Internal Structure Of The Sun Research Articles

Modelling of the convective plasma dynamics in the Sun: anelastic and Boussinesq MHD systems

This work deals with the mathematical modelling and asymptotic analysis of the plasma convective dynamics in the center of the Sun. The heat produced via thermonuclear fusion in the interior of the Sun is transported towards the surface first via radiation, and finally via convection. Convection is thought to be responsible for the generation of magnetic fields and is hence a very important phenomenon to be understood in detail in order to get more insight in the internal structure of the Sun. Anelastic and Boussinesq models are formally derived here from the underlying compressible MHD models and we shall prepare the ground for our future numerical works, based on asymptotic-preserving techniques.

Observations of global oscillations of the sun in the SOKOL experiment onboard the CORONAS-PHOTON satellite

The helioseismological experiment onboard the CORONAS-PHOTON satellite is intended for the study of characteristics and the internal structure of the Sun using the solar eigenmodes spectrum obtained by the measurement of fluctuations of the solar radiation intensity. This experiment is the continuation of investigations of solar global oscillations started onboard artificial satellites CORONAS-I and CORONAS-F. Measurements of fluctuations of the solar radiation intensity in seven optical ranges—from the near ultraviolet to infrared spectral regions—are carried out by the solar photometer SOKOL (SOlnechnye KOLebaniya (Solar Oscillations)) developed at IZMIRAN. Over an instrument operation period of more than 9 months, a large volume of the scientific information (more than 500 MB) has been obtained. The primary processing of obtained data was performed, and spectra of fluctuations of the solar radiation intensity were constructed. On the basis of part of the processed information obtained by the photometer SOKOL, and data of the experiment DIFOS (Differential Oscillations of the Sun) onboard the artificial satellite CORONAS-F, the dependence of the relative amplitude of oscillations on the wavelength of the observation was determined.

The Current Status of Asteroseismology

Stellar evolution, a fundamental bedrock of modern astrophysics, is driven by the physical processes in stellar interiors. While we understand these processes in general terms, we lack some important ingredients. Seemingly small uncertainties in the input physics of the models, e.g., the opacities or the amount of mixing and of interior rotation, have large consequences for the evolution of stars. The goal of asteroseismology is to improve the description of the interior physics of stars by means of their oscillations, just as global helioseismology led to a huge step forward in our knowledge about the internal structure of the Sun. In this paper we present the current status of asteroseismology by considering case studies of stars with a variety of masses and evolutionary stages. In particular, we outline how the confrontation between the observed oscillation frequencies and those predicted by the models allows us to pinpoint limitations of the input physics of current models and improve them to a level that cannot be reached with any other current method.

Mixing a Stellar Cocktail

Waves similar to those observed in Earth9s atmosphere may strongly influence the internal structure of the Sun and other stars.

The proportion of hot and cold regions in the polar solar corona during 1957–2002

Occultation observations of the intensity of the FeXIV 530.3 nm and FeX 637.4 nm forbidden lines detected at the Kislovodsk Mountain Station during 1957–2002, indicate long-term changes in the structure of the solar corona. The monthly average intensities of green (KI530.3) and red (KI637.4) spectral lines are calculated for all latitudes (0°–90°) and for a high-latitude zone (45°–90°). A strong correlation (r = 0.91) between the green KI530.3 line intensity and the Wolf numbers is found and used to fill gaps in the observations. The ratio KI637.4/KI530.3 takes on its maximum value at the solar minimum. The KI637.4p/KI530.3p ratio in the high-latitude solar zone (45°–90°) increased by more than a factor of two during 1957–2002. This means that the fraction of cool regions in the polar corona has more than doubled over these years. We suggest that this increase in the number of cool regions is related to an increase in the area of the polar solar zones occupied by magnetic field of a single polarity at the solar minimum, and possibly to an increase in the area occupied by polar coronal holes. This is associated with long-term variations in the internal structure of the Sun.

Detectability of large-scale flows in global helioseismic data – A numerical experiment

Convective motions affect the solar p-modes by shifting their frequencies. In comparison to the frequency splitting caused by the differential rotation, this is only a small additional effect. As the spatial resolution of the inversions for the differ- ential rotation becomes better, it is important to know how these additional frequency shifts modify the splitting coefficients and how these two effects might be disentangled. Therefore we carry out a numerical experiment. We use quasi-degenerate pertur- bation theory to create frequencies of p-modes that are affected by differential rotation and by large-scale flows. The simulated frequency sets are analyzed and inverted for differential rotation. We use changes in the (l ,ν ) coverage, the multiplets, and the inversion results as diagnostics to draw conclusions about the detectability of large-scale flows in global helioseismic data. The result is a detectability limit of the order of 10ms −1 for large-scale flows in the convection zone. A sectoral poloidal flow with greater amplitude will lead to a noticeable distortion of the rotation rate, while a zonal poloidal flow with greater amplitude will lead to distorted even-a coefficients and disrupted multiplets. In this study, we search for large-scale, long-lived struc- tures in the convection zone by using global helioseismol- ogy results combined with quasi-degenerate perturbation the- ory and numerical simulations. We concentrate on changes in the (l ,ν ) coverage, the multiplets, and the inversion results and derive detectability limits for the velocity of large-scale structures. The motivation for this study is two-fold. On the one hand, the internal structure of the Sun has been determined with in- creasing accuracy within the last few years. Especially with the availability of high-quality helioseismic data the properties of the differential rotation and its depth dependence have been studied in great detail by inverting oscillation data. Because of the improving resolution of the solar interior, it is necessary to know about smaller effects of physical processes in the sun on the solar oscillations and their influences on the inversion results.

