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  • Weitere Quellen  (23)
  • Solar Physics  (14)
  • Astrophysics  (6)
  • Space Sciences (General)  (3)
  • 1
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    Enhanced Spectral Anisotropies Near the Proton-Cyclotron Scale: Possible Two-Component Structure in Hall-FLR MHD Turbulence Simulations (2011)
    In:  CASI
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    Publikationsdatum: 2019-07-19
    Beschreibung: Recent analysis of the magnetic correlation function of solar wind fluctuations at 1 AU suggests the existence of two-component structure near the proton-cyclotron scale. Here we use two-and-one-half dimensional and three-dimensional compressible MHD models to look for two-component structure adjacent the proton-cyclotron scale. Our MHD system incorporates both Hall and Finite Larmor Radius (FLR) terms. We find that strong spectral anisotropies appear adjacent the proton-cyclotron scales depending on selections of initial condition and plasma beta. These anisotropies are enhancements on top of related anisotropies that appear in standard MHD turbulence in the presence of a mean magnetic field and are suggestive of one turbulence component along the inertial scales and another component adjacent the dissipative scales. We compute the relative strengths of linear and nonlinear accelerations on the velocity and magnetic fields to gauge the relative influence of terms that drive the system with wave-like (linear) versus turbulent (nonlinear) dynamics.
    Schlagwort(e): Solar Physics
    Materialart: GSFC.ABS.5714.2011 , American Geophysical Union 2011 Fall Meeting; Dec 04, 2011 - Dec 09, 2011; San Francisco, CA; United States
    Format: application/pdf
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  • 2
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    Turbulence and Global Properties of the Solar Wind (2010)
    In:  CASI
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    Publikationsdatum: 2019-07-19
    Beschreibung: The solar wind shows striking characteristics that suggest that it is a turbulent magnetofluid, but the picture is not altogether simple. From the earliest observations, a strong correlation between magnetic fluctuations and plasma velocity fluctuations was noted. The high corrections suggest that the fluctuations are Alfven waves. In addition, the power spectrum of the magnetic fluctuation showed evidence of an inertial range that resembled that seen in fully-developed fluid turbulence. Alfven waves, however, are exact solutions of the equations of incompressible magnetohydrodynamics. Thus, there was a puzzle: how can a magnetofluid consisting of Alfven waves be turbulent? The answer lay in the role of velocity shears in the solar wind that could drive turbulent evolution. Puzzles remain: for example, the power spectrum of the velocity fluctuations is less steep than the slope of the magnetic fluctuations. The plasma in the magnetic tail of Earth's magnetosphere also shows aspects of turbulence, as does the plasma in the dayside magnetosphere near the poles the dayside cusps. Recently, new analyses of high time resolution magnetic field data from Cluster have offered a glimpse of how turbulence is dissipated, thus heating the ambient plasma.
    Schlagwort(e): Solar Physics
    Materialart: Department of Physics and Astronomy Colloquium: "Viewing the Solar Wind as a Laboratory for the Study of Magnetofluid Turbulence"; Nov 15, 2010; Ames, IA; United States
    Format: application/pdf
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  • 3
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    Efforts to Simulate Solar Wind Turbulence (2007)
    In:  Other Sources
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    Publikationsdatum: 2019-07-19
    Beschreibung: A three-dimensional integration of the MHD equations in spherical coordinates has been developed that attempts to simulate a variety of solar wind conditions. These include the interaction of Alfven wave packets and the development of a turbulent cascade, the role of the heliospheric current sheet, the role of quasi-two-dimensional fluctuations in determining how magnetic field lines meander throughout the heliosphere, and the role of interstellar pickup ions in perturbing the solar wind in the outer heliosphere.
    Schlagwort(e): Solar Physics
    Materialart: Conference to be held in Kodaikanal, India and Oxford, United Kingdom, August 24 - September 9, 2007
    Format: text
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  • 4
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    Three-Dimensional Magnetohydrodynamic Modeling of the Solar Wind Including Pickup Protons and Turbulence Transport (2012)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: To study the effects of interstellar pickup protons and turbulence on the structure and dynamics of the solar wind, we have developed a fully three-dimensional magnetohydrodynamic solar wind model that treats interstellar pickup protons as a separate fluid and incorporates the transport of turbulence and turbulent heating. The governing system of equations combines the mean-field equations for the solar wind plasma, magnetic field, and pickup protons and the turbulence transport equations for the turbulent energy, normalized cross-helicity, and correlation length. The model equations account for photoionization of interstellar hydrogen atoms and their charge exchange with solar wind protons, energy transfer from pickup protons to solar wind protons, and plasma heating by turbulent dissipation. Separate mass and energy equations are used for the solar wind and pickup protons, though a single momentum equation is employed under the assumption that the pickup protons are comoving with the solar wind protons.We compute the global structure of the solar wind plasma, magnetic field, and turbulence in the region from 0.3 to 100 AU for a source magnetic dipole on the Sun tilted by 0 deg - .90 deg and compare our results with Voyager 2 observations. The results computed with and without pickup protons are superposed to evaluate quantitatively the deceleration and heating effects of pickup protons, the overall compression of the magnetic field in the outer heliosphere caused by deceleration, and the weakening of corotating interaction regions by the thermal pressure of pickup protons.
