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 BI1: Space and Astrophysical Plasmas I |
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Chair: William Matthaeus, University of Delaware Room: Adam's Mark Hotel Plaza Ballroom ABC |
Monday, October 24, 2005 9:30AM - 10:00AM |
BI1.00001: How Does Collisionless Magnetic Reconnection Work in the Presence of a Guide Magnetic Field? Invited Speaker: The dissipation mechanism of guide field magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as 2.5 and three-dimensional PIC simulations of guide-field magnetic reconnection. We will show that diffusion region scale sizes in moderate and large guide field cases are determined by electron Larmor radii, and that analytical estimates of diffusion region dimensions need to include description of the heat flux tensor. The dominant electron dissipation process appears to be based on thermal electron inertia, expressed through nongyrotropic electron pressure tensors. We will argue that this process remains viable in three dimensions by means of a detailed comparison of high resolution particle-in-cell simulations. [Preview Abstract] |
Monday, October 24, 2005 10:00AM - 10:30AM |
BI1.00002: A Fermi mechanism for the production of energetic electrons during magnetic reconnection Invited Speaker: The production of energetic electrons has been documented in observations of solar flares, magnetic reconnection in the Earth's magnetosphere and in laboratory tokamak experiments yet the understanding of these widespread observations remains poor. Simulations reveal that magnetic reconnection with a guide field leads to the growth and dynamics of multiple magnetic islands rather than a single large x-line. Above a critical energy electron acceleration is dominated by the Fermi-like reflection of electrons within the resulting magnetic islands rather than by the parallel electric fields associated with the x-line. Particles trapped within islands gain energy as they reflect from ends of contracting magnetic islands, slowly drift outwards and scatter as they undergo non-adiabatic motion near the magnetic separatrices. A Fokker-Planck equation for the distribution of energetic particles similar to that developed in shock acceleration theory is obtained by averaging over the particle interaction with many islands. Steady state solutions in reconnection geometry result from convective losses balancing the Fermi drive. Distribution functions take the form of a powerlaw whose spectral index depends on the mean aspect-ratio of the islands. In large systems the spectral index is the same as that obtained for high mach number shocks -- namely the conversion efficiency of magnetic energy into energetic electrons is high. The energy content of these particles is a consequence of their high mobility -- they can rapidly interact with many islands to reach high energy. The model is consistent with several key solar and magnetospheric observations: the production of large numbers of energetic electrons; the isotropy of the particle distributions at high energy and powerlaw distributions.\\ \\ In collaboration with H. Che, M. Swisdak and M. A. Shay. [Preview Abstract] |
Monday, October 24, 2005 10:30AM - 11:00AM |
BI1.00003: Two-fluid effects on 3D reconnection in the SSX experiment with comparisons to space data Invited Speaker: We report on several new experimental results from spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) with relevance to three dimensional reconnection in laboratory and space plasmas. First, we discuss a measurement of non-ideal terms of the generalized Ohm's law at a reconnection site of a weakly collisional laboratory plasma (recently reported in GRL, Cothran, et al). Time resolved vector magnetic field measurements on a 3D lattice (${\bf B}({\bf r}, t)$) enables evaluation of the various terms. Results show that the Hall term dominates everywhere (${\bf J \times B}$); resistive and electron inertia terms are small. The suggestion is that electron pressure supports the reconnection electric field at the neutral point. Second, we discuss experimental measurement of the in-plane Lorentz force and out-of- plane magnetic field associated with the Hall electric field near the reconnection zone. Both show a quadrupolar structure at the ion inertial scale. Earlier work at SSX has shown that formation of three- dimensional structure at the ion inertial scale is temporally and spatially correlated with the observation of superthermal, super-Alfv\'enic ions ions accelerated along the X-line normal to the local 2D plane of reconnection. Anomalous resistivity, while not ruled out, is not required to account for the results. Third, we report on recent velocity and temperature measurements of impurity ions using ion doppler spectroscopy (IDS). Bi-directional outflow at nearly the Alfv\'en speed is clearly observed. Each of these will be related to and compared with similar measurements in a solar or space context. [Preview Abstract] |
Monday, October 24, 2005 11:00AM - 11:30AM |
BI1.00004: MHD Eddy Viscosity: Testing the Concept with the Solar-Wind/Magnetosphere Coupling Data Base Invited Speaker: In Navier-Stokes fluids, the presence of upstream turbulence increases the viscous drag of an obstacle in a flow: this is known as the ``turbulence effect.'' The effect is explained by an eddy viscosity that enhances the viscous coupling, with the eddy viscosity controlled by the amplitude of the upstream turbulence. From solar-wind/magnetosphere data analysis it is known that activity in the Earth's magnetosphere is driven by the solar wind (chiefly as a consequence of reconnection, but also as a consequence of unexplained ``viscous effects''); the measured level of geomagnetic activity can be used as a measure of the strength of solar-wind/magnetosphere coupling. The solar wind is a turbulent flow with an MHD-turbulence amplitude that varies with time. We have statistically confirmed that there is a turbulence effect in solar-wind/magnetosphere coupling, where geomagnetic activity is observed to be greater when the solar-wind turbulence is louder. An expression for the MHD eddy viscosity of the turbulent solar wind that can be evaluated in terms of spacecraft-measured quantities is derived. Using this expression and a few decades of solar-wind measurements, cross correlations between geomagnetic activity and the eddy viscosity of the solar wind are performed. These cross correlations can yield (a) an experimental confirmation of the concept of MHD eddy viscosity and (b) a test of the validity, accuracy, and usefulness of the MHD-eddy-viscosity explanation of the turbulence effect in solar-wind/magnetosphere coupling. [Preview Abstract] |
Monday, October 24, 2005 11:30AM - 12:00PM |
BI1.00005: The Plasma Physics of Cosmic Ray Acceleration: Where We Stand After 30 Years Invited Speaker: Cosmic rays (CRs) are long believed to be accelerated in SNR shocks by the Fermi mechanism. Accelerated electrons have already been convincingly demonstrated to be the major TeV emitter in a number of SNRs. However, electrons make only 1-2\% of the entire CR spectrum. Therefore, distinguishing between leptonic and hadronic origin of the observed TeV emission is a key to the proof of the supernova origin of CRs. With the advent of the ground based Cerenkov telescopes, it is possible to tackle this difficult problem. The quality of the spectra measured is becoming sufficient to provide a sharp test of diffusive shock acceleration theory, which is built on a foundation from basic plasma physics and devised some thirty years ago. The first observational attempt to prove the proton acceleration in the only available candidate (SNR RXJ 1713.7-3946) has triggered hot debates in the community and clearly requires refinements of the theory. We discuss a new mechanism of spectrum formation in partially ionized dense gases, near SNRs. This is the most favorable situation for detecting hadrons as when they interact with dense gases, they generate gamma-radiation. Using a self-consistent analytic model of nonlinear diffusive shock acceleration, we calculate the spectra of protons and estimate the resulting gamma-ray emission occurring when the SNR shock approaches a molecular cloud. We show that the spectrum develops a break in the TeV range and that its GeV component is suppressed. These modifications to the standard theory occur because of the physics of particle and Alfven wave propagation inside the gas. Applications of the obtained spectra to the recent CANGAROO and HESS observations of the SNR RXJ 1713.7-3946 and key plasma physics problems at the forefront of cosmic ray research will be discussed. [Preview Abstract] |
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