Phase Differences and Gains between Intensity and Velocity in Low‐Degree Acoustic Modes Measured bySOHO

Helioseismic instruments aboard SOHO are making possible a more accurate way of investigating the internal structure of the Sun. Making use of the different techniques and characteristics of these instruments, it is possible to measure solar oscillations as variations of the photospheric velocity (GOLF, MDI) or as irradiance and radiance fluctuations (VIRGO, MDI). Among the other advantages of observing solar oscillations simultaneously with different instruments and techniques, the study of velocity and irradiance measurements provides information on nonadiabatic effects in the radiatively cooled solar atmosphere. The thermodynamical properties of the atmosphere determine a phase shift between intensity and velocity (downward positive) oscillations of -90° in the case of an adiabatic atmosphere. Here we compute the phase differences and gains between intensity and velocity acoustic modes measured by SOHO to quantify the nonadiabatic degree of the solar atmosphere. After correcting the observed phase differences of the solar background influence, we find not exactly an adiabactic behavior, but close to it. Finally, we compare our results with three different theoretical models of the solar atmosphere, finding the best agreement with a model that includes turbulent pressure associated with convection and fluctuations of the superadiabatic temperature gradient.

Computing Travel Time in Time-Distance Helioseismology

In time-distance helioseismology, the position of the peak of the envelope of a cross-correlation function is identified as the travel time of a wave packet between two spatially separated locations on the Sun. When a cross-correlation function is a sum of two or more cross-correlation functions, the following theorem forbids this identification: THEOREM. The envelope of the analytic signal of a sum of two real functions is not equal to the sum of the envelopes of the analytic signals of the individual functions, unless (1) the ratio of the two functions is equal to the ratio of their Hilbert transforms and the sum of the product of the two functions and the product of their Hilbert transforms is nonnegative or (2) the envelopes of the two functions are disjoint. The individual functions should be isolated before the travel times are identified with their envelope peaks. Typically, in time-distance helioseismology, the envelopes are Gaussians and are never disjoint, nor do the cross-correlation functions and their Hilbert transforms automatically satisfy condition 1. This Letter derives the conditions under which condition 1 is satisfied and the conditions under which the envelope of a sum can be approximated to the sum of the individual envelopes so that the errors committed are minimal. These precautionary measures need to be taken while computing travel times from solar oscillations, so that the subsequent theoretical interpretations that derive the internal structure of the Sun can be reliable.

The seismic Sun

Helioseismic techniques allow us to probe the interior of the Sun with very high precision and in the process test the physical inputs to stellar models. The picture of the Sun that has been built in this manner may be termed “The Seismic Sun”. After a brief discussion of some of the inversion techniques used in the process, our current view of the seismic Sun shall be reviewed. What we know so far suggests that the internal structure of the Sun can be represented by a standard model, however, one which has a smoother sound-speed and abundance variation than the solar models with the usual treatment of diffusion.

First Results on it p Modes from GOLF Experiment

The GOLF experiment on the SOHO mission aims to study the internal structure of the Sun by measuring the spectrum of global oscillations in the frequency range 10-7 to 10-2 Hz. Here we present the results of the analysis of the first 8 months of data. Special emphasis is put into the frequency determination of the p modes, as well as the splitting in the multiplets due to rotation. For both, we show that the improvement in S/N level with respect to the ground-based networks and other experiments is essential in achieving a very low-degree frequency table with small errors ∼ 2 parts in 10-5). On the other hand, the splitting found seems to favour a solar core which does not rotate slower than its surface. The line widths do agree with theoretical expectations and other observations.

Theoretical Foundations of Time-Distance Helioseismology

view Abstract Citations (28) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Theoretical Foundations of Time-Distance Helioseismology D'Silva, Sydney Abstract Time-distance helioseismology (Duvall et al.) measures the signal due to solar oscillations at any two points on the surface of the Sun and cross-correlates them to obtain the time taken by the signal to travel between the two points. The travel time provides information on the solar interior through which the oscillations propagate. Traditional helioseismology, on the other hand, studies the mode structure of the power spectrum of the oscillations, which also provides information on the internal structure of the Sun. In this paper, a theoretical basis for time-distance helioseismology is presented. Its departure from traditional helioseismology is described. The theory can be applied to any dispersive or nondispersive medium. In time-distance helioseismology, it provides a method of computing theoretical cross-correlation functions from solar models for the signals of acoustic-gravity waves measured at any two points on the solar surface. One of the applications of time-distance helioseismology will be to measure subsurface flows and rotation. The theory provides a method of computing theoretical cross-correlation functions from solar models in the presence of any subsurface flow, or rotation. Publication: The Astrophysical Journal Pub Date: October 1996 DOI: 10.1086/177843 Bibcode: 1996ApJ...469..964D Keywords: SUN: INTERIOR; SUN: OSCILLATIONS; SUN: ROTATION full text sources ADS |

