Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session UM7: Miniconference on the Plasma Physics of the Solar Wind: From Parker (1958) to the Present IV |
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Chair: Benjamin Chandran, University of New Hampshire Room: Pegasus |
Thursday, November 20, 2008 2:00PM - 2:25PM |
UM7.00001: Anisotropic fluxes of energy and cross-helicity to smaller scales in the solar wind Miriam Forman, Benjamin MacBride, Charles Smith, John Podesta The anisotropic MHD vector analog of Kolmogorov's 4/5 law for isotropic hydrodynamics describes transport of energy and cross-helicity to smaller scales by both velocity and magnetic fluctuations acting on both energy and cross-helicity. We have recently reported the first theoretical discussions of these fluxes in anisotropic context, and the first measurements of them in any plasma, using 8 years of magnetic and plasma data from the ACE spacecraft at L1 in the solar wind. These vector fluxes are signed third moments of fluctuations and are difficult to measure. Their scaling, magnitude and direction are the only direct measure of the existence and nature of a local anisotropic cascade of energy to smaller scales resulting from the non-linear terms in the MHD equations. \textit{This revelation is independent of any theoretical model of MHD turbulence}. The scaling of the energy and cross-helicity fluxes also provide the only direct and model-independent way to measure the turbulent energy and cross-helicity dissipation rates. [Preview Abstract] |
Thursday, November 20, 2008 2:25PM - 2:50PM |
UM7.00002: Flux tubes in the solar wind and perpendicular diffusion of energetic particles Gang Li, Gang Qin Recent studies showed that flux tubes may be very common in the solar wind and solar wind plasma resides in individual flux tubes. Observations show that upon cross the boundary, plama properties, in particular magnetic field directions often change substantially. The presence of these structures introduces a new source of solar wind turbulence intermittency and can affect the transport of energetic particles. We study the effects of flux tubes on the transport of energetic particles in the solar wind. We construct a toy model of the solar wind turbulence by including explicitly flux-tube-like structures and calculate numerically the particle diffusion coefficients by following single particle trajectory. we find that flux tubes in the solar wind can lead to stronger scatterings of particles in both directions parallel and perpendicular to the large scale background magnetic field. In particular a true diffusion in the large scale perpendicular direction (with respect to $B_0$) is obtained even when the local {\it intrinsic} turbulence in individual cells are of pure slab. [Preview Abstract] |
Thursday, November 20, 2008 2:50PM - 3:15PM |
UM7.00003: Flux-Tube Texture of the Solar Wind: Implications for the Scaling of Anisotropic Magnetic Fluctuation Spectra A. Bhattacharjee, C.-S. Ng, C.W. Smith, B. Vasquez Over the years, there has been a steady accumulation of observational evidence that the solar wind may be thought of as a network of individual magnetic flux tubes each with its own magnetic and plasma characteristics. As early as 1963, Parker referred to these tubes as magnetic and plasma ``filaments,'' and the picture has undergone several refinements since then [Bartley et al. 1966, Marliani et al. 1973, Tu and Marsch 1990, Bruno et al. 2001], culminating in the recent work of Borovsky [2008] who has suggested that these are fossil structures that originate at the solar surface. We use the weakly compressible MHD turbulence model [Bhattacharjee et al., 1998] to characterize the anisotropic magnetic fluctuation spectra observed by ACE. For a model of pressure-driven interchange turbulence in a generic solar wind flux tube, we use the Invariance Principle approach [Connor and Taylor 1977], to calculate explicitly the scaling of magnetic field fluctuations with plasma beta and other background plasma parameters. The theory predicts precise scaling laws for the magnetic fluctuation spectra parallel and perpendicular to the background magnetic field. We calculate the beta scaling of the variance anisotropy for electrostatic and electromagnetic pressure-driven turbulence in the solar wind, and demonstrate that they bracket well recent ACE observations. [Preview Abstract] |
Thursday, November 20, 2008 3:15PM - 3:40PM |
UM7.00004: MHD relaxation, intermittency and the origin of discontinuities in the solar wind W. Matthaeus, S. Servidio, A. Greco, P. Chuychai, P. Dmitruk Simulations of MHD turbulence show that several types of fast, local relaxation processes operate robustly. These include production of correlations associated with force-free states, Alfvenic states, and Beltrami flows. In MHD, all principle nonlinearities - Lorentz force, advection, magnetic induction, and the correlations among these -- are weakened by fast relaxation. This occurs in spatial patches bounded by near-discontinuities, and can be described by a local adaptation of global relaxation principles. Non-Gaussian statistics are implied, thus providing a natural real-space explanation of the origin of intermittency. Given the many indications that turbulence is active in the solar wind, one might also inquire as to whether the numerous discontinuities observed are related to intermittency generated by turbulence. We show by analysis using classical methods and intermittency statistics, applied to both simulations and to solar wind magnetic field data, that tangential discontinuities in the solar wind may be signatures of intermittent turbulence associated with the boundaries between interacting flux tubes. [Preview Abstract] |
Thursday, November 20, 2008 3:40PM - 4:05PM |
UM7.00005: Determination of anisotropic magnetic energy spectra using Ulysses data Sean Oughton, Timothy S. Horbury, Miriam A. Forman Observational results from a wavelet analysis of Ulysses magnetic field fluctuations are presented, indicating that the magnetic energy spectrum in the solar wind is anisotropic. The wavelet technique enables accurate scale-dependent determination of the local magnetic field direction, $ \bar{\bf{B}} $. The frequency power spectrum of magnetic field fluctuations is calculated, as a function of the angle $ \theta_{\bar{B}} $ between $ \bar{\bf{B}} $ and the radial (i.e., measurement) direction. Nine different 30-day intervals (of 1 second data) are analyzed, and in each case the spectral slope is $\approx 5/3$ for angles not ``close'' to the field-parallel direction (i.e., $0^\circ$), but transitions smoothly to $\approx 2$ as $\theta_{\bar{B}} \to 0^\circ$. Discussion of these results in terms of anisotropic models of MHD turbulence is also presented. [Preview Abstract] |
Thursday, November 20, 2008 4:05PM - 4:30PM |
UM7.00006: The role of cross-helicity in magnetohydrodynamic turbulence Stanislav Boldyrev, Fausto Cattaneo, Joanne Mason, Jean Carlos Perez We argue that conservation of cross-helicity plays a fundamental role in driven magnetohydrodynamic turbulence. Turbulent regions exhibit a scale-invariant hierarchy of positive and negative cross-helical domains (eddies). We propose that this structure is consistent with the scale-dependent dynamic alignment of the velocity and magnetic fluctuations, and with the exact analytic results known for MHD turbulence. We then discuss the spectra of both weak and strong cross-helical (or imbalanced) MHD turbulence, and compare the results with the solar wind observations. [Preview Abstract] |
Thursday, November 20, 2008 4:30PM - 4:55PM |
UM7.00007: Imbalanced Magnetohydrodynamic Turbulence: Numerics and Theory Jean C. Perez, Stanislav Boldyrev Recent results from theory and numerical simulations suggest that magnetohodrodynamic turbulence develops regions of imbalanced cascades, even when no overall imbalance is imposed. Imbalanced magnetohydrodynamic turbulence is believed to be present in the solar wind, where magnetic and velocity fluctuations are dominated by Alfv\'en waves moving outward from the sun. We present results of high resolution numerical simulations of balanced and imbalanced turbulent cascades, obtain turbulent spectra, and propose an analytic explanation of the results. [Preview Abstract] |
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