1. Evolution of the modelling of the inside of the Sun, from “standard models” to the more empirical approach used now, and based on neutrino astronomy, solar sismology, and the interpretation of active phenomena. 2. New concepts introduced in the set of physical equations valid inside the Sun, under the pressure of new observations. 3. Similar trends seeming to affect in a comparable way the modelling of the solar cycle. 4. Other stars, and the improved knowledge of solar evolution may help to understand many observed features of rotation, magnetism, etc...Conclusion: the Sun is “one”, - and it cannot safely be studied piecemeal.

As it is impossible to approach all the problems concerning the inside of the Sun, a number of questions will not be taken into consideration during the meeting. In this brief overview of the presently unsolved questions I shall insist on some special aspects of the solar properties: the variations of the solar radius, the generation of the solar wind, some interesting effects due to the presence of a strong gradient of 3He, the history of the rotating Sun. The presence of the planetary system suggests that the Sun might have been a T Tauri star, with an accretion disc and may have started on the mains sequence as a fast rotating star. A sketch is given of the possible consequences.

The main features of standard solar models, the logic of the calculations, and some of the important results concerning solar neutrinos experiments are given. The input parameters that cause the greatest uncertainties in the calculated neutrino fluxes are the nuclear rection rates, the chemical abundances, the radiative opacity, and the equation of state. This article is based, with permission of the publisher, on Chapters 1 and 4 of Neutrino Astrophysics by J. N. Bahcall, Cambridge University Press (1989).

In large regions of densities and temperatures the properties of plasmas are influenced by Coulombic forces. Here especially hydrogen and helium plasmas are studied. The region of nonideality is given and the main effects as screening, quantum corrections, level shifts and the cut off for the sum over states are discussed. Based on methods of classical and quantum statistics corrections to the ideal EOS are calculated. Interpolational expressions covering the whole density-temperature plane are given and phase transitions to highly ionized states are discussed. Level shifts due to nonideality are derived. Finally the ionization equilibrium and kinetics are considered theoretically focussing the attention on nonideality effects.

This review discusses the current situation for opacities at the solar center, the solar surface, and for the few million kelvin temperatures that occur below the convection zone. The solar center conditions are important because they are crucial for the neutrino production, which continues to be predicted about 4 times that observed. The main extinction effects there are free-free photon absorption in the electric fields of the hydrogen, helium and the CNO atoms, free electron scattering of photons, and the bound-free and bound-bound absorption of photons by iron atoms with two electrons in the 1s bound level. An assumption that the iron is condensed-out below the convection zone, and the opacity in the central regions is thereby reduced, results in about a 25 percent reduction in the central opacity but only a 5 percent reduction at the base of the convection zone. Furthermore, the p-mode solar oscillations are changed with this assumption, and do not fit the observed ones as well as for standard models. A discussion of the large effective opacity reduction by weakly interacting massive particles (WIMPs or Cosmions) also results in poor agreement with observed p-mode oscillation frequencies. The much larger opacities for the solar surface layers from the Los Alamos Astrophysical Opacity Library instead of the widely used Cox and Tabor values show small improvements in oscillation frequency predictions, but the largest effect is in the discussion of p-mode stability. Solar oscillation frequencies can serve as an opacity experiment for the temperatures and densities, respectively, of a few million kelvin and between 0.1 and 10 g/cm3. Current oscillation frequency calculations indicate that possibly the Opacity Library values need an increase of typically 15 percent just at the bottom of the convection zone at 3×106K. Opacities have uncertainties at the photosphere and deeper than the convection zone ranging from 10 to 25 percent. The equation of state that supplies data for the opacity calculations fortunately has pressure uncertainties of only about 1 percent, but opacity uncertainties will always be much larger. A discussion is given about opacity experiments that the stars provide. Opacities in the envelopes of the Hyades G stars, the Cepheids, δ Scuti variables, and the β Cephei variables indicate that significantly larger opacities, possibly caused by iron lines, seem to be required.

In an effort to reduce uncertainties in the theoretical radiative opacities a new code has been developed at LLNL which removes several of the approximation present in past calculations. Results from the new code with comparisons to other available opacity calculations are presented as well as experiments.

This paper constitutes a brief and exceedingly elementary review of the consraints imposed on models of the solar interior by comparisons between theoretical models of evolving stars and the observed characteristics of stars, including the Sun.

The present knowledge of the dynamics and structure of the solar convection zone is reviewed with the aim of checking current assumptions and conjectures against laboratory experiments and numerical modeling of thermal convection. Buoyancy is the only forcing considered. Rotation and magnetic fields are explicitly avoided. Nor are departures from planar geometry considered, except as regards large scale structures. Local theories are reviewed in section §2, hydrodynamic models in §3, non-local theories in §4, the global structure of the convection zone is discussed in §5 and the flow patterns in §6.

