The observations of solar oscillations provide an unrivalled, precise way of probing the solar interior. In this paper, I consider the observations and their interpretation in terms of the physics of the Sun. The oscillations that we are concerned with here are the so-called p modes, i.e. oscillations for which pressure is the restoring force. The modes for which gravity is the restoring force have yet to be unambiguously detected on the Sun. The observations are made either as Doppler velocity or as intensity and are, in general, very small effects. To get an impression of the precision required, consider that in integrated velocity the total signal is ~ 1 m s−1 with the strongest individual modes being about 15-20 cm s−1. The weakest, detected modes are of order a few mm s−1. When this signal is measured as a Doppler shift, v/c is a few parts in 1011. The observations are made by a variety of instruments on Earth or in Space which can be simply divided into those which observe the Sun as a star and those which image the solar surface into many pixels Although there are many different observers using many different techniques, in all cases one is analysing light emitted from a region relatively high in the atmosphere of the Sun. When one considers how these measurements can be interpreted in terms of the solar oscillations, two issues arise:

1. 1. Roughly where in the solar atmosphere are the lines formed?

2. 2. How different are the heights of formation for different lines?

Helioseismology seeks to infer the properties of the solar interior using measurements of the global normal mode oscillations as a function of spherical harmonic degree l, azimuthal order m, and radial order n as observed on the solar surface. The frequencies, v0(l, m, n), of the modes are influenced by the physical conditions of the solar plasma through which the p-mode (pressure) waves propagate, while the power, P(l, m, n), and line widths, Γ(l, m, n), provide clues about the excitation and damping of the oscillations. These mode parameters are extracted from observations of the solar surface using a long and complex data reduction procedure. It is thus important to precisely describe the steps in the reduction and to assess the influence of the choices on the resulting inferred physical conditions.

We determine the structure of the solar radiative zone with the imposition of the sound speed profile and the depth of the convection zone obtained from helioseismic analysis. We discuss the neutrino fluxes and capture rates using the resultant seismic solar model. We find that the seismic solar model cannot resolve the solar neutrino problem. The hydrogen and helium profiles of the Sun are obtained as a part of the solutions. We find that hydrogen is reduced in the core as expected in the theory of stellar evolution.

Lithium is an excellent tracer of mixing in stars as it is destroyed (by nuclear reactions) at a temperature around ~ 2.5 × 106 K. The lithium destruction zone is typically located in the radiative region of a star. If the radiative regions are stable, the observed surface value of lithium should remain constant with time. However, comparison of the meteoritic and photospheric Li abundances in the Sun indicate that the surface abundance of Li in the Sun has been depleted by more than two orders of magnitude. This is not predicted by solar models and is a long standing problem. Observations of Li in open clusters indicate that Li depletion is occurring on the main sequence. Furthermore, there is now compelling observational evidence that a spread of lithium abundances is present in nearly identical stars. This suggests that some transport process is occurring in stellar radiative regions. Helioseismic inversions support this conclusion, for they suggest that standard solar models need to be modified below the base of the convection zone. There are a number of possible theoretical explanations for this transport process. The relation between Li abundances, rotation rates and the presence of a tidally locked companion along with the observed internal rotation in the Sun indicate that the mixing is most likely induced by rotation. The current status of non-standard (particularly rotational) stellar models which attempt to account for the lithium observations are reviewed.

The axis of rotation of the Sun's surface is inclined from the normal to the ecliptic by 7°.25. Is that true also of the rotation of the rest of the Sun? Knowledge of the direction of the angular momentum is pertinent to studies of the formation of the solar system. Moreover, Bai and Sturrock (1993) have recently interpreted temporal variations in the spatial distribution of solar flares as the outcome of the interaction of the Sun's envelope with an obliquely rotating core. We report here an attempt to determine the principal seismic axes of oscillation of the dipole and quadrupole p modes from LOI data obtained as a component of the VIRGO investigation on the spacecraft SOHO. We find that formally their most likely orientation is somewhat closer to being normal to the ecliptic than is the axis of the surface rotation. However, the uncertainty in the determination well encompasses the possibility of them being parallel to the surface rotation axis, yet it does not reject (at a level marginally greater than one standard deviation) the possibility that the Sun's angular momentum is parallel to that of the rest of the solar system.

