A radiochemical 71Ga−71 Ge experiment to determine the integral flux of neutrinos from the sun has been constructed at the Baksan Neutrino Observatory in the USSR. Measurements have begun with 30 tonnes of gallium. The experiment is being expanded with the addition of another 30 tonnes. The motivation, experimental procedures, and present status of this experiment are presented.

We discuss bounds on neutrino masses using an analysis based on direct measurements, cosmological bounds, oscillation experiments, the solar neutrino puzzle and theoretical considerations on neutrino decays. We present four possible solutions for the mass range of the three neutrino flavors. We outline experiments which can distinguish among these solutions and discuss their implications for the cosmological dark matter problem.

Conditions and dynamics of the resonant neutrino conversion are described. We discuss the applications of the effect to the solar neutrinos as well as present status of the conversion inside the Sun. The influence of different matter density perturbations on the conversion is considered, and in this connection the possible effects of parametric and stochastic enhancement of the influence are remarked.

There have been several excellent reviews of observational helioseismology in recent years. These include the reviews by Harvey (1988), Libbrecht (1988), and van der Raay (1988) presented at a recent conference in Tenerife. The present effort will concentrate on the progress made on solar rotation recently.

The Sun is a resonant cavity that supports many (~ 107) modes of oscillation. The modes that are most easily observed are the acoustic or p-modes. The eigenfunctions for these modes are:

fnl(r) is the radial part of the separable eigenfunction where r is the radial coordinate measured from the center of the star. n is the number of radial nodes in the eigenfunction. Ylm (θ,ϕ) is the spherical harmonic function, where θ is the colatitude and ϕ is the longitude. The spherical harmonic degree l is the number of nodes of the spherical harmonic measured along a great circle that makes an angle with the equator. The azimuthal order m is the number of nodes around the equator. The frequency of the eigenmode, vnlm, depends on the mode. Much of our information derived about the solar interior from helioseismology comes from the measurement of these frequencies.

Helioseismology from a single ground-based observatory is severely compromised by the diurnal rising and setting of the Sun. This causes sidelobes to appear in the helioseismic power spectrum at multiples of ± 11.57 μHz from each solar line, contaminating the spectrum and rendering mode identification and frequency measurement extremely difficult. The difficulty can be overcome in three ways — observing from a fully sunlit orbit in space, observing from the Polar regions, or observing with a network of stations placed around the Earth. This paper discusses the networks that are either currently in operation or being planned. These include the Global Oscillation Network Group (GONG) project, the Birmingham network, the IRIS network of the University of Nice, and the SCLERA network of the University of Arizona. The scientific objectives and instrumentation of these networks are briefly described. Theoretical predictions for network performance are compared with actual results. The problem of merging simultaneous data from multiple instruments is discussed, as well as the relationship of the networks with the helioseismology experiments on the SOHO space mission.

IPHIR (Interplanetary Helioseismology by IRradiance measurements) is a solar irradiance experiment on the USSR planetary mission PHOBOS to Mars and its satellite Phobos. The experiment was built by an international consortium including PMOD/WRC, LPSP, SSD/ESA, KrAO and CRIP. The sensor is a three channel sunphotometer (SPM) which measures the solar spectral irradiance at 335, 500 and 865 nm with a precision of better than 1 part-per-million (ppm). It is the first experiment dedicated to the investigation of solar oscillations from space. The results presented here are from a first evaluation of data gathered during 160 days of the cruise phase of PHOBOS II, launched on July, 12th 1988. The long uninterrupted observation produces a spectrum of the solar p-mode oscillations in the 5-minute range with a very high signal-to-noise ratio, which allows an accurate determination of frequencies and line shapes of these modes.

In view of their costs, space-borne instruments should be considered only for their exclusive capabilities in helio-and asteroseismology. Space-borne high resolution spectrometers and photometers operate free of atmospheric pertubations, can be put on special orbits offering continuous (uninterrupted) observations and therefore offer the best opportunity for high signal-over-noise ratio. The recent data obtained on board the Phobos-2 mission clearly evidences this fact. The ESA-NASA SOHO observatory will be the first mission of its kind carrying a comprehensive set of instruments to analyse the gravity and acoustic modes of solar oscillations over an uninterrupted period of at least 2 years. Projects also exist to observe oscillations of the solar diameter. Long term observation of the solar constant may provide a clue to the understanding of the origins of the solar cycle. Simultaneous out of eliptic measurements may nicely complement our data set and offer unambiguous views on the asymmetries of the solar interior. Space observations are probably the only means to get access to the deep solar interior through the detections of g modes. They offer the only prospect in the exploitation of asteroseismology over a larger number of stars.