This contribution discusses observational aspects of the evolution of individual structures of solar convection.

It has been shown, that mesogranulation is a convective phenomenon that fits well into the gap between granulation and supergranulation. Apparently this observation justifies the view that the three members of the granulation family represent sections of a broad continuum of convective motions spanning the range of sizes from a yet unknown fraction of 1 Mm to about 50 Mm. Nevertheless, power spectra of velocity and brightness fluctuations exhibit three maxima, separated by intervals with significantly less power near 3 Mm and 7.5 Mm. Do these gaps give reasons for reconsidering the old idea, that each of the three characteristic scales has its own source layer at a certain depth in the convection zone?

Power spectra of the granular energy distribution near the observational limit of spatial resolution suggest a continuous transfer of kinetic energy to smaller eddies by turbulent decay of the larger scale elements. Morphological studies of granular evolution and a comparison of the observed spectral line bisectors with theoretical predictions seem to disprove this idea. These observations imply either that the turbulent cascade, if it exists, is buried in the spatially unresolved part of the power distribution, or that radiative losses ultimately limit the life time of individual granules on all scales.

The confrontation between theoretical predictions and observations requires an estimate of the uncertainties of these predictions. Recent results on nuclear reaction rates and photospheric abundances are analysed, in a classical model framework. The role of opacities in the determination of the helium content, the neutrino fluxes prediction and the adiabatic sound speed is discussed. Comments on extra phenomena as mass loss, turbulent mixing and WIMPS are also presented, in the light of very recent seismological results.

A short review of some of the non–standard models proposed in these last two decades is presented. Their main physical assumptions are shown, as well as the way they meet or do not meet the various observational constraints.

Two of the outstanding problems in astrophysics are the solar neutrino problem and the missing mass problem. The “solar cosmion”, a weakly interacting massive particle, could solve both problems. Several particle physics models have been suggested for the solar cosmion.

The solar cosmion may have other interesting astrophysical effects. It will alter the predicted helioseismology spectrum, effect horizontal branch evolution and may alter the mass-radius relationship in low mass stars. These considerations constrain solar cosmion properties.

Several laboratories have begun an active experimental search for the solar cosmion. The UCSB-UCB-Saclay silicon experiment in the Oroville mine has already placed stringent limits on solar cosmions that couple to matter through spin-independent interactions. A planned Saclay experiment may either detect or rule out the existence of “solar cosmions”.

The Sun is an powerful laboratory for exploring particle physics beyond the standard model. Even if “solar cosmions” do not exist, the Sun can help “illuminate” the search for other weakly interacting particles posited as solutions to the missing mass problem.

An overview of the solar neutrino projects is given, with an emphasis on the complementarity of the different experiments (gallium, indium, heavy water,...) to solve the solar neutrino problem that was raised by the chlorine and the Kamiokande results. The separation of the different sources of neutrinos in the Sun would contribute significantly to the astrophysical understanding of the Sun. Some of the planned experiments could be able to pinpoint neutrino oscillations (within a wide range of parameters) almost independently of solar models. Projects which are particularly sensitive to a variation of the neutrino flux with time are also discussed.

A report on the results obtained from the chlorine radiochemical solar neutrino experiment in the Homestake mine, Lead, SD. Over the period 1970-1988 a neutrino capture rate of 2.3 ± 0.3 SNU was observed. This rate is discussed in relation: to the theoretical standard solar model, the results from the Kamiokande II experiment, and variations in the solar neutrino flux.

The observation of 8B solar Neutrinos in the Kamiokande-II detector is presented. Based on 450 days of data in the time period of January 1987 through May 1988, the measured flux obtained with Ee ≥ 9.3 MeV was 0.46 ± 0.13 (stat) ± 0.08 (sys) of the value predicted by the standard solar model. The detector and analysis methods were improved since June 1988 and the background level has been decreased by a factor of about three since then.

The GALLEX collaboration aims at the detection of solar neutrinos in a radiochemical experiment employing 30 tons of Gallium in form of concentrated aqueous Gallium-chloride solution. The detector is primarily sensitive to the otherwise inaccessible pp-neutrinos. Details of the experiment have been repeatedly described before [1-7]. Here we report the present status of implementation in the Laboratori Nazionali del Gran Sasso (Italy). So far, 12.2 tons of Gallium are at hand. The present status of development allows to start the first full scale run at the time when 30 tons of Gallium become available. This date is expected to be January, 1990.