The new results obtained from the observation of solar oscillations over the past decade, have a direct impact on our knowledge of the Sun's interior. As a consequence, a great interest in helioseismology has arisen and is reflected in the development of new observational projects as well as new analyse and inversion techniques. In this review we will describe the present ground-based observational programmes, which, unlike the space ones, are mostly designed to produce high quality data over very long time spans (up to solar cycle time scales). The characteristics of the various observational programmes, single-site and network, will be described together with their performances, the main results obtained up to now, and some other logistical aspects.

The Taiwan Oscillation Network (TON) is a ground-based network measuring solar intensity oscillations for the study of the internal structure of the Sun. So far, four telescopes have been installed at Teide Observatory (Tenerife), Huairou Solar Observing Station (near Beijing), Big Bear Solar Observatory (California), and Tashkent (Uzbekistan). The TON telescopes take K-line full-disk solar images of diameter 1000 pixels at a rate of one image per minute. The TON high-resolution data is specially suitable to study local helioseismology. Here, we present recent results of three topics on local helioseismology from TON data: (1) Inference of Subsurface Magnetic Field From Absorption Coefficients of p-modes in Active Regions, (2) Subsurface Structure of Emerging Flux Regions From Helioseismology, and (3) Flow Around Sunspots From Measurements of Frequency Shift.

The helioseismology started in 1960 with the discovery of oscillations of the velocity field on the solar surface by Leighton, 1960. The mechanism producing these oscillations remained unexplained for the next ten years until Ulrich, 1970, and independently Leibacher and Stein, 1971, demonstrated that acoustic waves trapped beneath the solar surface could generate such oscillations. According to that theory the Sun should oscillate globally and not only locally as seen in the pionnering observations. First Deubner, 1975, and then Claverie et al., 1979, and Grec et al., 1980, demonstrated the existence of global oscillations of intermediate and low degrees. These observations analysed in terms of k-ω diagram and power spectrum led to the famous p-modes ridges and spectrum lines which are at the base of any inference of the solar structure. All these observations where carried out from the ground. Nevertheless ground-based observations at that time (early eighties) were suffering some limitations which could be over taken from space. This is the reason why the eighties saw the intensive development both of ground-based networks and space missions for helioseismology resulting in 1995 in the existence of four networks: BISON, IRIS, GONG and TON (see Pallé, 1996) and the launch of the SOHO satellite (Domingo et al., 1995). In the next section I review the limitations of ground-based observations and those of space missions. In section 3, I describe in a chronological order the past space helioseismology missions (from 1980 to 1994). Furthermore I discuss, in section 4, the present status of space helioseismology with SOHO and finally in section 5 the co-ordination of space missions and ground-based solar observations. In the last section the future and the prospects of space helioseismology are reviewed.

GOLF is designed to measure the Global Oscillations of the integrated solar disk, by determining the line-of-sight velocity of the photosphere as a function of time, over the frequency range 10−7 to 10−2 Hz.

First results from the 4-6 months observations of the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) on the ESA/NASA Mission SOHO (Solar and Heliospheric Observatory) are reported. The time series are evaluated in terms of solar irradiance variability, solar background noise characteristics and p-mode oscillations. The solar irradiance is modulated by the passage of active regions across the disk, but not all of the modulation is straightforwardly explained in terms of sunspot flux blocking and facular enhancement. The observed p-mode frequencies are more-or-less in agreement with earlier measurements, but it is interesting to note that systematic differences seem to exist between the observations in different colours. There is also evidence that magnetic activity plays a significant role in the dynamics of the oscillations beyond its modulation of the resonant frequencies. Moreover, by comparing the amplitudes of different components of p-mode multiplets, each of which are influenced differently by spatial inhomogeneity, we have found that activity enhances excitation.

The SOI/MDI experiment on SOHO has some unique capabilities for time-distance helioseismology. The great stability of the images observed without benefit of an intervening atmosphere is quite striking. It has made it possible for us to detect the time of flight for separations of points as small as 2.4 Mm in the high-resolution mode of MDI (0.6 arcsec/pixel). This has enabled us to detect the supergranulation flow beneath the surface. Using a tomographic inversion technique, we can now study the 3-dimensional evolution of the flows near the solar surface.

