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21.
Summary Accurate measurements of observed frequencies of solar oscillations are providing a wealth of data on the properties of the solar interior. The frequencies depend on solar structure, and on the properties of the plasma in the Sun. Here we consider in particular the dependence on the thermodynamic state. From an analysis of the equations of stellar structure, and the relevant aspects of the properties of the oscillations, we argue that in the convection zone one can isolate information about the equation of state which is relatively unaffected by other uncertainties in the physics of the solar interior. We review the different treatments that have been used to describe the thermodynamics of stellar plasmas. Through application of several of these to the computation of models of the solar envelope we demonstrate that the sensitivity of the observed frequencies is in fact sufficient to distinguish even quite subtle features of the physics of solar matter. This opens up the possibility of using the Sun as a laboratory for statistical mechanics, under conditions that are out of reach in a terrestrial laboratory. 相似文献
22.
JØrgen Christensen-Dalsgaard 《Solar physics》2004,220(2):137-168
The physics of solar and stellar oscillations determines their observable properties: frequencies, amplitudes, lifetimes,
line asymmetries and phase relations. In the solar case these quantities have been measured, often with high precision, and
much has been learned about the properties of the solar interior, and the properties of the oscillations. With recent advances
in observational techniques, such seismic investigations are now being extended to solar-like oscillations in distant stars.
I provide a brief overview of the basic properties of stellar oscillations, and of the information about stellar properties
that may be inferred from them, concentrating mostly on the low-degree modes for which information may be expected for distant
stars. In addition, I consider the current state of investigations of solar-like oscillations in other stars, and the prospects
for an improved understanding of the physics of such oscillations. 相似文献
23.
The observational basis for asteroseismology is being dramatically strengthened, through more than two years of data from the CoRoT satellite, the flood of data coming from the Kepler mission and, in the slightly longer term, from dedicated ground-based facilities. Our ability to utilize these data depends on further development of techniques for basic data analysis, as well as on an improved understanding of the relation between the observed frequencies and the underlying properties of the stars. Also, stellar modelling must be further developed, to match the increasing diagnostic potential of the data. Here we discuss some aspects of data interpretation and modelling, focusing on the important case of stars with solar-like oscillations. 相似文献
24.
We examine the influence of nonadiabatic effects on the modes of an isothermal stratified magnetic atmosphere. The present investigation is a continuation of earlier work by Hasan and Christensen-Dalsgaard (1992) and Banerjee, Hasan, and Christensen-Dalsgaard (1995, 1996), where the interaction of various elementary modes in a stratified magnetized atmosphere was studied in the purely adiabatic limit. The inclusion of radiative dissipation based on Newton's law of cooling demonstrates the importance of this effect in the study of magnetoatmospheric waves. We analyze the physical nature of magnetoacoustic gravity (or MAG) oscillations in the presence of Newtonian cooling and find that the eigenfrequency curves in the diagnostic diagram, as in the previous analysis, undergo avoided crossings. However, the qualitative nature of the mode interaction is strongly influenced by radiative dissipation, which leads to strong mode damping in the avoided-crossing regions. We demonstrate this effect for the interaction between the Lamb mode and a magnetic mode. Our results could be important in the analysis of waves in flux tubes on the Sun. 相似文献
25.
S. Turck-Chièze S. Basu A. S. Brun J. Christensen-Dalsgaard A. Eff-Darwich I. Lopes F. Pérez Hernández G. Berthomieu J. Provost R. K. Ulrich F. Baudin P. Boumier J. Charra A. H. Gabriel R. A. Garcia G. Grec C. Renaud J. M. Robillot T. Roca Cortés 《Solar physics》1997,175(2):247-265
After 8 months of nearly continuous measurements the GOLF instrument, aboard SOHO, has detected acoustic mode frequencies
of more than 100 modes, extending from 1.4 mHz to 4.9 mHz. In this paper, we compare these results with the best available
predictions coming from solar models. To verify the quality of the data, we examine the asymptotic seismic parameters; this
confirms the improvements achieved in solar models during the last decade.
Using the GOLF set of frequencies for l=0, 1, 2, 3 combined with the LOWL second year data set for l > 3 we then carry out
inversions to infer properties of the solar core.
This largely confirms the previous results down to around 0.1 R⊙, while there remain differences, even closer to the centre,
where the present study shows an extreme sensitivity of the inversion results to the values of the frequencies. We finally
consider physical processes which may influence directly or indirectly the solar core structure. 相似文献
26.
Howe R Christensen-Dalsgaard J Hill F Komm RW Larsen RM Schou J Thompson MJ Toomre J 《The Astrophysical journal》2000,534(2):L163-L166
We discuss the spectrum arising from synchrotron emission by fast cooling (FC) electrons, when fresh electrons are continually accelerated by a strong blast wave, into a power-law distribution of energies. The FC spectrum has so far been described by four power-law segments divided by three break frequencies nusa相似文献
27.
Chun-Guang Zhang Li-Cai Deng Da-Run Xiong Jrgen Christensen-Dalsgaard 《天文和天体物理学研究(英文版)》2013,(9):1127-1140
Local mixing-length theory is incapable of describing nonlocal phenomena in stellar convection, such as overshooting. Therefore standard solar models constructed with local mixing-length theory significantly deviate from the Sun at the boundaries of the convection zone, where convection becomes less efficient and nonlocal effects are important. The differences between observed and computed frequencies mainly come from the region near the surface, while the localized difference in sound speed is just below the convective envelope. We compute a solar envelope model using Xiong’s nonlocal convection theory, and carry out helioseismic analysis. The nonlocal model has a smooth transition at the base of the convection zone, as revealed by helioseismology. It reproduces solar frequencies more accurately, and reduces the localized difference in sound speed between the Sun and standard solar models. 相似文献
28.
