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  • 1
    Electronic Resource
    Electronic Resource
    Alfvénicity of fluctuations associated with the Kelvin–Helmholtz instability (2000)
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 7 (2000), S. 2995-3003 
    Details
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The velocity shear which exists between three layers in an ideal plasma is studied. This configuration is modeled as a jet (or, strictly speaking, a wake) embedded in a uniform medium using a magnetohydrodynamics (MHD) code developed for astrophysical jet simulations. Weak and strong magnetic fields are considered both inside and outside the jet with a shear Mach number of 6. The shear can be Kelvin–Helmholtz (KH) unstable and evolve into a new less sheared pattern. There exists extensive literature on the KH instability which is extended by quantitatively describing the MHD properties of the fluctuations associated with the instability. To do so, a time series analysis of the fluctuations at various points inside and adjacent to the jet is performed. Specifically, points either in the center of the jet or just outside the transition layer—the initial location of the shear layer are considered. In the nonlinear stage, the perturbation is found to be a sum of the fast magnetosonic mode, slow magnetosonic mode, and the Alfvén component. To quantitatively evaluate the fluctuations, the normalized cross-helicity and Elsässer ratios are calculated, which in turn measure the degree of Alfvénicity. Fully nonlinear fluctuations are found to be more Alfvénic than magnetosonic in the low β case (β(approximate)0.833) as compared with high β case (β(approximate)13.3). This is in contrast to linear modes generated by the KH instability, which are magnetosonic modes. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.874151
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  • 2
    Electronic Resource
    Electronic Resource
    Meridional flow in the solar wind in the presence of latitudinally dependent boundary conditions (1975)
    Springer
    Solar physics 45 (1975), S. 255-266 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The influence of latitudinally dependent boundary conditions on the large radius values of meridional flow in the distant solar wind is examined through a double perturbation expansion of the magnetohydrodynamic equations. A general result is derived for the meridional velocity which allows arbitrary specification of radial velocity, radial magnetic field, and mass flux, as a function of colatitude at some coronal reference surface. Three specific examples are treated, including the model of Pneuman and Kopp (1971). The latter example indicates that there may be flow toward the equator at large radii, as opposed to the pure equatorial divergence of internally generated motion due to a flow which is latitudinally uniform at the reference radius. A solar cycle effect most probably averages the boundary conditions so that only the equatorial divergence from an average spherically symmetric corona is seen in comet-tail observations. This may also explain the off-and-on-again nature of the meridional gradient in the radial velocity of the solar wind as seen in radio scintillation observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00152236
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  • 3
    Electronic Resource
    Electronic Resource
    Solar wind: The quasi-radial approximation and its limitations (1975)
    Springer
    Solar physics 40 (1975), S. 487-499 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The mathematical basis for approximating the solar wind expansion as nearly radial is examined and defined, removing earlier restrictions thought to occur in the presence of a magnetic field and large variations in latitude. The equations and side conditions governing quasi-radial flow are derived and solved for a simple example to illustrate how this technique can be used for global models of the solar wind.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00162393
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  • 4
    Electronic Resource
    Electronic Resource
    Probing the local interstellar medium (1985)
    [s.l.] : Nature Publishing Group
    Nature 317 (1985), S. 702-703 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] As the solar wind flows into the LISM, it undergoes a transition to subsonic flow and then expands until it is dispersed within the LISM. Using Voyager data, Kurth et al.1 reported on 2-3-kHz radio emission at 18 AU, which they suggest is radiation at the second harmonic of the plasma frequency at ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/317702a0
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  • 5
    Electronic Resource
    Electronic Resource
    Magnetohydrodynamic simulation of a streamer beside a realistic coronal hole (1994)
    Springer
    Space science reviews 70 (1994), S. 295-298 
    Details
    ISSN: 1572-9672
    Keywords: Streamers ; Coronal Holes ; MHD Simulations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract Existing models of coronal streamers establish their credibility and act as the initial state for transients. The models have produced satisfactory streamer simulations, but unsatisfactory coronal hole simulations. This is a consequence of the character of the models and the boundary conditions. The models all have higher densities in the magnetically open regions than occur in coronal holes (Noci,et al., 1993).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00777883
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  • 6
    Electronic Resource
    Electronic Resource
    The Geometric Spreading of Coronal Plumes and Coronal Holes (1998)
    Springer
    Solar physics 180 (1998), S. 231-246 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract The geometric spreading in plumes and in the interplume region in coronal holes is calculated, using analytic and numerical theoretical models, between 1.0 and 5.0 R⊙. We apply a two-scale approximation that permits the rapid local spreading at the base of plumes (fl) to be evaluated separately from the global spreading (fg) imposed by coronal hole geometry. We show that fl can be computed from a potential-field model and fg can be computed from global magnetohydrodynamic simulations of coronal structure. The approximations are valid when the plasma beta is small with respect to unity and for a plume separation small with respect to a solar radius.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1005001618698
