共查询到20条相似文献，搜索用时 562 毫秒
1.
A procedure is outlined for estimating the influence of large-scale convective eddies on the wave patterns of five-minute oscillations of high degree. The method is applied to adiabatic oscillations, with frequency ω and wave number k, of a plane-parallel polytropic layer upon which is imposed a low-amplitude convective flow. The distortion to the k - ω relation has two constituents: one depends on the horizontal component of the convective velocity and has a sign which depends on the sign of ω/k; the other depends on temperature fluctuations and is independent of the sign of ω/k. The magnitude of the distortion is just at the limit of present observational sensitivity. Thus there is reasonable hope that it will be possible to reveal some aspects of the large-scale flow in the solar convection zone. 相似文献
2.
In local helioseismology, numerical simulations of wave propagation are useful to model the interaction of solar waves with perturbations to a background solar model. However, the solution to the linearised equations of motion include convective modes that can swamp the helioseismic waves that we are interested in. In this article, we construct background solar models that are stable against convection, by modifying the vertical pressure gradient of Model S (Christensen-Dalsgaard et al., 1996, Science 272, 1286) relinquishing hydrostatic equilibrium. However, the stabilisation affects the eigenmodes that we wish to remain as close to Model S as possible. In a bid to recover the Model S eigenmodes, we choose to make additional corrections to the sound speed of Model S before stabilisation. No stabilised model can be perfectly solar-like, so we present three stabilised models with slightly different eigenmodes. The models are appropriate to study the f and p 1 to p 4 modes with spherical harmonic degrees in the range from 400 to 900. Background model CSM has a modified pressure gradient for stabilisation and has eigenfrequencies within 2% of Model S. Model CSM_A has an additional 10% increase in sound speed in the top 1 Mm resulting in eigenfrequencies within 2% of Model S and eigenfunctions that are, in comparison with CSM, closest to those of Model S. Model CSM_B has a 3% decrease in sound speed in the top 5 Mm resulting in eigenfrequencies within 1% of Model S and eigenfunctions that are only marginally adversely affected. These models are useful to study the interaction of solar waves with embedded three-dimensional heterogeneities, such as convective flows and model sunspots. We have also calculated the response of the stabilised models to excitation by random near-surface sources, using simulations of the propagation of linear waves. We find that the simulated power spectra of wave motion are in good agreement with an observed SOHO/MDI power spectrum. Overall, our convectively stabilised background models provide a good basis for quantitative numerical local helioseismology. The models are available for download from http://www.mps.mpg.de/projects/seismo/NA4/ . 相似文献
3.
4.
The paper contains a numerical simulation of the nonlinear coupling between the kinetic Alfvén wave and the ion acoustic wave for an intermediate β-plasma (m e/m i?β?1). For this study, we have introduced the nonlinear ponderomotive force (due to the finite frequency (ω 0<ω ci) kinetic Alfvén wave) in the derivation of the ion acoustic wave. The main aim of the present paper is to study the nonlinear effects associated with the different driving finite frequencies (ω 0<ω ci) of the pump kinetic Alfvén wave on the formation of localized structures and a turbulent spectrum applicable to the solar wind around 1 AU. As a result, we found that the different driving frequencies of the pump kinetic Alfvén wave affect the formation of the localized structures. We have also studied the turbulent scaling which follows (～k ?3.6) for ω 0/ω ci≈0.2, (～k ?3.4) for ω 0/ω ci≈0.3 and (～k ?3.2) for ω 0/ω ci≈0.4, at small scales. Further, we have also found that different finite driving frequencies of the pump kinetic Alfvén wave affect the turbulence scaling at small scales, which may affect the heating of the plasma particles in solar wind. The present study is correlated with the observation made by the Cluster spacecraft for the solar wind around 1 AU. 相似文献
5.
An attempt is made to infer the structure of the solar convection zone from observedp-mode frequencies of solar oscillations. The differential asymptotic inversion technique is used to find the sound speed in
the solar envelope. It is found that envelope models which use the Canuto-Mazzitelli (CM) formulation for calculating the
convective flux give significantly better agreement with observations than models constructed using the mixing length formalism.