The precisely measured frequencies of solar oscillations provide us with a unique tool to probe the solar interior with sufficient accuracy. These frequencies are principally determined by the dynamical quantities like sound speed, density or the adiabatic index of the solar material and a primary inversion of the observed frequencies yields the sound speed and density profiles inside the Sun (Gough et al. 1996). The equations of thermal equilibrium enable us to determine the temperature and chemical composition profiles, but for this additional prescriptions regarding the input physics (i.e., opacities, equation of state and nuclear energy generation rate) are required (Shibahashi 1993; Antia & Chitre 1995; Shibahashi & Takata 1996; Kosovichev 1996). This information in turn can be used to calculate the neutrino fluxes, and the seismic models can thus be used to explore the possibility of an astrophysical solution to the solar neutrino problem (Roxburgh 1996; Antia & Chitre 1997).

For deriving p-mode parameters from m, v diagrammes, one has to treat correctly the statistics of the observation. The correct statistical treatment of these diagrammes was first achieved by Schou (1992) (PhD thesis, Aarhus University). Fitting p-mode spectra requires 4 major steps:

1. 1. Compute the mode leakage matrices

2. 2. Compute mode covariance matrices from the previous matrices

3. 3. Compute the noise covariance matrices

4. 4. Compute and maximize the likelihood of the observation

Here, we compare the low-degree solar p-mode frequencies returned from the analysis of two, contemporaneous, independent helioseismological data sets collected during 1996. The first comprises Doppler velocity observations of the 770-nm line of potassium, made in integrated sunlight by the six-station, terrestrial Birmingham Solar-Oscillations Network (BiSON). The second consists of irradiance distribution measurements of the solar disc, made at 500 nm, by the Luminosity Oscillations Imager (LOI), which is part of the VIRGO experiment on the ESA/NASA SOHO satellite. The starting date is 27 March 96. The power spectra corresponds to an 8-month time series. Shorter time series of 4 months were also used to verify the starting dates and the temporal behaviour of the p modes.

BiSON currently consists of four fully- and two semi-automated observatories, dedicated to the round-the-clock collection of low-degree (low-l) helioseismological data. A complete historical record of the network can be found in Chaplin et al. (1996). The annual duty cycles achieved by BiSON have improved steadily since our sixth site was commissioned in 1992. Over the period 1992 – 1996 inclusive, we have recorded annual duty cycles of: 54. 68. 78. 74 and 78 per cent respectively.

The Global Oscillation Network Group (GONG) is an international, community-based helioseismology project conducting a detailed study of the internal structure and dynamics of the Sun. GONG is operating a six-station network of full-disk solar imagers exploring the range from ℓ = 0 to about ℓ = 200. Full network operation began in October 1995. The observation duty cycle has averaged about 87% over the first 684 days of full operation, and single-site data loss due to instrument down time is less than 2%. The data management group is processing the network data at pace with the collection rate. Here we present a comparison of spectra obtained in Doppler velocity, total spectral intensity, and modulation (a mixture of equivalent width and magnetic field strength).

The aim of the present work is the detection of solar g-modes, making use of their spatial and temporal properties, by means of a new observational strategy. The basic data, gathered at the Observatorio del Teide in 1993, consists on daily solar velocity measurements taken continuous and sequentially at six different and symmetric positions on the solar disk. By correlating the time series obtained from different positions, and considering the geometrical properties of different modes (l, m) on the Sun‘s surface, some of them can selectively be eliminated or enhanced. In particular, the main spectral features present in the resulting power spectra must have precise phase relations if they correspond to global solar g-modes.

