The rotation of the sun affects the wave propagation in the solar cavity. The measurement of the effect can be used to infer the internal rotation of the sun, and indeed much of what we know today about the dynamics of the solar interior has been derived from observation and interpretation of the rotationally induced splitting of solar p-mode frequencies. Inversion has proven to be a very powerful tool in such investigations, and will remain at the centre of our effort in studying solar rotation from helioseimic data. Study of the flow inside the sun is not only important in its own right but is vital for improving our understanding of solar activity and its cycle. Now that both GONG and SOHO are operational and are accumulating data, there is no doubt that we are about to learn a great deal more about the dynamical structure of our own star.

The Medium-l Program of the Michelson Doppler Imager (MDI) instrument on board SOHO provides continuous observations of oscillation modes of angular degree, l, from 0 to ∼ 300. The initial results show that the noise in the Medium-l oscillation power spectrum is substantially lower than in ground-based measurements. This enables us to detect lower amplitude modes and, thus, to extend the range of measured mode frequencies. The MDI observations also reveal the asymmetry of oscillation spectral lines. The line asymmetries agree with the theory of mode excitation by acoustic sources localized in the upper convective boundary layer. The sound-speed profile inferred from the mean frequencies gives evidence for a sharp variation at the edge of the energy-generating core. In a thin layer just beneath the convection zone, helium appears to be less abundant than predicted by theory. Inverting the multiplet frequency splittings from MDI, we detect significant rotational shear in this thin layer.

We report inferences for the Sun's internal rotation from GONG months 4-10 averaged power spectra. In keeping with the international collaborative nature of the GONG project, the results presented here are based on the work of several groups around the world inverting the GONG data and sharing their results via the world-wide web. These groups are at the Observatoire de la Côte d'Azur, Nice (T. Corbard, G. Berthomieu, J. Provost); Theoretical Astrophysics Center, Aarhus (J. Christensen-Dalsgaard, F. Pijpers); Center for Astrophysics, Cambridge MA (A. Eff-Darwich, S. Korzennik); QMW, London (R. Howe, M. Thompson, in collaboration with J. Schou, Stanford); Institute of Astronomy, Cambridge (T. Sekii, D. Gough); University of Sydney (D. Burtonclay, Li Yan, P. Wilson); and Tata Institute of Fundamental Research, Bombay (H. Antia, S. Chitre).

An overview of the multiple facets of Philippe Delache's scientific achievements is presented, from his early involvement in experimental work and space research, through his re-orientation towards radiative transfer, astrophysics, and then the study of the global Sun and its internal structure, and the study of the interior of Jupiter. The constant concern of P. Delache for a proper and correct mathematical analysis of the problems and for modelling, are outlined in this brief overview. The generous character, the profound and complex personality, as well as the human qualities of this very talented scientist, among which modesty is very prominent, are presented in this description of a remarkable career which ended much too early.

Measurements of the solar energy throughout the solar spectrum and understanding its variability provide important information about the physical processes and structural changes in the solar interior and in the solar atmosphere. Solar irradiance measurements (both bolometric and at various wavelengths) over the last two decades have demonstrated that the solar radiative output changes with time as an effect of the waxing and waning solar activity. Although the overall pattern of the long-term variations is similar in the entire spectrum and at various wavelengths, being higher during high solar activity conditions, remarkable differences exist between the magnitude and shape of the observed changes. These differences arise from the different physical conditions in the solar atmosphere where the irradiances are emitted. The aim of this paper is to discuss the solar-cycle-related long-term changes in solar total and UV irradiances. The space-borne irradiance observations are compared to ground-based indices of solar magnetic activity, such as the Photometric Sunspot Index, full disk magnetic flux, and the Mt. Wilson Magnetic Plage Strength Index. Considerable part of the research described in this paper was stimulated by the discussions with the late Philippe Delache, who will always remain in the heart and memory of the author of this paper.

