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  • 1
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    Quasi-periodic transverse plasma flow associated with an evolving MHD vortex street in the outer heliosphere (1993)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: We study a transverse plasma flow induced by the evolution of a Karman vortex street using a Chebyshev-Fourier spectral algorithm to solve both the compressible Navier-Stokes and MHD equations. The evolving vortex street is formed by the nonlinear interaction of two vortex sheets initially in equilibrium. We study spatial profiles of the total plasma velocity, the density, the meridional flow angle and the location of sector boundaries and find generally good agreement with Voyager 2 measurements of quasi-periodic transverse flow in the outer heliosphere. The pressure pulses associated with the meridional flows in the simulation are too small, although they are correctly located, and this may be due to the lack of any 'warp' in the current sheet in this model. A strong, flow-aligned magnetic field, such as would occur in the inner heliosphere, is shown to lead to weak effects that would be masked by the background interplanetary turbulence. We also study the plasma and magnetic transport resulting from the meridional flow and find that deficits of magnetic quantities do occur near the ecliptic. While the effect is relatively small, it is in general agreement with the most recent analysis of 'flux deficit' in the outer heliosphere.
    Keywords: ASTROPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 98; A8; p. 13,233-13,246.
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  • 2
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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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  • 3
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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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  • 4
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    Do interplanetary Alfven waves cause auroral activity? (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A recent theory holds that high-intensity, long-duration, continuous auroral activity (HILDCAA) is caused by interplanetary Alfven waves propagating outward from the sun. A survey of Alfvenic intervals in over a year of ISEE 3 data shows that while Alfvenic intervals often accompany HILDCAAs, the reverse is often not true. There are many Alfvenic intervals during which auroral activity (measured by high values of the AE index) is very low, as well as times of high auroral activity that are not highly Alfvenic. This analysis supports the common conclusion that large AE values are associated with a southward interplanetary field of sufficient strength and duration. This field configuration is independent of the presence of Alfven waves (whether solar generated or not) and is expected to occur at random intervals in the large-amplitude stochastic fluctuations in the solar wind.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research (ISSN 0148-0227); 95; 4327-433
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  • 5
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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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  • 6
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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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  • 7
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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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  • 8
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    Magnetic field waves at Uranus (1991)
    In:  CASI
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    Publication Date: 2019-06-28
    Description: The proposed research efforts funded by the UDAP grant to the BRI involve the study of magnetic field waves associated with the Uranian bow shock. This is a collaborative venture bringing together investigators at the BRI, Southwest Research Institute (SwRI), and Goddard Space Flight Center (GSFC). In addition, other collaborations have been formed with investigators granted UDAP funds for similar studies and with investigators affiliated with other Voyager experiments. These investigations and the corresponding collaborations are included in the report. The proposed effort as originally conceived included an examination of waves downstream from the shock within the magnetosheath. However, the observations of unexpected complexity and diversity within the upstream region have necessitated that we confine our efforts to those observations recorded upstream of the bow shock on the inbound and outbound legs of the encounter by the Voyager 2 spacecraft.
    Keywords: LUNAR AND PLANETARY EXPLORATION
    Type: NASA-CR-192113 , NAS 1.26:192113
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  • 9
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    Numerical simulation of solar wind and magnetospheric phenomena (1987)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The nature of the nonlinear evolution of Kelvin-Helmholtz instability in the presence of sheared magnetic fields was investigated via numerical simulation. Models of the magnetosheath-magnetopause boundary in earth's tail and stream interaction regions in the inner heliosphere were studied. The development of the instability is influenced strongly by the orientation and magnitude of the magnetic field. Large vortical structures that resemble observations in the earth's tail can form while other cases generate turbulent spectra that provide insight into the generation of Alfven turbulence in the solar wind.
    Keywords: SPACE SCIENCES (GENERAL)
    Type: ESA, Proceedings of the 21st ESLAB Symposium on Small Scale Plasma Processes in the Solar Chromosphere(Corona, Interplanetary Medium and Planetary Magnetospheres; p 115-124
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  • 10
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    Entropy Generation Across Earth's Bow Shock (2011)
    In:  CASI
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    Publication Date: 2019-07-19
    Description: Earth's bow shock is a transition layer that causes an irreversible change in the state of plasma that is stationary in time. Theories predict entropy increases across the bow shock but entropy has never been directly measured. Cluster and Double Star plasma experiments measure 3D plasma distributions upstream and downstream of the bow shock that allow calculation of Boltzmann's entropy function H and his famous H-theorem, dH/dt ~ O. We present the first direct measurements of entropy density changes across Earth's bow shock. We will show that this entropy generation may be part of the processes that produce the non-thermal plasma distributions is consistent with a kinetic entropy flux model derived from the collisionless Boltzmann equation, giving strong support that solar wind's total entropy across the bow shock remains unchanged. As far as we know, our results are not explained by any existing shock models and should be of interests to theorists.
    Keywords: Geophysics
    Type: SH53B-2052 , GSFC.ABS.5717.2011 , American Geophysical Union 2011 Fall Meeting; Dec 04, 2011 - Dec 09, 2011; San Francisco, CA; United States
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