This poster illustrates the calibration procedure and the analysis pipeline developed for the helioseismology data acquired with the VAMOS (Velocity And Magnetic Observations of the Sun) instrument. The VAMOS, based on the MOF (Magneto-Optical Filter) technology, and its operation are discussed in detail elsewhere (Cacciani et al., 1997 and Moretti et al., 1997).

We have used a differential technique to look for the signature of g-modes at the limb of full-disk intensity images of the Sun. Our spectra show tentative evidence for a set of peaks that are equally spaced in period, as predicted by asymptotic theory. The period spacing that we find is ~ 27.5 minutes. If interpreted as ℓ = 1 g-modes, this implies To ~ 39.0 minutes.

The frequencies of solar p-modes are usually computed by fitting Lorentzian profiles to the observed signals’ Fourier spectra. We offer a new technique to estimate mode parameters: direct modeling by autoregressive stochastic processes, in conjunction with Singular Spectrum Analysis.

Although the rate of production of nuclear energy in the sun's core almost certainly increases smoothly on time scales shorter than and comparable to the solar age, the mechanisms transporting this energy to the photosphere may be subject to more erratic fluctuations. Indeed space observations establish that the total solar irradiance in the direction of the earth is variable on time scales of days to years. I discuss energy transport by convection and magnetic fields with consideration of the possibility of energy storage through changes in the solar radius. Convection is the primary means of energy transport below the solar surface and must be involved in any modulation of energy flow. The large scale and long duration properties of solar convection arising from zones below the solar surface can be well studied using the Michelson-Doppler Imager instrument on SOHO. New evidence of long duration convective structures based on the MDI data is presented. These patterns appear to be related to the torsional oscillations observed at the Mt. Wilson Observatory.

The internal structure of stars is governed by hydrostatic support, the distribution of the chemical elements, the transport of energy by radiation and convection, and the liberation of energy by nuclear reactions. The evolution of stars is primarily determined by the changing composition due to the nuclear burning of elements in the central parts of the star, and the redistribution of the products of these reactions by mixing processes. The dominant mixing process is convection: it governs the extent of the mixed cores in moderate and large mass main sequence stars and their subsequent evolution, it mixes nuclear processed material into the envelopes of giants affecting the composition of material ejected into the interstellar medium, thereby affecting the chemical (and luminosity) evolution of galaxies. Understanding convection is essential if one is to understand the evolution of stars. Here I am concerned with convection in stellar cores and in particular with the extension of these cores by the penetration of convective motions into the surrounding stable layers affecting the internal structure and enlarging the chemically mixed region, which in turn affects the subsequent evolution. I briefly discuss a number of approaches to this problem: isochrone fitting of clusters and binary stars; simple theoretical models, the integral constraint, numerical simulation and what we can hope to get from asteroseismological observations of individual stars and of clusters and stellar groups.

We determine the structure of the solar convective envelope by solving the basic equations for mass conservation and hydrostatic equilibrium with the imposition of the sound-speed profile determined from helioseismology and the equation of state. The solution is required to match with the structure of the radiative core, which is also determined with the imposition of the sound speed profile. The helium abundance is obtained as a part of the solutions.

We study the influence of the structure and physics of the uppermost superadiabatic layers on the oscillation spectrum of p-modes. These modes are trapped inside the Sun by reflecting at the upper layers of the Sun, so these layers are important for the oscillation spectrum. Indeed, the main difference between the computed and observed frequencies seems to come from these layers (Christensen-Dalsgaard, 1985; Christensen-Dalsgaard and Thompson, 1997).

Measurements of the total solar irradiance (TSI) during the last 18 years from spacecraft are reviewed. Corrections are determined for the early measurements made by the HF radiometer within the ERB experiment on NIMBUS7 and the factor to refer ACRIM II to the ACRIM I irradiance scale. With these corrections a composite TSI is constructed for the period from 1978-1997. This time series is compared with a model that combines a magnetic brightness proxy with observed sunspot darkening and explains nearly 90% of the observed short and longterm variance. Possible, but still unverified degradation of the radiometers hampers conclusions about irradiance changes on decadal time scales and longer.