The giant planets Jupiter and Saturn belong to the interesting category of possible goals for remote seismic analysis. Their first seismic observations and their analysis were attempted in 1987 and 1991 respectively, under Philippe Delache's initiative. The theoretical analysis of giant planets seismology reveals the strong signature of the dense planetary core and the tiny one of the hydrogen plasma phase transition. The asymptotic formalism makes possible to obtain pertinent information for the observation of planetary oscillations and for their analysis. Specific observational techniques were developed to detect the seismic signature of giant planets. However, the first observations (Schmider et al. 1991, Mosser et al. 1993) of Jovian oscillations remain tentative. Even if the Jovian origin of the signal is beyond doubt, the interpretation in terms of Jovian global modes remains speculative. The collision of comet SL9 onto Jupiter provided an unexpected and unique opportunity to search for oscillations excited by the cometary impacts (Mosser et al. 1996). Seismic observations of Saturn remain negative so far. Therefore, this review focuses on Jupiter. Finally, the almost 10-years long experience of seismic observations of Jupiter and Saturn has not yet provided new constraints for planetary interior models. However, guidelines for future observational projects dedicated to Jovian seismology can be drawn. The different techniques of observation are compared, and observational requirements are precisely described.

Philippe Delache brought me to solar physics in late June 1992. The first things he began with was the diameter of the Sun as it is measured at the “plateau de Calern”, and my first work was to analyze this time series in order to separate the “noise” from the pertinent information (section 2). Wet then applied the technique developed for this purpose to the study of the 11-year solar cycle as seen in the Wolf sunspot number (section 3) and I continued in collaboration with Judit Pap with the study of total irradiance and some other solar indicators which could help to understand its variations (section 4).

Helioseismology provides us with the tools to probe solar activity. So that we can consider how the solar oscillations are influenced by that activity, we first consider the phenomena that we associate with the active Sun. The surface of the Sun is not quiet but shows evidence of convection on a wide range of scales from a few hundred kilometres through to several tens-of-thousands of kilometres. The surface temperature shows signs of the convection structures with the temperature in the bright granules being some 100 K to 200 K hotter than the surrounding dark lanes. Sunspots, which are regions of high magnetic field that suppress convective flows, are clearly visible to even quite crude observations. They are several tens-of-thousands of kilometres in diameter and about 2000 K cooler than their surroundings. Ultraviolet and X-ray pictures from satellites show that the higher layers of the solar atmosphere are very non-uniform with bright regions of high activity. Contemporaneous magnetograms show that these regions are associated with sunspots. Flares - regions of magnetic reconnections - are seen at all wavelengths from X-ray through the visible to radio. They are the non-thermal component of the radio emission of the Sun. There are many other indicators of activity on the Sun.

The stochastic excitation of solar oscillations due to turbulent convection is reviewed. A number of different observational results that provide test for solar p-mode excitation theories are described. I discuss how well the stochastic excitation theory does in explaining these observations. The location and properties of sources that excite solar p-modes are also described. Finally, I discuss why solar g-modes should be linearly stable, and estimate the surface velocity amplitudes of low degree g-modes assuming that they are stochastically excited by the turbulent convection in the sun.

Time series of simultaneous high resolution filtergram observations in white light and in CaK2 have been employed to study the dynamical interdependence of the ‘low’ and the ‘high’ layers of the solar atmosphere, the 3-D propagation of perturbations in the stratification, and the excitation of p-mode and other oscillations. Impulsive and oscillatory perturbations have been isolated in the wavenumber and frequency regime, and their mutual interaction was also studied.

Several of our findings challenge interpretation and deserve further study.

1. • The occurrence of a granule can be clearly recognized in the upper chromosphere.

2. • A granule induces low brightness and low oscillation amplitudes at the CaK2 level, whereas the opposite happens at the intergranular border.

3. • It is not always evident that the ‘source’ of the perturbation is located at the lower, i.e. the photospheric layer.

4. • It is far from being clear which components of the various perturbations contribute to a net outward mechanical energy flux.

