Showing papers by "William H. Matthaeus published in 2005"

Anisotropy in Fast and Slow Solar Wind Fluctuations

Abstract: Using 5 years of spacecraft data from near Earth orbit, we investigate the correlation anisotropy of solar wind magnetohydrodynamic-scale fluctuations and show that the nature of the anisotropy differs in fast (>500 km s-1) and slow (<400 km s-1) streams. In particular, fast streams are relatively more dominated by fluctuations with wavevectors quasi-parallel to the local magnetic field, while slow streams, which appear to be more fully evolved turbulence, are more dominated by quasi-perpendicular fluctuation wavevectors.

Spatial correlation of solar-wind turbulence from two-point measurements

Abstract: Interplanetary turbulence, the best studied case of low frequency plasma turbulence, is the only directly quantified instance of astrophysical turbulence. Here, magnetic field correlation analysis, using for the first time only proper two-point, single time measurements, provides a key step in unraveling the space-time structure of interplanetary turbulence. Simultaneous magnetic field data from the Wind, ACE, and Cluster spacecraft are analyzed to determine the correlation (outer) scale, and the Taylor microscale near Earth's orbit.

Radial evolution of cross helicity in high‐latitude solar wind

Abstract: [1] We employ a turbulence transport theory to explain the high-latitude radial evolution of cross helicity, or Alfvenicity, observed by the Ulysses spacecraft. Evolution is slower than at low latitudes due to weakened shear driving.

Parallel and perpendicular cascades in solar wind turbulence

Abstract: . MHD-scale fluctuations in the velocity, magnetic, and density fields of the solar wind are routinely observed. The evolution of these fluctuations, as they are transported radially outwards by the solar wind, is believed to involve both wave and turbulence processes. The presence of an average magnetic field has important implications for the anisotropy of the fluctuations and the nature of the turbulent wavenumber cascades in the directions parallel and perpendicular to this field. In particular, if the ratio of the rms magnetic fluctuation strength to the mean field is small, then the parallel wavenumber cascade is expected to be weak and there are difficulties in obtaining a cascade in frequency. The latter has been invoked in order to explain the heating of solar wind fluctuations (above adiabatic levels) via energy transfer to scales where ion-cyclotron damping can occur.Following a brief review of classical hydrodynamic and magnetohydrodynamic (MHD) cascade theories, we discuss the distinct nature of parallel and perpendicular cascades and their roles in the evolution of solar wind fluctuations.

Generalized Ohm's law in a 3-D reconnection experiment

Abstract: [1] We report the measurement of non-ideal terms of the generalized Ohm's law at a reconnection site of a weakly collisional laboratory magnetohydrodynamic plasma. Results show that the Hall term dominates the measured terms; resistive and electron inertia terms are small. We suggest that electron pressure (not measured) supports the observed quasistatic reconnection rate, and that anomalous resistivity, while not ruled out, is not required to account for the results.

Direct comparisons of compressible magnetohydrodynamics and reduced magnetohydrodynamics turbulence

Abstract: Direct numerical simulations of low Mach number compressible three-dimensional magnetohydrodynamic (CMHD3D) turbulence in the presence of a strong mean magnetic field are compared with simulations of reduced magnetohydrodynamics (RMHD). Periodic boundary conditions in the three spatial coordinates are considered. Different sets of initial conditions are chosen to explore the applicability of RMHD and to study how close the solution remains to the full compressible MHD solution as both freely evolve in time. In a first set, the initial state is prepared to satisfy the conditions assumed in the derivation of RMHD, namely, a strong mean magnetic field and plane-polarized fluctuations, varying weakly along the mean magnetic field. In those circumstances, simulations show that RMHD and CMHD3D evolve almost indistinguishably from one another. When some of the conditions are relaxed the agreement worsens but RMHD remains fairly close to CMHD3D, especially when the mean magnetic field is large enough. Moreover, the well-known spectral anisotropy effect promotes the dynamical attainment of the conditions for RMHD applicability. Global quantities (mean energies, mean-square current, and vorticity) and energy spectra from the two solutions are compared and point-to-point separation estimations are computed. The specific results shown here give support to the use of RMHD as a valid approximation of compressible MHD with a mean magnetic field under certain but quite practical conditions.

Suppressed diffusive escape of topologically trapped magnetic field lines

Abstract: Many processes in astrophysical plasmas are directly related to magnetic connection in the presence of turbulent fluctuations. Even statistically homogeneous turbulence can contain closed topological structures that inhibit otherwise random transport of field line trajectories, thus temporarily trapping certain trajectories. When a coherent random field perturbation is added, the trapped field lines can escape diffusively but at a suppressed rate that is much lower than what would be estimated based on the perturbation field alone. Here we demonstrate both trapping and escape, and show, using a novel quasi-linear theory, how to compute the suppressed diffusion that affects the escape from the trapping structure. The effect is relevant to understanding filamentary magnetic connection in interplanetary space and the observed dropouts in moderately energetic particles from impulsive solar flares. Expressed here in terms of a magnetic field line random walk, this phenomenon also has analogies in a broad range of dynamical systems that evolve as an incompressible flow in phase space with a coherent perturbation.

Fluid and kinetic structure of magnetic merging in the Swarthmore Spheromak Experiment

Abstract: Received 1 July 2005; revised 27 September 2005; accepted 26 October 2005; published 6 December 2005. [1] Measurement of the in-plane Lorentz force and the outof-plane magnetic field associated with the Hall electric field near the reconnection zone in the Swarthmore Spheromak Experiment (SSX) confirms expectations, based on simulation, theory and spacecraft data, that the quadrupolar out-of-plane magnetic field is a signature of collisionless effects in magnetic reconnection with a weak guide field. Citation: Matthaeus, W. H., C. D. Cothran, M. Landreman, and M. R. Brown (2005), Fluid and kinetic structure of magnetic merging in the Swarthmore Spheromak Experiment, Geophys. Res. Lett., 32, L23104, doi:10.1029/ 2005GL023973.

Radial evolution of cross helicity at low and high latitudes in the solar wind

Abstract: We employ a turbulence transport theory to the radial evolution of the solar wind at both low and high latitudes. The theory includes cross helicity, magnetohydrodynamic (MHD) turbulence, and driving by shear and pickup ions. The radial decrease of cross helicity, observed in both low and high latitudes, can be accounted for by including sufficient shear driving to overcome the tendency of MHD turbulence to produce Alfvénic states. The shear driving is weaker at high latitudes leading to a slower evolution. Model results are compared with observations from Ulysses and Voyager.

A two-component phenomenology for the evolution of MHD turbulence

Abstract: Incompressible MHD turbulence with a mean magnetic field B0 develops anisotropic spectral structure and can be simply described only by including at least two distinct fluctuation components. These are conveniently referred to as “waves,” for which propagation effects are important, and “quasi-2D” turbulence, for which nonlinear effects dominate over propagation ones. The quasi2D component has wavevectors approximately perpendicular to B0. These two idealized ingredients capture the essential physics of propagation (high frequency fluctuations) and strong turbulence (low frequency fluctuations.) Here we present a two-component energycontaining range phenomenology for the evolution of homogeneous MHD turbulence.