The SOHO probe was successfully launched on December 2nd, 1995. The performances of the Atlas II flight, the trajectory and the final injection in the Halo orbit around the L1 Lagrangian point left on board a large amount of hydrazine, allowing the possibility for a mission extension later than the 2 planned years. The operations of the GOLF experiment started on January 16th for a period devoted to the initial tests and to the adjustments of the thermal settings. The effective solar observations started on February 18thand are still running. For the studies presented here below, the data set ends in mid-September. All tables and figures come from the compilation of the data analysis made in several institutes with different methods, and some complementary or additional results are displayed in the poster booklet published from this symposium.

The SoHO/MDI experiment generates a continous record of the solar limb brightness using 1.96″ pixels. Because there is no atmospheric blurring, these data allow measurements of solar limb brightness and shape changes with a precision that has not been achieved from the ground. The first results of a 1 month astrometric timeseries from the MDI solar structure program will be described here.

The unprecedented combination of spatial resolution and stability achieved by the Solar Oscillations Investigation/Michelson Doppler Imager on SOHO has opened up new opportunities for the analysis of solar surface oscillations of high spatial frequencies. In this regime the oscillations are essentially plane waves, amenable to the techniques of ring-diagram analysis of their three-dimensional power spectra. This approach holds the promise of measuring fluid motions and possibly magnetic fields in spatially-resolved structures within the uppermost levels of the convective envelope, a region unresolved by the global modes. Atmospheric g-modes trapped above the photosphere may also be detectable. We review the first results of plane-wave analysis of various types of SOI data and comparisons with the analyses of comparable ground-based datasets.

In the last two decades the large amount of accurate helioseismic data has open a new possibility to sound the stellar interiors and to test the stellar evolution theory. This has particularly impulsed a critical reexamination of the basic assumptions and physical inputs in solar modelling. The present status of theoretical solar models is presented and discussed in relation with the observational constraints: helioseismology, lithium depletion and measured neutrino fluxes.

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.

This paper is an interim report of our inferences about the hydrostatic structure of the Sun, following the first report of the GONG team in Science (Gough et al., 1996). That work confirms that the spherically averaged structure of the Sun is more or less in agreement with current standard solar models. However, there remain some significant deviations which we regard as important clues to the existence of dynamical phenomena which are not taken into account in standard solar modelling.

The p-mode frequencies obtained by the VIRGO instrument have been inverted to derive the solar core structure. Two sets of frequencies have been used in the inversions. The sets are derived from different time series (the second containing the first), using different procedures for fitting the spectra. The influence of the correlations between the p-mode frequencies has been implemented in the inversion procedure. The two data sets are in good agreement with each other, and show no evidence that the sun is significantly different from the latest available standard solar model.

We present a method of making a solar model based on the helioseismic data. We first invert the observed eigenfrequencies to determine the sound speed profile, and then solve the basic equations governing the stellar structure with the imposition of the determined sound-speed profile. This approach is different from that of the standard solar model in the sense that the ‘seismic’ solar model is a snapshot model of the sun constructed without any assumption about the history of the sun. We invert the data obtained at the South Pole by the Bartol/NSO/NASA group along with BISON, HLH, and LOWL data. Finally we estimate the neutrino fluxes of the seismic model.

Many observations indicate that some mixing occurs in stellar radiation zones: in massive stars, chemical elements resulting from nuclear burning in the core are detected at the surface, and in solar-type stars lithium is depleted with age. Since all mixing processes transport also momentum, the depletion of lithium should be linked with the loss of angular momentum through the stellar wind, and there are indeed signs of such a correlation in the behavior of tidally-locked binaries. Moreover, any transport process leaves its signature in the internal rotation profile, and this can help greatly in its identification. After reviewing the main transport mechanisms which have been considered so far, our present conclusion is that the uniform rotation observed in the radiative interior of the Sun is probably achieved by the action of internal waves emitted at the base of the convective envelope. It remains to be verified whether these waves contribute directly to the mixing, or whether they act only through the shaping of the rotation profile, which in turn determines the mixing through meridian circulation and turbulent diffusivity.