disturbances initiated by the process propagate into the surrounding solar wind plasma. There is as yet no hard evidence that indicates that reconnection in the solar wind ever produces substantial particle acceleration. [Preview Abstract] |
Tuesday, November 3, 2009 9:55AM - 10:15AM |
GM9.00002: The Tectonics Model of Coronal Heating: Unsteady Dynamics and Scaling in Statistical Steady State C.S. Ng, L. Lin, A. Bhattacharjee The tectonics model of coronal heating, proposed by Priest et al. [Astrophys. J., {\bf 576}, 533 (2002)] envisions coronal heating caused by a hierarchy of current sheets produced by the movement of a myriad of flux elements in the magnetic carpet covering the Sun. We have recently obtained new scaling results in two dimensions (2D) suggesting that the heating rate becomes independent of resistivity in a {\it statistical} steady state [C. S. Ng and A. Bhattacharjee, Astrophys. J., {\bf 675}, 899 (2008)]. Our numerical work has now been extended to 3D. Random photospheric footpoint motion is applied to obtain converged average coronal heating rates. In the large Lundquist number limit, we find that the heating rate is independent of the Lundquist number, with average magnetic energy saturating at a constant level due to the formation of strong current layers and subsequent disruptions. In this talk, we will present our latest numerical results from large-scale 3D simulations, and discuss differences with previous scaling laws. [Preview Abstract] |
Tuesday, November 3, 2009 10:15AM - 10:40AM |
GM9.00003: Energy conversion by magnetic reconnection Joachim Birn, Michael Hesse The energy release and conversion by magnetic reconnection is discussed on the basis of magnetohydrodynamic (MHD) and particle-in-cell (PIC) simulations. Contrary to common assumptions, the energy conversion to enthalpy flux (convected heat) plays a major role, which typically exceeds the conversion to bulk kinetic energy flux. Direct Joule dissipation plays only a minor role in the energy release and transfer. These results are found consistently between PIC and MHD simulations, provided that the imposed resistivity is strongly localized or sufficiently small in magnitude. Approximate conservation of entropy on magnetic flux tubes is responsible for governing the structure of final equilibrium in closed systems. [Preview Abstract] |
Tuesday, November 3, 2009 10:40AM - 11:00AM |
GM9.00004: Dipolarization fronts as a distinctive feature of transient reconnection in the magnetotail: Particle simulations with open boundaries and THEMIS observations M. Sitnov, A. Runov, V. Angelopoulos, M. Swisdak, P. Guzdar, A. Divin, V. Sergeev, S. Ohtani, B. Mauk, J. Bonnell, J. McFadden, D. Larson, K.-H. Glassmeier, U. Auster Particle simulations with open boundary conditions revealed an interesting new phenomenon in the outflow regions of the collisionless reconnection pattern. It was found that transient reconnection may result in the formation of dipolarization fronts (DFs), regions of the strong magnetic flux pileup propagating with the speed of the order of the Alfven speed in the reconnection exhausts. The flux pileup is accompanied by a drop of the plasma density making the front similar to a plasma bubble. Similar phenomena have been observed in the tail of the Earth's magnetosphere by Geotail, Cluster and other missions. Here we present the results of simulations of DFs using the explicit massively parallelized full-particle code P3D [Zeiler et al., JGR, 107, 1230, 2002] with open boundaries [Divin et al., GRL, 34, L09109, 2007] and compare them with observations of DFs in the magnetosphere using five-satellite THEMIS mission. DFs are found to be microscopic shock-like boundaries with the thickness of the order of the ion inertial length that propagate over a macroscopic distance up to ten Earth radii. [Preview Abstract] |
Tuesday, November 3, 2009 11:00AM - 11:20AM |
GM9.00005: A Hall-MHD model for dipolarization fronts P.N. Guzdar, M.I. Sitnov, A.B. Hassam, M. Swisdak In recent studies of transient reconnection, dipolarization fronts which are characterized by a steep increase or jump in the z component of the earth's tail magnetic field, have been reported in kinetic simulations with open boundaries as well as in many satellite observation, including the most recent five-probe THEMIS mission. One possible interpretation of these jumps is the formation of supersonic shocks using the standard MHD Rankine-Hugoniot relations. The observed jumps in the measured quantities like the density, magnetic field and velocity components are compared with the different kinds of oblique shocks that can occur in MHD. Using these jump conditions obtained from MHD, an initial value code for the 1D Hall-MHD system of equations is then solved to investigate the role of Hall physics on these shocks. Because of the strong coupling between the z component and an out-of-plane y component of the magnetic field in Hall-MHD, a discontinuous y component develops when the initial condition does not have a B$_{y}$ component. The sharp shock discontinuity leads to the generation of whistler waves which have different propagation characteristics on either side of the shock boundary. [Preview Abstract] |