The Seismic Structure of the Sun

Global Oscillation Network Group data reveal that the internal structure of the sun can be well represented by a calibrated standard model. However, immediately beneath the convection zone and at the edge of the energy-generating core, the sound-speed variation is somewhat smoother in the sun than it is in the model. This could be a consequence of chemical inhomogeneity that is too severe in the model, perhaps owing to inaccurate modeling of gravitational settling or to neglected macroscopic motion that may be present in the sun. Accurate knowledge of the sun's structure enables inferences to be made about the physics that controls the sun; for example, through the opacity, the equation of state, or wave motion. Those inferences can then be used elsewhere in astrophysics.

J. Homer Lane and the Internal Structure of the Sun

J. Homer Lane and the Internal Structure of the Sun

Frequency stability of solar oscillations

Changes in the internal structure of the Sun over the 11-year magnetic activity cycle could be reflected in the eigenfrequencies of the acoustic p-modes. The first tentative experimental evidence was presented in 19841 and subsequently an analysis of ACRIM solar intensity data2 suggested a decrease of frequencies of the 5-min solar p-modes between 1980 and 1984 of ∼0.4 μHz. Recently3–6 further experimental data have provided conflicting results; frequency increases, decreases and stability have all been reported.

Oscillations and pulsations in the sun and stars (invited review)

The discovery of the solar global oscillations, and their identification to many radial and nonradial eigenmodes of the Sun, have opened a new important field of research called ‘solar seismology’, in which we may probe the internal structure of the Sun by using its oscillations. In recent years, pulsations and oscillation-related phenomena have also been discovered in many stars which were hitherto regarded as non-pulsating stars. They include white dwards (ZZ Ceti-stars), Ap-stars, early-type stars with slow and rapid rotation (53 Per and ζ Oph-stars, respectively). Developments in high spectral and high time-resolution observations will be expected to reveal variability in many other stars as well, in near future.

The Sun as an inconstant star

It has been thought for a long time that the luminosity of the Sun has remained constant since the Sun evolved into the Main Sequence stage almost 4.5 billion yr ago. However, many of recent data obtained from the isotopic analyses in the tree rings, meteoritic and lunar samples have shown that the luminosity and the activity of the Sun must have been varied for such long years. It seems that the one of the most important discoveries on the variability of the Sun is that of the Maunder Minimum (1645–1715), during which the solar activity had been extremely weak so that no sunspot had been observed for almost seventy years. Furthermore, this minimum was almost coincident with the severest period of the ‘Little Ice Age’ having covered the Earth from the early 14th to the middle 19th centuries. These results suggest a possible connection between the long-term variation of the Earth's climate and that of the solar activity. The Sun shines as emitting continuously the nuclear energy as light quanta. As well known, this energy is almost constantly being released from the thermonuclear reactions taking place in the central core of the Sun. Whenever the efficiency of these reactions changes due to some mechanisms to occur inside the Sun, the light emissivity from the Sun, namely, the Sun's luminosity, would change accordingly. Thus some change in the physical processes inside the Sun may always induce various kinds of variability as related to the rearrangement of the internal structure of the Sun. As a result of this kind of change, the Earth's climatic condition also seems to be critically influenced in association with the variation of the Sun's luminosity. Since it seems that the mean level of the solar activity for a long time, say, 100 yr, is dependent on the long-term change in the physical processes inside the Sun as related to the variation of the solar luminosity, the Earth's climatic condition may be necessarily changeable as dependent on the long-term variation of the solar activity. Some evidence is here shown by reviewing the historical records on the climatic change. A brief account is finally given on the possible origin of the inconstancy in the solar luminosity and activity.

Free oscillations of the sun and the giant planets

The current state of research on the free oscillations of the sun is reviewed. Observational data on oscillations with periods from 5 to 160 min published through the end of 1980 are described. The interpretation of these data in connection with research on the internal structure of the sun is discussed. A theory of the free oscillations in the linear adiabatic approximation is described; differential rotation is taken into account. The principles for classifying the theoretical normal modes are discussed. The problem of the excitation of the solar oscillations is outlined. The theoretical normal-mode spectra of Jupiter and Saturn are discussed.

Solar Activity in the Corona

When I began my studies of solar radio astronomy, Dr J. L. Pawsey, who then led the radio astronomy group in the Division of Radiophysics, CSIRO, explained to me that the internal structure of the Sun was ‘well understood’, thanks to a lack of conflicting observational data, but that for the observable layers of the Sun, the photosphere, chromosphere and corona, a great many mysteries remained. I am sure that he would have been amused by the recent discovery that there are not enough neutrons coming from the core of the Sun. I shall devote most of my talk to matters about which we are fairly certain, but often I will only be able to give part of the story because the details have not yet emerged from the wealth of solar mysteries.

A Model for the Internal Structure of the Sun.

A Model for the Internal Structure of the Sun.