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  • 1980-1984  (7)
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  • 1
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    Stellar convection theory. III - Dynamical coupling of the two convection zones in A-type stars by penetrative motions (1981)
    Institute of Physics
    Details
    Publication Date: 1981-09-01
    Print ISSN: 0004-637X
    Electronic ISSN: 1538-4357
    Topics: Physics
    Published by Institute of Physics
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    PAPER CURRENT
  • 2
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    On the identification of normal modes of oscillation from observations of the solar periphery (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: The decomposition of solar oscillations into their constituent normal modes requires a knowledge of both the spatial and temporal variation of the perturbation to the Sun's surface. The task is especially difficult when only limited spatial information is available. Observations of the limb darkening function, for example, are probably sensitive to too large a number of modes to permit most of the modes to be identified in a power spectrum of measurements at only a few points on the limb, unless the results are combined with other data. A procedure was considered by which the contributions from quite small groups of modes to spatially well resolved data obtained at any instant can be extracted from the remaining modes. Combining these results with frequency information then permits the modes to be identified, at least if their frequencies are low enough to ensure that modes of high degree do not contribute substantially to the signal.
    Keywords: SOLAR PHYSICS
    Type: NASA-CR-173667 , NAS 1.26:173667
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    Nonlinear anelastic modal theory for solar convection (1983)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Solar envelope models are developed using single-mode anelastic equations as a description of turbulent convection which provide estimates for the variation with depth of the largest convective cellular flows, with horizontal sizes comparable to the total depth of the convection zone. These models can be used to describe compressible motions occurring over many density scale heights. Single-mode anelastic solutions are obtained for a solar envelope whose mean stratification is nearly adiabatic over most of its vertical extent because of the enthalpy flux explicitly carried by the big cell, while a subgrid scale representation of turbulent heat transport is incorporated into the treatment near the surface. It is shown that the single-mode equations allow two solutions for the same horizontal wavelength which are distinguished by the sense of the vertical velocity at the center of the three-dimensional cell. It is found that the upward directed flow experiences large pressure effects which can modify the density fluctuations so that the sense of the buoyancy force is changed, with buoyancy braking actually achieved near the top of the convection zone. It is suggested that such dynamical processes may explain why the amplitudes of flows related to the largest scales of convection are so weak in the solar atmosphere.
    Keywords: SOLAR PHYSICS
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    Stellar convection theory. III - Dynamical coupling of the two convection zones in A-type stars by penetrative motions (1981)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The thermal convection occurring over many density scale heights in an A-type star outer envelope, encompassing both the hydrogen and helium convectively unstable zones, is examined by means of anelastic modal equations. The single-mode anelastic equations for such compressible convection display strong overshooting of the motions into adjacent radiative zones, which would preclude diffusive separation of elements in the supposedly quiescent region between the two unstable zones. In addition, the anelastic solutions reveal that the two zones of convective instability are dynamically coupled by the overshooting motions. The two solutions that the nonlinear single-mode equations admit for the same horizontal wavelength are distinguished by the sense of the vertical velocity at the center of the three-dimensional cell. It is suggested that strong horizontal shear flows should be present just below the surface of the star, and that the large-scale motions extending into the stable atmosphere would appear mainly as horizontal flows.
    Keywords: ASTROPHYSICS
    Type: AD-A112460 , AFGL-TR-82-0089 , Astrophysical Journal; vol. 248
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    Nonlinear modal analysis of penetrative convection (1982)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: It is pointed out that thermal convection in many astrophysical and geophysical settings occurs in an unstable layer bounded above and below by regions which are stably stratified. The convective motions may extend a substantial distance into the adjacent stable zones. If the motions have little direct effect upon the mean stratification of the stable zone, then they are usually referred to as convective overshooting. The primary objective of the present investigation is to study the dynamics of overshooting at very large Rayleigh numbers, mainly with stellar applications in mind. Numerically this is only feasible if severe simplifications are made in the description of what are likely to be turbulent motions. The approach employed in the investigation utilizes nonlinear modal equations in which the vertical and temporal structure of the convection is described accurately at the expense of the horizontal structure. A summary of the salient properties of penetrative convection is provided on the basis of the results of the conducted studies.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: Geophysical and Astrophysical Fluid Dynamics; 22; 1-2,; 1982
    Format: text
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  • 6
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    Penetrative cellular convection in a stratified atmosphere (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: In the present investigation of penetrative convection within a simple compressible model, the middle one of the three layers of differing stratification prior to the onset of convection is a convectively unstable polytrope bounded above and below by two stably stratified polytropes. One- and two-mode steady solutions with hexagonal planforms have been studied for Rayleigh numbers up to aobut 1000 times critical, and for a range of Prandtl numbers, horizontal wavenumbers, and stratifications. These indicate that the penetration into the lower stable layer by downward plumes is substantially larger in a stratified medium than in a Boussinesq fluid, and produces an extended region of adiabatic stratification. The strong asymmetry between upward and downward penetration in compressible media has major implications for the mixing of stable regions above and below stellar convection zones.
    Keywords: ASTROPHYSICS
    Type: AD-A162982 , AFGL-TR-85-0326 , Astronomy and Astrophysics (ISSN 0004-6361); 140; 1
    Format: text
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    NASA TECHNICAL REPORTS
  • 7
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    On the identification of normal modes of oscillation from observations of the solar periphery (1984)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The decomposition of solar oscillations into their constituent normal modes requires a knowledge of both the spatial and temporal variation of the perturbation to the sun's surface. The task can be especially difficult when only limited spatial information is available. Observations of the limb-darkening function, for example, are probably sensitive to too large a number of modes to permit most of the modes to be identified in a power spectrum of measurements at only a few points on the limb, unless the results are combined with other data. In this paper a procedure is considered by which the contributions from quite small groups of modes to spatially well resolved data obtained at any instant can be extracted from the remaining modes. Combining these results with frequency information then permits the modes to be identified, at least if their frequencies are low enough to ensure that modes of high degree do not contribute substantially to the signal.
    Keywords: SOLAR PHYSICS
    Type: Astronomy Express (ISSN 0265-5365); 1; 9-24
    Format: text
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    NASA TECHNICAL REPORTS
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