This inference can be drawn from both the scaled frequency differences and the sound speed difference. The sound speed in
the CM envelope model is within 0.2% of that in the Sun except in the region withr > 0.99R
⊙. The envelope models are extended below the convection zone, to find some evidence for the gravitational settling of helium
beneath the base of the convection zone. It turns out that for models with a steep composition gradient below the convection
zone, the convection zone depth has to be increased by about 6 Mm in order to get agreement with helioseismic observations. 相似文献
6.
7.
A. G. W. Cameron 《Astrophysics and Space Science》1975,32(1):215-229
A number of white dwarf models have been calculated which correspond to various radial and nonradial modes of vibration with eigenfrequencies in agreement with the observed pulsation frequencies of the X-ray sources Hercules X-1 and Centaurus X-3. Most of the white dwarf models have hot interiors, but for calculational purposes these were simplified so that the bulk of the interior was isothermal, and the surface layers were designed to produce an energy generation rate of 1037 erg s?1 and to transport this energy continuously to the surface by radiative transfer. Cold white dwarfs have a fairly large spread of masses corresponding to the different overtone modes with the given eigenfrequencies, but in the hot models this spread of masses is greatly reduced, for both radial and nonradial modes. It is concluded that if the pulsating X-ray sources are hot white dwarfs, the mass of Cen X-3 probably lies in the range 0.7–1.2M ⊙, and the mass of Her X-1 probably lies in the range 1.1–1.25M ⊙ (in accord with observation). 相似文献
8.
We study high-order acoustic modes which reside in the outer layers of the solar interior. Magnetic field effects are not taken into account in this paper as we wish first to filter out how the modal frequencies depend on physical characteristics of a particular model structure of the Sun. In particular, we are interested in how the modal frequencies of solar global oscillations depend on the thickness of the convection layer and on the temperature gradient of the solar interior below. The model we use consists of three planar layers: an isothermal atmosphere, while the convection layer and the interior have temperature gradients that are adiabatic and sub-adiabatic, respectively. The presence of a convection layer with a finite thickness brings in additional modes while the variations in temperature gradient of the interior cause shifts in eigenfrequencies that are more pronounced for the p modes than for the g modes. These shifts can easily be of the order of several hundreds of Hz, which is much larger than the observational accuracy. 相似文献
9.
Two-dimensional power spectra of solar five-minute oscillations display prominent ridge structures in (k, ω) space, where k is the horizontal wavenumber and ω is the temporal frequency. The positions of these ridges in k and ω can be used to probe temperature and velocity structures in the subphotosphere. We have been carrying out a continuing program of observations of five-minute oscillations with the diode array instrument on the vacuum tower telescope at Sacramento Peak Observatory (SPO). We have sought to establish whether power spectra taken on separate days show shifts in ridge locations; these may arise from different velocity and temperature patterns having been brought into our sampling region by solar rotation. Power spectra have been obtained for six days of observations of Doppler velocities using the Mgi λ5173 and Fei λ5434 spectral lines. Each data set covers 8 to 11 hr in time and samples a region 256″ × 1024″ in spatial extent, with a spatial resolution of 2″ and temporal sampling of 65 s. We have detected shifts in ridge locations between certain data sets which are statistically significant. The character of these displacements when analyzed in terms of eastward and westward propagating waves implies that changes have occurred in both temperature and horizontal velocity fields underlying our observing window. We estimate the magnitude of the velocity changes to be on the order of 100 m s-1; we may be detecting the effects of large-scale convection akin to giant cells. 相似文献
10.
The continuous high spatial resolution Doppler observation of the Sun by the Solar Dynamics Observatory/Helioseismic and Magnetic Imager allows us to compute a helioseismic k–ω power-spectrum diagram using only oscillations inside a sunspot. Individual modal ridges can be clearly seen with reduced power in the k–ω diagram that is constructed from a 40-hour observation of a stable and round sunspot. Comparing this with the k–ω diagram obtained from a quiet-Sun region, one sees that inside the sunspot the f-mode ridge is more reduced in power than the p-mode ridges, especially at high wavenumbers. The p-mode ridges all shift toward lower wavenumber (or higher frequency) for a given frequency (or wavenumber), implying an increase of phase velocity beneath the sunspot. This is probably because the acoustic waves travel across the inclined magnetic field of the sunspot penumbra. Line-profile asymmetries exhibited in the p-mode ridges are more significant in the sunspot than in the quiet Sun. Convection inside the sunspot is also highly suppressed, and its characteristic spatial scale is substantially larger than the typical convection scale of the quiet Sun. These observational facts demand a better understanding of magnetoconvection and interactions of helioseismic waves with magnetic field. 相似文献
11.