A. H. Gabriel J. Charra G. Grec J.-M. Robillot T. Roca Cortés S. Turck-Chièze R. Ulrich S. Basu F. Baudin L. Bertello P. Boumier M. Charra J. Christensen-Dalsgaard M. Decaudin H. Dzitko T. Foglizzo E. Fossat R. A. García J. M. Herreros M. Lazrek P. L. Pallé N. Pétrou C. Renaud C. Régulo 《Solar physics》1997,175(2):207-226
GOLF in-flight commissioning and calibration was carried out during the first four months, most of which represented the cruise
phase of SOHO towards its final L1 orbit. The initial performance of GOLF is shown to be within the design specification,
for the entire instrument as well as for the separate sub-systems. Malfunctioning of the polarising mechanisms after 3 to
4 months operation has led to the adoption of an unplanned operating sequence in which these mechanisms are no longer used.
This mode, which measures only the blue wing of the solar sodium lines, detracts little from the detection and frequency measurements
of global oscillations, but does make more difficult the absolute velocity calibration, which is currently of the order of
20%. Data continuity in the new mode is extremely high and the instrument is producing exceptionally noise-free p-mode spectra.
The data set is particularly well suited to the study of effects due to the excitation mechanism of the modes, leading to
temporal variations in their amplitudes. The g modes have not yet been detected in this limited data set. In the present mode
of operation, there are no indications of any degradation which would limit the use of GOLF for up to 6 years or more. 相似文献
29.
D. Koch W. Borucki G. Basri T. Brown D. Caldwell J. Christensen-Dalsgaard W. Cochran E. Dunham T. N. Gautier J. Geary R. Gilliland J. Jenkins Y. Kondo D. Latham J. Lissauer D. Monet 《Astrophysics and Space Science》2006,304(1-4):391-395
The Kepler Mission is a photometric space mission that will continuously observe a single 100 square degree field of view (FOV) of the sky of more than 100,000 stars in the Cygnus-Lyra region for four or more years with a precision of 14 parts per million (ppm) for a 6.5 hour integration including shot noise for a twelfth magnitude star. The primary goal of the mission is to detect Earth-size planets in the habitable zone of solar-like stars. In the process, many eclipsing binaries (EB) will also be detected. Prior to launch, the stellar characteristics will have been determined for all the stars in the FOV with K<14.5. As part of the verification process, stars with transits (about 5%) will need to have follow-up radial velocity observations performed to determine the component masses and thereby separate grazing eclipses caused by stellar companions from transits caused by planets. The result will be a rich database on EBs. The community will have access to the archive for uses such as for EB modeling of the high-precision light curves. A guest observer program is also planned for objects not already on the target list. 相似文献
30.
G. Houdek W.J. Chaplin T. Appourchaux J. Christensen-Dalsgaard W. Däppen Y. Elsworth D.O. Gough G.R. Isaak R. New M.C. Rabello-Soares 《Monthly notices of the Royal Astronomical Society》2001,327(2):483-487
Measurements of both solar irradiance and p-mode oscillation frequencies indicate that the structure of the Sun changes with the solar cycle. Balmforth, Gough & Merryfield investigated the effect of symmetrical thermal disturbances on the solar structure and the resulting pulsation frequency changes. They concluded that thermal perturbations alone cannot account for the variations in both irradiance and p-mode frequencies, and that the presence of a magnetic field affecting acoustical propagation is the most likely explanation of the frequency change, in the manner suggested earlier by Gough & Thompson and by Goldreich et al. Numerical simulations of Boussinesq convection in a magnetic field have shown that at high Rayleigh number the magnetic field can modify the preferred horizontal length scale of the convective flow.
Here, we investigate the effect of changing the horizontal length scale of convective eddies on the linewidths of the acoustic resonant mode peaks observed in helioseismic power spectra. The turbulent fluxes in these model computations are obtained from a time-dependent, non-local generalization of the mixing-length formalism. The modelled variations are compared with p-mode linewidth changes revealed by the analysis of helioseismic data collected by the Birmingham Solar-Oscillations Network (BiSON); these low-degree (low- l ) observations cover the complete falling phase of solar activity cycle 22. The results are also discussed in the light of observations of solar-cycle variations of the horizontal size of granules and with results from 2D simulations by Steffen of convective granules. 相似文献
Here, we investigate the effect of changing the horizontal length scale of convective eddies on the linewidths of the acoustic resonant mode peaks observed in helioseismic power spectra. The turbulent fluxes in these model computations are obtained from a time-dependent, non-local generalization of the mixing-length formalism. The modelled variations are compared with p-mode linewidth changes revealed by the analysis of helioseismic data collected by the Birmingham Solar-Oscillations Network (BiSON); these low-degree (low- l ) observations cover the complete falling phase of solar activity cycle 22. The results are also discussed in the light of observations of solar-cycle variations of the horizontal size of granules and with results from 2D simulations by Steffen of convective granules. 相似文献