Tuesday, November 3, 2009 11:20AM - 11:40AM |
GM9.00006: What Causes Electron Holes During Magnetic Reconnection and What Can We Learn From Them Martin V. Goldman, David L. Newman, Giovanni Lapenta, Andre Divin, Francesco Califano, Haihong Che Weak bipolar electrostatic fields are commonly observed in association with magnetic reconnection. Recent attention has focused on their origin due to nonlinear evolution of electrostatic instabilities.\footnote{Goldman, M.~V., D.~L.~Newman, and P. Pritchett, \textit{GRL}, \textbf{35}, doi:10.1029/2008GL035608 (2008).}$^,$\footnote{Newman, D.~L.~and M.~V.~Goldman, SM31B-1735, AGU Fall Meeting (2008).}$^,$\footnote{Che, H., J.~F.~Drake, M.~Swisdak, and P.~H.~Yoon, \textit{PRL}, \textbf{102}, 145004 (2009).} We present evidence from both older and new reconnection simulations for the SPATIAL dependence of electrostatically unstable electron distributions along the separatrix during guide-field magnetic reconection. Particle distributions further from the reconnection region tend to be Buneman (electron-ion) unstable, while distributions closer to the reconnection region tend to be two-stream (electron-electron) unstable. It may be possible to infer properties of the particle distributions from measurements of the speed, half-width, amplitude and aspect ratio of weak electron holes. [Preview Abstract] |
Tuesday, November 3, 2009 11:40AM - 12:00PM |
GM9.00007: Dissipation of Thin Current Sheets Interacting with Nonlinear Alfven Waves in Relativistic Plasmas Edison Liang, Guy Hilburn, Hui Li, Wei Liu We present results from 2.5D and 3D particle-in-cell simulations of the interaction of thin current sheets with nonlinear Alfven waves in relativistic plasmas. We find that the Alfven waves cause the current sheet to bend and kink and enhance its dissipation rate. The electrons are heated by several competing mechanisms, eventually forming a double Maxwellian population. The cooler Maxwellian is heated mainly by Alfven turbulence cascade while the hotter Maxwellian is heated by the drift kink instability. The implications of these results for the kinetic dissipation of turbulence driven by the magnetorotational instability (MRI) will be discussed. Potential applications to the emissions of low-luminosity black hole accretion flows will be studied. [Preview Abstract] |
Tuesday, November 3, 2009 12:00PM - 12:20PM |
GM9.00008: Evolving Magnetic Reconnection in Well Confined Plasmas with Low Collisionalities$*$ B. Coppi There are two kinds of modes, producing large scale magnetic islands in well confined plasmas with low degrees of collisionality. These have phase velocities of opposite signs and are expected to emerge following the excitation of other modes as they cannot be found to be linearly unstable. One type is the ``drift-tearing'' [1] mode with a phase velocity in the direction of the electron diamagnetic velocity ($\textup{v}_{de}$) and the other is classified as an ``inductive'' mode [2] with a phase velocity in the direction of $\textup{v}_{di}$. The ``drift-tearing'' can be excited after a mode that has the effect of decreasing the ratio of the longitudinal to the transverse electron thermal conductivity, like the ``micro-reconnecting'' mode discussed in Ref. [3]. The second type requires the previous excitation of a pressure gradient driven mode [4] that has a flow velocity in the $\textup{v}_{di}$ direction. Moreover, a mode-particle resonance with a high energy particle population [1] is involved in the growth of both the primary and the secondary (reconnecting) mode. Recent experimental observations [4] are consistent with these conclusions. Sawtooth oscillations that involve periodic reconnection events and modes that are related to those described earlier are discussed. *Sponsored in part by the U.S. DoE. [1] B. Coppi, \textit{Phys. Fluids} \textbf{8}, 2273 (1965) [2] B. Coppi, Bull. \textit{Am. Phys. Soc} \textbf{45}, 366 (2000) [3] B. Coppi, in ``Collective Phenomena etc.'' pg. 59, Eds. G. Bertin \textit{et. al., Publ. World Scientific} (2007) [4] P. Buratti \textit{et al.} Paper 02.007, 2009 E.P.S. Conference [Preview Abstract] |
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