In this paper we study the dependence on depth and latitude of the solar angular velocity produced by a meridian circulation in the convection zone, assuming that the main mechanism responsible for setting up and driving the circulation is the interaction of rotation with convection. We solve the first order equations (perturbation of the spherically symmetric state) in the Boussinesq approximation and in the steady state for the axissymmetric case. The interaction of convection with rotation is modelled by a convective transport coefficient k c = k co + ?k c2 P 2(cos θ) where ? is the expansion parameter, P 2 is the 2nd Legendre polynomial and k c2 is taken proportional to the local Taylor number and the ratio of the convective to the total fluxes. We obtain the following results for a Rayleigh number 103 and for a Prandtl number 1:
- A single cell circulation extending from poles to the equator and with circulation directed toward the equator at the surface. Radial velocities are of the order of 10 cm s?1 and meridional ones of the order of 150 cm s?1.
- A flux difference between pole and equator at the surface of about 5 percent, the poles being hotter.
- An angular velocity increasing inwards.
- Angular velocity constant surfaces of spheroidal shape. The model is consistent with the fact that the interaction of convection with rotation sets up a circulation (driven by the temperature gradient) which carries angular momentum toward the equator against the viscous friction. Unfortunately also a large flux variation at the surface is obtained. Nevertheless it seems that the model has the basic requisites for correct dynamo action.
12.
Ts. G. Tsvetkov 《Astrophysics and Space Science》1983,89(2):435-445
Models of Delta Scuti stars are tested against radial pulsations in the four lowest modes, using a linear, nonadiabatic approximation. It is shown that the theoretical blue edge locations of the instability strip on the H-R diagram depend strongly (ΔlogT e?0.03) on both opacity interpolation and variations of the envelope helium abundanceY. Several other factors have small effect (Δ logT e?0.01): variations of the heavy element abundanceZ, the use of various opacity tables, the treatment of convection, and the number of envelope mass zones. A comparison with both results obtained by various authors and observations is performed. The locations of the observed blue edge and our theoretical third-harmonic ones are consistent. A nonlinear dependence of the theoretical blue edge locations on envelope helium abundanceY is derived. A star withY?0.2 may pulsate within the instability strip, if it is not near the blue edge. 相似文献
13.
Analytical models of solar atmospheric magnetic structures have been crucial for our understanding of magnetohydrodynamic (MHD) wave behaviour and in the development of the field of solar magneto-seismology. Here, an analytical approach is used to derive the dispersion relation for MHD waves in a magnetic slab of homogeneous plasma enclosed on its two sides by non-magnetic, semi-infinite plasma with different densities and temperatures. This generalises the classic magnetic slab model, which is symmetric about the slab. The dispersion relation, unlike that governing a symmetric slab, cannot be decoupled into the well-known sausage and kink modes, i.e. the modes have mixed properties. The eigenmodes of an asymmetric magnetic slab are better labelled as quasi-sausage and quasi-kink modes. Given that the solar atmosphere is highly inhomogeneous, this has implications for MHD mode identification in a range of solar structures. A parametric analysis of how the mode properties (in particular the phase speed, eigenfrequencies, and amplitudes) vary in terms of the introduced asymmetry is conducted. In particular, avoided crossings occur between quasi-sausage and quasi-kink surface modes, allowing modes to adopt different properties for different parameters in the external region. 相似文献
14.
