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  • 1
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    An evolving MHD vortex street model for quasi-periodic solar wind fluctuations (1992)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: Magnetohydrodynamic (MHD) simulation are used to provide a dynamical basis for the 'vortex street' model of the quasi-periodic meridional flow observed by Voyager 2 in the outer heliosphere. Various observations suggest the existence near the current sheet at solar minimum, of a vorticity distribution of two opposite shear layers with an antisymmetric staggered velocity pattern due to structured high-speed wind surrounding low-speed equatorial flow. It is shown that this flow pattern leads to the formation of a highly stable vortex street through the nonlinear interaction of the two shear layers. Spatial profiles of various simulated parameters (velocity, density, meridional flow angle and the location of magnetic sector boundaries) and their relative locations in the quasi-steady vortex street are generally in good agreement with the observations.
    Keywords: SOLAR PHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 19; 14, J
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  • 2
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    Magnetohydrodynamic turbulence in the solar wind (1995)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: The fluctuations in magnetic field and plasma velocity in solar wind, which possess many features of fully developed magnetohydrodynamic (MHD) turbulence, are discussed. Direct spacecraft observations from 0.3 to over 20 AU, remote sensing radio scintillation observations, numerical simulations, and various models provide complementary methods that show that the fluctuations in the wind parameters undergo significant dynamical evolution independent of whatever turbulence might exist in the solar photosphere and corona. The Cluster mission, with high time resolution particle and field measurements and its variable separation strategies, should be able to provide data for answering many questions on MHD turbulence.
    Keywords: SOLAR PHYSICS
    Type: ESA, Proceedings of the Cluster Workshops on Data Analysis Tools, and Physical Measurements and Mission-Oriented Theory; p 137-147
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  • 3
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    Anisotropy and minimum variance directions of solar wind fluctuations in the outer heliosphere (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Voyager 2 magnetic field and plasma data are examined over time intervals of 1 to 12 hours in the heliospheric range of 1 to 10 AU to study the evolution of the anisotropy of solar wind fluctuations. Consistent with previous results, the directions of minimum variance vectors of magnetic fluctuations are found to be close to the mean magnetic field direction with an increasing component along the field at larger scales. At large radial distances there is more spread in the minimum variance directions than at smaller radial distances. The power in smaller-scale fluctuations in the magnetic field components perpendicular to the local mean field B(0) is in the ratio of about 5:1 near 1 AU at the scale of 1 hour but decreases to about 3:1 further out. No evidence for selective enhancement of out-of-the-ecliptic components of fluctuations is found. In contrast to results for field fluctuations, analysis of velocity fluctuations shows that the minimum variance direction systematically remains more radially oriented and becomes increasingly less oriented along B(0) with increasing heliocentric distance. The velocity fluctuations are generally more isotropic than the magnetic fluctuations. The observations cannot be explained by a superposed wave picture, and thus are consistent with the view that nonlinear turbulent evolution is responsible for the anisotropy in the fluctuations.
    Keywords: SOLAR PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 96; 3779-378
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  • 4
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    Spectral evolution and cascade constant of solar wind Alfvenic turbulence (1989)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The theoretical model of Tu (1988) for the turbulent evolution of solar wind fluctuations assumed the constancy of the ratio alpha(1) between the energy in inward and outward propagating Alfven waves. Here, this model is extended to reflect the observed evolution in the propagation directions of the interplanetary fluctuations. The radial dependence of alpha(1) is determined by direct time domain correlations of the normalized cross helicity. The theoretical results match the observations at least as well as those of the previous, constant alpha(1), model. Measured values of alpha(1) are then used to find the value of the cascade constant that determines the overall level of the energy spectrum. The value of 1.25 for this constant is very close to the value observed in fluid turbulence if it is assumed that the correspondence between fluid and magnetofluid theories should be made for uncorrelated (zero cross helicity) MHD turbulence.
    Keywords: SOLAR PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 94; 13575-13
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  • 5
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    Turbulence and waves in the solar wind (1991)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Studies of turbulence and waves in the solar wind is discussed. Consideration is given to the observations and theory concerning the origin and evolution of interplanetary MHD fluctuations and to the observations, theory, and simulations of compressive fluctuations. Particular attention is given to extrapolations to near-sun and polar fields regions. Results obtained on turbulence at comets and magnetic turbulence of low-frequency waves excited by unstable distributions of ions are discussed.
    Keywords: SOLAR PHYSICS
    Type: Reviews of Geophysics Supplement (ISSN 8755-1209); 29; 932-943
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  • 6
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    Magnetohydrodynamic simulation of the radial evolution and stream structure of solar-wind turbulence (1991)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A unified interpretation of observations of interplanetary fluctuations is presented in terms of nearly incompressible magnetohydrodynamics. Incompressive effects explain the rapid evolution of turbulence in slow wind containing the heliospheric current sheet. The relative constancy of the spectrum of 'inward propagating' fluctuations compared to the rapid decline in 'outward' fluctuations results from incompressive spectral transfer combined with strong dissipation of the outward fluctuations. Secondary compressive effects account for nearly pressure-balanced structures and the density fluctuation levels.
    Keywords: SOLAR PHYSICS
    Type: Physical Review Letters (ISSN 0031-9007); 67; 3741-374
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  • 7
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    Test particle acceleration in turbulent reconnecting magnetic fields (1988)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: The effect of turbulence on particle acceleration in a MHD field was investigated by computing test particle trajectories in turbulent MHD reconnecting fields, including reconnection simulations at different magnetic Reynolds numbers. The dynamics of individual particles were investigated making it possible to examine the acceleration mechanism in great detail. It was found that turbulence influences the acceleration in two ways. It enhances the reconnection electric field while producing a stochastic electric field that gives rise to momentum diffusion; and it produces magnetic 'bubbles' and other irregularities that can temporarily trap test particles in the strong reconnection electric field for times comparable to the magnetofluid characteristic time.
    Keywords: PLASMA PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 93; 14383-14
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  • 8
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    Velocity shear generation of solar wind turbulence (1992)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: A two-dimensional incompressible MHD spectral code is used to show that shear-driven turbulence is a possible means for producing many observed properties of the evolution of the magnetic and velocity fluctuations in the solar wind and, in particular, the evolution of the cross helicity ('Alfvenicity') at small scales. It is shown that large-scale shear can nonlinearly produce a cascade to smaller scale fluctuations even when the linear Kelvin-Helmholtz mode is stable, and that a roughly power law inertial range is established by this process. The evolution found is similar to that seen in some other simulations of MHD turbulence.
    Keywords: SOLAR PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 97; A11; p. 17,115-17,130.
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  • 9
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    Evidence for the presence of quasi-two-dimensional nearly incompressible fluctuations in the solar wind (1990)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: Assuming that the slab and isotropic models of solar wind turbulence need modification (largely due to the observed anisotropy of the interplanetary fluctuations and the results of laboratory plasma experiments), this paper proposes a model of the solar wind. The solar wind is seen as a fluid which contains both classical transverse Alfvenic fluctuations and a population of quasi-transverse fluctuations. In quasi-two-dimensional turbulence, the pitch angle scattering by resonant wave-particle interactions is suppressed, and the direction of minimum variance of interplanetary fluctuations is parallel to the mean magnetic field. The assumed incompressibility is consistent with the fact that the density fluctuations are small and anticorrelated, and that the total pressure at small scales is nearly constant.
    Keywords: SOLAR PHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 20673-20
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