Multimode pulsations have been discovered in a large variety of main sequence stars and white dwarfs. There are abundant and accurate data on p- and g- mode frequencies in these objects. So far, however, use of these data for sounding interiors has been very limited. The main obstacle is the difficulty in identification of detected modes that is in assignment of spherical harmonic degree, l, azimuthal order, m, and radial order, n, to the modes detected in power spectra of variable stars. This has never been a problem in helioseismology. Even in the case of whole disc measurements a simple structure of power spectra enables an unambiguous mode identification. Such a situation is very rare in asteroseismology. Perhaps oscillating white dwarfs PG 1159-035 and GD 358 are the only examples. Methods of determining the l and m from observational data on the excited modes are available. However, their implementation to – typically low amplitude – multimode pulsators is difficult. In most cases mode identification cannot be separated from determination of stellar parameters.

The improvement in observational techniques to study pulsations in solarlike stars is discussed. For higher mass stars the problem is to allocate specific eigenmodes to a set of observed frequencies. Additional observables can be used to sort out the present ambiguities.

The scientific objectives and the observational strategy of asteroseismology from space are presented. The projects proposed in different contexts are briefly reviewed, with a particular emphasis on the COROT experiment, now accepted for a launch in 2001 in the framework of the french “Petites Missions” program.

The European Union Network ANTENA started to work in October 1993. During these last three years, several collaborative projects have been undertaken. ANTENA has offered a very good opportunity for most of the European people doing asteroseismology to work together. The asteroseismological networks STEPHI and STACC have run within the framework of the project, obtaining fairly good results. New instrumentation has also been developed, such as the Four-Channel Stellar Photometer.

The theoretical potential of white dwarf asteroseismology is summarized. It is shown how one can derive fundamental parameters on the internal structure and evolution of these stars. The analysis of the non-radial g-modes permits in principle to determine the total mass, the rotation rate, the magnetic field strength. The mass of the outer layers, left on top of the carbon/oxygen core, can be determined as well as the structure of the transition zone between the core and the outer layers, giving an “a posteriori” unique information on the efficiency of the previous mass loss episodes. When measurable, the rate of change of the pulsation periods gives direct access to the evolutionary time scale and to the chemical composition of the core. These theoretical expectations are compared with the observations of variable white dwarfs in the three known instability strips for the planetary nebulae nuclei and PG1159 stars, for the DB and DA white dwarfs. Emphasis is put on results obtained from multi-sites photometric campaigns. Prospects on both theoretical developments and observations conclude the review.

Ground-based observational networks are now able to reliably detect more than 20 frequencies for individual δ Scuti stars. The ground-based Delta Scuti Network specializes in the intensive study of individual stars for several months with dedicated telescopes on different continents. With the new quantity and quality of data, the observational limit to extracting 50 or more frequencies in a single δ Scuti star is no longer determined only by the signal/noise ratio, but by the available frequency resolution.

Recent results on the δ Scuti stars FG Vir and 4CVn are presented. 4CVn exhibits some of the characteristics of δ Scuti stars: some close spacings are observed, pulsations occur in specific frequency regions with relatively sharp borders, and show large amplitude variations. This presents an important asteroseismological diagnostic, and can help us to improve our understanding of for example rotation and excitation mechanisms.

We also report a new Gamma Doradus variable with two excited g modes (HD 108100, F2V).

Chemically peculiar magnetic (Ap) stars in the Instability Strip shouldn't really pulsate (since the helium in their very stable atmospheres appears to have gravitationally settled below the He II ionisation zone). And if they did pulsate, they should have periods of a few hours like the δ Scuti variables, since the Ap stars have comparable mean densities and their atmospheric peculiarities wouldn't strongly affect the global eigenfunctions of low-degree p-modes.

I have been charged to summarize the mood of the symposium, although not to summarize the scientific details, for they are best learned by reading the proceedings. To the best of my ability, I shall comment on those details in a manner that Philippe Delache might spontaneously have done had it not been incumbent upon him to cover everything. Therefore I shall be somewhat personal. I shall make no attempt to offer a balanced view of the observations and calculations that have been reported, but instead concentrate on how they might be viewed. In so doing I bear in mind that Philippe was not one to tow the party line. He often ignored the democratic means by which unfortunately so-called scientific truth is too often established today, and instead showed more interest in the unusual. Although such behaviour can sometimes lead one into danger, as it did with regard to Philippe's proposed explanation of the infamous 160-minute oscillation of the sun (Arvonny, 1983), it is also the most common road to true discovery.