A strong coupling between convection and pulsations is known to play a major role in the disappearance of unstable modes close to the red edge of the classical Cepheid instability strip. As mean-field models of time-dependent convection rely on weakly-constrained parameters (see, e.g., Baker in Physical Processes in Comets, Stars and Active Galaxies, p. 105, 1987), we tackle this problem by the means of 2-D Direct Numerical Simulations (DNS) of the κ-mechanism with convection. Using a linear stability analysis, we first determine the physical conditions favourable for the κ-mechanism to occur inside a purely-radiative layer. Both the instability strips and the nonlinear saturation of unstable modes are then confirmed by the corresponding DNS. We next present the new simulations with convection, where a convective zone and the driving region overlap. The coupling between the convective motions and acoustic modes is then addressed by using projections onto an acoustic subspace. 相似文献
15.
L. S. Marochnik 《Astrophysics and Space Science》1983,89(1):61-75
The solar system's position in the Galaxy is an exclusive one, since the Sun is close to the corotation circle, which is the place where the angular velocity of the galactic differential rotation is equal to that of density waves displaying as spiral arms. Each galaxy contains only one corotation circle; therefore, it is an exceptional place. In the Galaxy, the deviation of the Sun from the corotation is very small — it is equal to ΔR/R ⊙≈0.03, where ΔR=R c ?R ⊙,R c is the corotation distance from the galactic center andR ⊙ is the Sun's distance from the galactic center. The special conditions of the Sun's position in the Galaxy explain the origin of the fundamental cosmogony timescalesT 1≈4.6×109 yr,T 2?108 yr,T 3?106 yr detected by the radioactive decay of various nuclides. The timescaleT 1 (the solar system's ‘lifetime’) is the protosolar cloud lifetime in a space between the galactic spiral arms. The timescaleT 2 is the presolar cloud lifetime in a spiral arm.T 3 is a timescale of hydrodynamical processes of a cloud-wave interaction. The possibility of the natural explanation of the cosmogony timescales by the unified process (on condition that the Sun is near the state of corotation) can become an argument in favour of the fact that the nearness to the corotation is necessary for the formation of systems similar to the Solar system. If the special position of the Sun is not incidental, then the corotation circles of our Galaxy, as well as those of other galaxies, are just regions where situations similar to ours are likely to be found. 相似文献
16.
本文采用改进后的平行Shooting方法，即连续正交归一化方法，通过增加系统的秩和把微分方程从线性变成非线性的方法，使得病态方程变成非病态方程，克服了平行Shooting方法（不连续正交归一化方法）的主要困难：一是选择径向网格点数目的问题，如果被检验振动模的径向节点数大或小，所需要的网格点数则多或少；二是在远离阈值的时候，尚未有合适的方法提高计算精度的问题．数值计算出了标准太阳模型的绝热非径向太阳振动的理论本征频率谱．从数值计算的结果发现改进后的平行Shooting方法比线性化的Henyey方法和平行Shooting方法收敛性更好且精度更高．其误差可以达到1％的精度． 相似文献
17.
A. J. R. Prentice 《Earth, Moon, and Planets》1978,19(3):341-398
A theory for the origin of the solar system, which is based on ideas of supersonic turbulent convection and indicates the possibility that the original Laplacian hypothesis may by valid, is presented. We suggest that the first stage of the Sun's formation consisted of the condensation of CNO ices (i.e. H2O, NH3, CH4,...) and later H2, including He as impurity atoms, at interstellar densities to from a cloud of solid grains. These grains then migrate under gravity to their common centre of mass giving up almost two orders of magnitude of angular momentum through resistive interaction with residual gases which are tied, via the ions, to the interstellar magnetic field. Grains rich in CNO rapidly dominate the centre of the cloud at this stage, both giving up almost all of their angular momentum and forming a central chemical inhomogeneity which may account for the present low solar neutrino flux (Prentice, 1976). The rest of the grain cloud, when sufficiently compressed to sweep up the residual gases and go into free fall, is not threatened by rotational disruption until its mean size has shrunk to about the orbit of Neptune. When the central opacity rises sufficiently to halt the free collapse at central density near 10?13 g cm?3, corresponding to a mean cloud radius of 104 R ⊙, we find that there is insufficient gravitational energy, for the vaporized cloud to acquire a complete hydrostatic equilibrium, even if a supersonic turbulent stress arising from the motions of convective elements becomes important, as Schatzman (1967) has proposed. Instead we suggest that the inner 3–4% of the cloud mass collapses freely all the way to stellar size to release sufficient energy to stabilize the rest of the infalling cloud. Our model of the