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 GZ1: Mini-conference on Reconnection and Turbulence in Fluids and Plasmas II |
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Chair: Norman Zabusky, Rutgers University Room: Adam's Mark Hotel Plaza Ballroom D |
Tuesday, October 25, 2005 2:00PM - 2:30PM |
GZ1.00001: Magnetic Reconnection and Anomalous Resistivity R. Kulsrud Magnetic reconnection is often invoked in astrophysics to explain desirable properties of magnetic fields. For example, it is used to remove small scale fields in galactic dynamos, to generate jets by Taylor relaxation, and in the generation of fields for gamma ray bursts. However, most theories of reconnection lead to too slow a rate for these purposes. As Petschek has pointed out, the critical limiting process is the slowness of the mass outflow from the reconnection region. The only way around this seems to be to enhance outflow is by a funnel provided by a Petschek-like model which is based on slow shocks. An anomalous resistivity due to instabilities can vary rapidly in space and thus lead to shocks emerging from a short diffusion region. This process is explored based on the detection and analyis of a obliquely propagating lower hybrid drift instability. This instability has been detected in the MRX and prove to be strong enough to produce the anomalous resistivity that is measured there. A rate for the modified version of Petschek reconnection is derived and applied to the above mentioned astrophysical applications. [Preview Abstract] |
Tuesday, October 25, 2005 2:30PM - 3:00PM |
GZ1.00002: Prospects for Turbulent Reconnection in Solar Flares Terry Forbes High-resolution images of solar flares often show features that are consistent with reconnection plasma flows into and out of a large scale current sheet. The flows into the sheet range from a few km/s up to 50 km/s, corresponding to inflow Alfv\'{e}n Mach numbers in the range from 0.001 to 0.05. The flows out of the sheet have been observed to exceed 600 km/s, but maybe even faster since existing instruments do not have sufficient time resolution to observe flows in excess of this speed. The outflowing plasma typically exhibits fluctuations that are suggestive of turbulence, and similar fluctuations sometimes occur in flare simulations if the magnetic Reynolds number is sufficiently large. Whether the observed or simulated fluctuations are consistent with turbulence is yet to be established. If they are, then an important question that needs to be addressed is the source of the turbulence. One possibility is that the turbulence is generated internally within the current sheet, but another possibility is that it is generated externally. In the latter case the observed fluctuations could result simply from the relative amplification that occurs when low level fluctuations are convected into the weak field region of the current sheet. [Preview Abstract] |
Tuesday, October 25, 2005 3:00PM - 3:30PM |
GZ1.00003: Formation and Dynamics of Thin Current Sheets in the Geomagnetic Tail Joachim Birn, M. Hesse, M. Hoshino, J. Huba, G. Lapenta, P. Pritchett, K. Schindler, L. Yin Satellite observations, theory, and computer simulations show that the formation of a thin current sheet, or the thinning of the magnetotail current sheet to less than the ion inertia length, is necessary to cause fast reconnection. We present MHD theory and simulations that demonstrate that external deformations of the magnetotail, imposed by the solar wind, can lead to the formation of a thin current sheet embedded in the near tail and the loss of MHD equilibrium. Using a variety of fluid and particle simulations, we further compare current sheet thinning and the onset of fast reconnection in response to deformations of a relatively thick current sheet. We find that PIC, hybrid, and Hall-MHD simulations lead to the same fast reconnection rates, apparently independent of the dissipation mechanism, as earlier simulations starting from a perturbed thin current sheet (``GEM challenge''). The similarity of the final states indicates that entropy conservation is satisfied similarly in fluid and kinetic approaches and that Joule dissipation plays only a minor role in the overall energy transfer. [Preview Abstract] |
Tuesday, October 25, 2005 3:30PM - 3:45PM |
GZ1.00004: BREAK
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Tuesday, October 25, 2005 3:45PM - 4:15PM |
GZ1.00005: Recent Developments in the theory of the turbulent MHD dynamo David Montgomery, Pablo Mininni, Annick Pouquet Turbulent dynamos amplify arbitrarily small magnetic fields in conducting MHD fluids. The theoretical/computational problem is typically formalized in 3D rectangular periodic boundary conditions. An external ``forcing function'' is added to mimic a relevant laboratory, geophysical, or astrophysical mechanical process. Inducing dynamo action by forcings that generate familiar simple flows (e.g., helical ``Roberts flow,'' discussed here) is surprisingly easy, with or without mechanical helicity injection. For an ``unforced kinematic dynamo,'' any turbulent velocity field that will produce enstrophy will amplify magnetic fields for low enough resistivity and viscosity. Triply periodic boundary conditions limit the realism of the fields generated, and also are on occasion inconsistent with Maxwell's equations. We are attempting to move beyond 3D periodicity by developing a wholly spectral code based on an expansion of the fields in spherical Chandrasekhar-Kendall eigenfunctions of the curl. [Preview Abstract] |
Tuesday, October 25, 2005 4:15PM - 4:45PM |
GZ1.00006: The Onset of Fast Magnetic Reconnection: A Catastrophe Model Paul Cassak, Michael Shay, James Drake The Hall MHD model produces rates of reconnection fast enough to explain observations (unattainable with the Sweet-Parker model of resistive MHD). However, there is no accepted mechanism for what triggers the sudden onset of Hall reconnection from a quiescent state. We present a model in which a decrease in the resistivity past a critical value induces a catastrophic transition from the Sweet-Parker to the Hall configuration. The crucial point is that for a wide range of resistivities, both the Sweet-Parker and Hall configurations are valid and stable, but below a critical resistivity, the Sweet-Parker solution no longer exists. This can be shown by a simple scaling analysis and we present the results of two-fluid simulations confirming the theory. The catastrophic transition occurs due to a decrease in the resistivity below the critical value due to external heating (or alternately an increase in the upstream magnetic field strength). The temperature at which this transition would occur in a solar flare is consistent with the known coronal value. The effects of a guide field will also be discussed. [Preview Abstract] |
Tuesday, October 25, 2005 4:45PM - 5:15PM |
GZ1.00007: Current Sheet Formation in Compressible MHD: Analytical and Numerical Results Bhimsen Shivamoggi, David Rollins Current sheet formation near a hyperbolic magnetic neutral line is considered. Effects of compressibility are included. A finite-time singularity is exhibited. Analytical and numerical results are given. [Preview Abstract] |
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