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  • 7
    Electronic Resource
    Electronic Resource
    Numerical simulation of mass injection for the formation of prominence magnetic field configurations (1988)
    Springer
    Solar physics 115 (1988), S. 93-106 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We have studied symmetric mass injection into a dipole magnetic field using a 2-D ideal magnetohydrodynamic (MHD) numerical model as a simulation of the formation of Kippenhahn-Schlüter (K-S)-type quiescent prominence (QP) magnetic field configurations. The result shows that there is an optimum magnetic field strength for QP formation, suggesting why prominences do not form above every neutral line. In the model, we varied the injection velocity, density, and magnetic field strength to find the optimum conditions for the formation of a K-S-type field configuration. Such a configuration is assumed to be necessary for QP formation in which injected plasma accumulates and subsequently condenses on magnetic field lines. The condensed plasma would, then, be supported by magnetic field against gravity. We find that a weaker magnetic field strength is more favorable for the condensation but that a stronger field is more favorable for the support against gravity. These two conflicting conditions lead to an optimum field strength for a QP formation by mass injection.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00146232
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  • 8
    Electronic Resource
    Electronic Resource
    A two-dimensional MHD global coronal model: Steady-state streamers (1993)
    Springer
    Solar physics 147 (1993), S. 55-71 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We describe a two-dimensional time-dependent, numerical, magnetohydrodynamic model for the determination of the physical properties of coronal streamers from the top of the transition zone (R⊙ = 1) to 15R⊙. Four examples are given: for dipole, quadrupole, and hexapole initial field topologies. The computed parameters are density, temperature, velocity, and magnetic field. In addition to the properties of the solutions, their accuracy is discussed. We use the model as the basis for a general discussion of the way boundary conditions are specified in this and similar simulations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00675487
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  • 9
    Electronic Resource
    Electronic Resource
    Lα intensity in coronal streamers (1993)
    Springer
    Solar physics 147 (1993), S. 73-96 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract White-light images are presently the primary source of information on physical conditions in the solar corona at distances greater than a few tenths of a solar radius above the limb. As a consequence, we still only have an incomplete description of structures extending beyond the solar limb. In particular, streamers, although observed for decades, represent a poorly known phenomenon. SOHO, to be launched in 1995, will be able to make long-term observations of these features up to heights of a fewR⊙, both in white light and UV. In this paper we present simulations of Lα intensity in coronal streamers, based on the two-dimensional (2-D) model developed by Wanget al. (1992, 1993) via a time-dependent numerical relaxation approach. Because the model is 2-D, we make ana priori hypothesis about the extension of streamers in the third dimension. Lα data, obtained from a rocket (Kohlet al., 1983), allowed us to identify a shape which fits the observations. We consider streamers with different magnetic field configurations and at different position angles with respect to the plane of the sky to illustrate how different regions along the line of sight contribute to the emergent intensity. Our purpose is twofold: to provide guidelines for UVCS observational operations and to explore the parameter space in order to understand the role of geometric factors and of the physical state of the corona in determining the overall streamer brightness. We conclude by showing how the results guide the future development of streamer models.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00675488
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  • 10
    Electronic Resource
    Electronic Resource
    On the formation of coronal cavities (1985)
    Springer
    Solar physics 102 (1985), S. 165-176 
    Details
    ISSN: 1573-093X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract We present a theoretical study of the formation of a coronal cavity and its relation to a quiescent prominence. We argue that the formation of a coronal cavity is initiated by the condensation of plasma which is trapped by the coronal magnetic field in a closed streamer and which then flows down to the chromosphere along the field lines due to lack of stable magnetic support against gravity. The existence of a coronal cavity depends on the coronal magnetic field strength; with low strength, the plasma density is not high enough for condensation to occur. Furthermore, we suggest that prominence and cavity material is supplied from the chromospheric level. Whether a coronal cavity and a prominence coexist depends on the magnetic field configuration; a prominence requires stable magnetic support. We initiate the study by considering the stability of condensation modes of a plasma in the coronal streamer model obtained by Steinolfson et al. (1982) using a 2-D, time dependent, ideal MHD computer simulation; they calculated the dynamic interaction between outward flowing solar wind plasma and a global coronal magnetic field. In the final steady state, they found a density enhancement in the closed field region with the enhancement increasing with increasing strength of the magnetic field. Our stability calculation shows that if the density enhancement is higher than a critical value, the plasma is unstable to condensation modes. We describe how, depending on the magnetic field configuration, the condensation may produce a coronal cavity and/or initiate the formation of a prominence.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00154046
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