    Schlagwort(e): Solar Physics
    Materialart: GSFC.JA.7114.2012 , The Astronomical Journal; 754; 40
    Format: text
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  • 5
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    Scale Dependence of Magnetic Helicity in the Solar Wind (2011)
    In:  Other Sources
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    Publikationsdatum: 2019-07-13
    Beschreibung: We determine the magnetic helicity, along with the magnetic energy, at high latitudes using data from the Ulysses mission. The data set spans the time period from 1993 to 1996. The basic assumption of the analysis is that the solar wind is homogeneous. Because the solar wind speed is high, we follow the approach first pioneered by Matthaeus et al. by which, under the assumption of spatial homogeneity, one can use Fourier transforms of the magnetic field time series to construct one-dimensional spectra of the magnetic energy and magnetic helicity under the assumption that the Taylor frozen-in-flow hypothesis is valid. That is a well-satisfied assumption for the data used in this study. The magnetic helicity derives from the skew-symmetric terms of the three-dimensional magnetic correlation tensor, while the symmetric terms of the tensor are used to determine the magnetic energy spectrum. Our results show a sign change of magnetic helicity at wavenumber k approximately equal to 2AU(sup -1) (or frequency nu approximately equal to 2 microHz) at distances below 2.8AU and at k approximately equal to 30AU(sup -1) (or nu approximately equal to 25 microHz) at larger distances. At small scales the magnetic helicity is positive at northern heliographic latitudes and negative at southern latitudes. The positive magnetic helicity at small scales is argued to be the result of turbulent diffusion reversing the sign relative to what is seen at small scales at the solar surface. Furthermore, the magnetic helicity declines toward solar minimum in 1996. The magnetic helicity flux integrated separately over one hemisphere amounts to about 10(sup 45) Mx(sup 2) cycle(sup -1) at large scales and to a three times lower value at smaller scales.
    Schlagwort(e): Astrophysics
    Materialart: GSFC.JA.4743.2011 , The Astrophysical Journal; 739; 9
    Format: text
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  • 6
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    Observations of Electron Vorticity in the Inner Plasmasheet and Its Relationship to Reconnection (2011)
    In:  CASI
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    Publikationsdatum: 2019-07-19
    Beschreibung: Spatial derivatives of the electron moments can be estimated using data from the four Cluster spacecraft. Using spatial derivatives of the velocity we have computed the vorticity in the plasmasheet for several crossings. What we have found is that vorticity appears to be a common feature in the inner plasmasheet. We will show a number of examples. In at least some of the observations the vorticity is well correlated with the passage of Cluster through the ion diffusion region of known reconnection events. That most of the vorticity events observed are reconnection related cannot be dismissed and in fact observations of vorticity may provide a means to locate times when the Cluster spacecraft are magnetically connected to regions where reconnection is taking place. Understanding the role and source of the vorticity should advance our understanding of the dissipation of the turbulence associated with reconnection. In the course of the presentation we will also touch on the methods used to estimate the spatial derivatives as well as the limitations and assumptions involved.
    Schlagwort(e): Astrophysics
    Materialart: SM54A-03 , GSFC.ABS.5720.2011 , American Geophysical Union (AGU) 2011 Fall Meeting; Dec 05, 2011 - Dec 09, 2011; San Francisco, CA; United States
    Format: application/pdf
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  • 7
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    Magnetohydrodynamic Turbulence and Reconnection in the Magnetotail (2011)
    In:  CASI
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    Publikationsdatum: 2019-07-19
    Beschreibung: We have used a global MHD simulation with high spatial resolution to investigate the origin and properties of turbulence in the plasma sheet. In this simulation we imposed a steady southward IMF with a magnitude of 5 nT at the upstream simulation boundary for more than three hours followed by ninety minutes of northward IMF of the same magnitude. The solar wind number density was 20 cm-3, the thermal pressure was 20 pPa, and the velocity was 500 km/s in the x direction. The moderately high dynamic pressure confined the magnetotail to the high-grid resolution region. Even for these nominal solar wind parameters and steady driving the plasma sheet became turbulent. The power spectral densities and probability distribution functions computed from the simulations were comparable to those obtained from spacecraft observations. The largest scale vortices were associated with reconnection outflows and, in the southward IMF case, with the diversion of high speed flows in the near-Earth region. Both time and space domain analyses revealed that there were three scales present, the large scale of the driving processes, the intermediate inertial scale and the dissipative scale.
