28,918 research outputs found
Transport of Solar Wind Fluctuations: A Two-Component Model
No full textWe present a new model for the transport of solar wind fluctuations which treats them as two interacting incompressible components: quasi-two-dimensional turbulence and a wave-like piece. Quantities solved for include the energy, cross helicity, and characteristic transverse length scale of each component, plus the proton temperature. The development of the model is outlined and numerical solutions are compared with spacecraft observations. Compared to previous single-component models, this new model incorporates a more physically realistic treatment of fluctuations induced by pickup ions and yields improved agreement with observed values of the correlation length, while maintaining good observational accord with the energy, cross helicity, and temperature
A reduced magnetohydrodynamic model of coronal heating in open magnetic regions driven by reflected low-frequency waves
No full textA reduced magnetohydrodynamic (RMHD) description is employed to examine a suggestion made by W. H. Matthaeus and colleagues in 1999 that coronal heating might be sustained by a cascade of low-frequency MHD turbulence. Here RMHD simulations show that the low-frequency cascade to high transverse wavenumbers can be driven by an externally maintained flux of low-frequency propagating Alfvén waves, in combination with reflection caused by an inhomogeneous background medium. The simulations show that the suggestions made previously on the basis of a phenomenology are indeed realizable. In addition, the effect is seen to sensitively depend on the presence of reflection, as the background turbulence level needed to maintain the cascade can be sustained only when reflection is imposed. The steady level of turbulence is insensitive to the initial seed turbulence level (provided it is nonzero). Consequences of this model for realistic models of coronal heating in open field-line regions are discussed
Turbulence transport throughout the heliosphere
No full textWe employ a turbulence transport model to compute distributions of turbulence throughout the heliosphere. The model determines the radial dependence of three (coupled) quantities that characterize interplanetary turbulence, the energy per unit mass, the cross helicity or Alfvénicity, and a similarity length scale. A fourth integrated quantity, the plasma temperature, is modified by heat deposition due to turbulent dissipation. The model includes advection, expansion, and reflection effects as well as the tendency toward dynamic alignment, and a von Kármán type dissipation function that represents decay of turbulence due to cascade to small scales. Two types of forcing are also featured, one a simple model of stream shear, and the other a driving in the outer heliosphere associated with wave energy injection due to pickup protons of interstellar origin. Parameters for the model have been tuned using observation data from Voyager and Ulysses. We analyze the constraining observations to provide boundary conditions and parameters that vary with heliocentric latitude, with some extrapolations. The fully assembled model permits the computation of the distribution of turbulence throughout the entire heliosphere, and we present solutions for several appropriate parameter sets
Reply to “Comment on ‘Evolution of energy-containing turbulent eddies in the solar wind’ by W. H. Matthaeus, S. Oughton, D. H. Pontius Jr., and Y. Zhou”
No full textIn their preceding comment on our paper [Matthaeus et al., 1994] (hereinafter reffered to as paper 1), Tu and Marsh [this issue] object vehemently to what they perceive as undeserved criticism of the class of solar wind turbulence models developed by Tu and coworkers[Tu et al., 1984, Tu, 1987, 1988]. Our intent was not to slight the valuable contributions made in those groundbreaking theories for incorporating turbulence into models for spatial transport. Rather, we wished to point out certain shortcomings in existing theories and suggest how they might be improved
The influence of a mean magnetic field on three-dimensional magnetohydrodynamic turbulence
No full textBuilding on results from two-dimensional magnetohydrodynamic (MHD) turbulence (Shebalin, Matthaeus & Montgomery 1983), the development of anisotropic states from initially isotropic ones is investigated numerically for fully three-dimensional incompressible MHD turbulence. It is found that when an external d.c. magnetic field (B₀) is imposed on viscous and resistive MHD systems, excitations are preferentially transferred to modes with wavevectors perpendicular to B₀). The anisotropy increases with increasing mechanical and magnetic Reynolds numbers, and also with increasing wavenumber. The tendency of B₀ to inhibit development of turbulence is also examined
Parallel and perpendicular cascades in solar wind turbulence
No full textMHD-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
Dynamical age of solar wind turbulence in the outer heliosphere
No full textIn an evolving turbulent medium, a natural timescale can be defined in terms of the energy decay time. The time evolution may be complicated by other effects such as energy supply due to driving, and spatial inhomogeneity. In the solar wind the turbulence appears not to be simply engaging in free decay, but rather the energy level observed at a particular position in the heliosphere is affected by expansion, “mixing,” and driving by stream shear. Here we discuss a new approach for estimating the “age” of solar wind turbulence as a function of heliocentric distance, using the local turbulent decay rate as the natural clock, but taking into account expansion and driving effects. The simplified formalism presented here is appropriate to low cross helicity (non-Alfvénic) turbulence in the outer heliosphere especially at low helio-latitudes. We employ Voyager data to illustrate our method, which improves upon the familiar estimates in terms of local eddy turnover times
Impact of Hall effect on energy decay in magnetohydrodynamic turbulence
No full textWe examine numerically the influence of Hall effect corrections to Ohm's law upon the decay of homogeneous compressible magnetohydrodynamic turbulence and conclude that there are no significant differences in global decay rate associated with the Hall effect. This affirms expectations that energy decay is controlled by the large-scale eddies
Depression of nonlinearity in decaying isotropic MHD turbulence
No full textSpectral method simulations show that undriven magnetohydrodynamic turbulence spontaneously generates coherent spatial correlations of several types, associated with local Beltrami fields, directional alignment of velocity and magnetic fields, and antialignment of magnetic and fluid acceleration components. These correlations suppress nonlinearity to levels lower than what is obtained from Gaussian fields, and occur in spatial patches. We suggest that this rapid relaxation leads to non-Gaussian statistics and spatial intermittency. © 2008 The American Physical Society.Fil: Servidio, S.. University of Delaware; Estados UnidosFil: Matthaeus, W. H.. University of Delaware; Estados UnidosFil: Dmitruk, Pablo Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina. University of Delaware; Estados Unido
Anisotropy in fast and slow solar wind fluctuations
No full textUsing 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) slow (<400 km/) 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.Fil: Dasso, Sergio Ricardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Milano, L. J.. University of Delaware; Estados UnidosFil: Matthaeus, W. H.. University of Delaware; Estados UnidosFil: Smith, C. W.. University of New Hampshire; Estados Unido
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