early solar nebula thus consists of a small dense quasi-stellar core surrounded by a vast tenuous but opaque turbulent convective envelope. Following an earlier paper (Prentice, 1973) we show how the supersonic turbulent stress \((\rho _t v_t ^2 ) = \beta \rho GM(r)/r\) , where β is called the turbulence parameter, ρ is the gas density andM(r) the mass interior to radiusr causes the envelope to become very centrally condensed (i.e. drastically lowers its moment-of-inertia coefficientf) and leads to a very steep density inversion at its photosurface, as well as causing the interior to rotate like a solid body. As the nebula contracts conserving its angular momentum the ratio θ of centrifugal force to gravitational force at the equator steadily increases. In order to maintain pressure equilibrium at its photosurface, material is extruded outwards from the deep interior of the envelope to form a dense belt of non-turbulent gases at the equator which are free of turbulent viscosity. If the turbulence is sufficiently strong, we find that when θ→1 at equatorial radiusR e=R0, corresponding to the orbit of Neptune, the addition of any further mass to the equator causes the envelope to discontinuously withdraw to a new radiusR e>R0, leaving behind the circular belt of gas at the Kepler orbitR 0. The protosun continues to contract inwards, again rotationally stabilizing itself by extruding fresh material to the equator, and eventually abandoning a second gaseous ring at radiusR 1, and so on. If the collapse occurs homologously the sequence of orbital radiiR n of the system of gaseous Laplacian rings satisfy the geometric progression $$R_n /R_{n + 1} = [1 + m/Mf]^2 = constant, n = 0, 1,2, \ldots ,$$ analogous to the Titius-Bode Law of planetary distances, wherem denotes the mass of the disposed ring andM the remaining mass of the envelope. Choosing a ratio of surface to central temperature for the envelope equal to about 10?3 and adjusting the turbulence parameter β～~0.1 so thatR n/Rn+1 matches the observed mean ratio of 1.73, we typically findf=0.01 and that the rings of gas each have about the same mass, namely 1000M ⊕ of the solar material. Detailed calculations which take into account non-homologous behaviour resulting from the changing mass fraction of dissociated H2 in the nebula during the collapse do not appreciably disturb this result. This model of the contracting protosun enables us to account for the observed physical structure and mass distribution of the planetary system, as well as the chemistry. In a later Paper II we shall examine in detail the condensation of the planets from the system of gaseous rings. 相似文献
18.
S. Nawar 《Earth, Moon, and Planets》1983,29(2):99-105
With a sensitive photoelectric photometer, observations of the sky twilight brightness have been carried out at different positions in the sky during high solar activity period. The measurements have been obtained using blue and red wide band glass filters centered at 4410 and 7900 Å, respectively. The variation of the (B-R) colour index of the sky twilight with Sun's depression have been investigated at different altitudes in the sky above the horizon and various bearing angles from the solar vertical. 相似文献
19.
Hiromoto Shibahashi 《Solar physics》1991,133(1):117-125
We devise two asymptotic inversion methods of inferring the density distribution in the outer part of the Sun from the p-mode frequency spectrum of the Sun. One of them is based on an integral equation of nonlinear inversion, while the other reduces the problem to a linear form which is derived from comparison of the observed eigenfrequencies of the true Sun and the theoretical eigenfrequencies of a solar model. 相似文献
20.
The stability of linear convective and acoustic modes in solar envelope models is investigated by incorporating the thermal and mechanical effects of turbulence through the eddy transport coefficients. With a reasonable value of the turbulent Prandtl number it is possible to obtain the scales of motion corresponding to granulation, supergranulation and the five-minute oscillations. Several of the acoustic modes trapped in the solar convection zone are found to be overstable and the most unstable modes, spread over a region centred predominantly around a period of 300 s with a wide range of horizontal length scales, are in reasonable accord with the observed power-spectrum of the five-minute oscillations. It is demonstrated that these oscillations are driven by a simultaneous action of the -mechanism and the radiative and turbulent conduction mechanisms operating in the strongly superadiabatic region in the hydrogen ionization zone, the turbulent transport being the dominant process in overstabilizing the acoustic modes. 相似文献