    Schlagwort(e): Space Sciences (General)
    Materialart: SM53A-06 , GSFC.ABS.5719.2011 , American Geophysical Union (AGU) 2011 Fall Meeting; Dec 05, 2011 - Dec 09, 2011; San Francisco, CA; United States
    Format: application/pdf
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  • 8
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    Magnetofluid Simulations of the Global Solar Wind Including Pickup Ions and Turbulence Modeling (2011)
    In:  CASI
    Details
    Publikationsdatum: 2019-07-19
    Beschreibung: I will describe a three-dimensional magnetohydrodynamic model of the solar wind that takes into account turbulent heating of the wind by velocity and magnetic fluctuations as well as a variety of effects produced by interstellar pickup protons. In this report, the interstellar pickup protons are treated as one fluid and the protons and electrons are treated together as a second fluid. The model equations include a Reynolds decomposition of the plasma velocity and magnetic field into mean and fluctuating quantities, as well as energy transfer from interstellar pickup protons to solar wind protons that results in the deceleration of the solar wind. The model is used to simulate the global steady-state structure of the solar wind in the region from 0.3 to 100 AU. Where possible, the model is compared with Voyager data. Initial results from generalization to a three-fluid model is described elsewhere in this session.
    Schlagwort(e): Astrophysics
    Materialart: SH43D-02 , American Geophysical Union 2011 Fall Meeting; Dec 04, 2011 - Dec 09, 2011; San Francisco, CA; United States
    Format: application/pdf
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  • 9
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    Theory and Simulations of Solar System Plasmas (2011)
    In:  CASI
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    Publikationsdatum: 2019-07-19
    Beschreibung: "Theory and simulations of solar system plasmas" aims to highlight results from microscopic to global scales, achieved by theoretical investigations and numerical simulations of the plasma dynamics in the solar system. The theoretical approach must allow evidencing the universality of the phenomena being considered, whatever the region is where their role is studied; at the Sun, in the solar corona, in the interplanetary space or in planetary magnetospheres. All possible theoretical issues concerning plasma dynamics are welcome, especially those using numerical models and simulations, since these tools are mandatory whenever analytical treatments fail, in particular when complex nonlinear phenomena are at work. Comparative studies for ongoing missions like Cassini, Cluster, Demeter, Stereo, Wind, SDO, Hinode, as well as those preparing future missions and proposals, like, e.g., MMS and Solar Orbiter, are especially encouraged.
    Schlagwort(e): Space Sciences (General)
    Materialart: European Geosciences Union General Assembly 2011; Apr 02, 2011 - Apr 09, 2011; Vienna; Austria
    Format: application/pdf
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  • 10
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    Cluster/Peace Electrons Velocity Distribution Function: Modeling the Strahl in the Solar Wind (2008)
    In:  Other Sources
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    Publikationsdatum: 2019-07-19
    Beschreibung: We present a study of kinetic properties of the strahl electron velocity distribution functions (VDF's) in the solar wind. These are used to investigate the pitch-angle scattering and stability of the population to interactions with electromagnetic (whistler) fluctuations. The study is based on high time resolution data from the Cluster/PEACE electron spectrometer. Our study focuses on the mechanisms that control and regulate the pitch-angle and stability of strahl electrons in the solar wind; mechanisms that are not yet well understood. Various parameters are investigated such as the electron heat-flux and temperature anisotropy. The goal is to check whether the strahl electrons are constrained by some instability (e.g., the whistler instability), or are maintained by other types of processes. The electron heat-flux and temperature anisotropy are determined by fitting the VDF's to a spectral spherical harmonic model from which the moments are derived directly from the model coefficients.
    Schlagwort(e): Solar Physics
    Materialart: American Geophysical Union; Dec 14, 2008 - Dec 21, 2008; San Francisco, CA; United States
    Format: text
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