The solar oscillation frequencies provide our only means of obtaining detailed information about conditions inside the Sun. Here I give a brief overview of the relevant properties of solar models and solar oscillations, and present examples of the dependence of the oscillation frequencies on the structure of the model. Furthermore I discuss some results obtained so far from analysis of observed frequencies.

A procedure for inverting helioseismic data to determine the hydrogen abundance in the radiative interior of the sun is briefly described. Using Backus-Gilbert optimal averaging, the variation of sound speed, density and hydrogen abundance in the energy-generating core is estimated from low-degree p-mode frequencies. The result provides some evidence for there having been some redistribution of material during the sun’s main-sequence evolution. The inversion also suggests that the evolutionary age of the sun is perhaps some 10 per cent greater than the generally accepted value, and that the solar neutrino flux, based on standard nuclear and particle physics, is about 75 per cent of the standard-model value.

Solar oscillations provide a probe of the internal magnetic field of the Sun if the field has sufficient intensity. Using the oscillation data of Libbrecht, we find evidence for a 2 ± 1 megagauss quadrupole toroidal field centered at 0.7 of the solar radius which is barely beneath the base of the convection zone. This field, by its location and symmetry, may be associated with the dynamo that drives the Sun’s 22-year activity cycle.

Solar cycle variation of the frequencies and of the power of solar acoustic oscillations are investigated. Integrated sunlight data from 1977 to 1988 obtained at the Observatorio del Teide (Izaña, Tenerife), using a resonant scattering spectrophotometer, is analyzed in 60 day time strings and their power spectra are calculated from 2 to 3.8 mHz. To study the frequency variation, each power spectrum is cross-correlated with the one corresponding to the 1981 series and the shifts of the centroids of the cross-correlation peaks are calculated. The results show a clear variation in frequency of the cross-correlation peaks of −0.37 ± 0.04 μHz peak to peak as solar activity cycle goes from maximum to minimum. Moreover, this effect is found to depend on the l value of the modes, being absent for l = 0 and of 0.42 ± 0.06 μHz for l = 1. These results can be interpreted as an amplitude modulation between modes of the same multiplet, probably as a consequence of the action of strong magnetic fields. As low l modes penetrate deeply into the Sun’s interior, these observations suggest changes in its structure correlated with the solar activity cycle. When the power of the modes is calculated, using the same series as before, and its change along the solar cycle is studied, a variation of ~ 40% is found, the power being higher when solar activity is at its minimum. If this effect is independent of the l value of the p-modes, the results can be interpreted in terms of a change in the efficiency of the excitation mechanism of such modes. Indeed, if turbulent convection is such a mechanism, a change in the characteristic size of the granulation would account for the observed effect. Alternatively, another explanation could be a selective change in the efficiency of the excitation and/or damping mechanisms of the l ≤ 3 modes in front of other l value modes.

Stellar acoustic oscillation frequencies will likely be accurately observed in the near future, in analogy to the well-known solar five-minute oscillation frequencies. Of course, we will never expect the wealth of solar data, which is a result of spatial resolution. We will therefore not be able to solve the inverse problem, that is to probe physical quantities as functions of depth, and the low number of anticipated observed frequencies will make an unambiguous mode identification difficult. Despite this restriction to the forward problem, however, observed stellar oscillation frequencies will become valuable constraints for the determination of stellar parameters. One should not forget that the present knowledge of stellar ages and compositions relies on the calibration of theoretical models (matching effective temperature and luminosity). Additional observational constraints will improve these calibrations, even if the theoretical models themselves are not questioned. We hope, however, that the observation of stellar oscillation frequencies will also lead to improvements in the physics of stellar models, in analogy to the solar case. Again, of course, stellar seismologists will be less ambitious than helioseismologists, since there are more open parameters in stellar models. However, stellar observations will allow tests of models with different age and composition.

The increasing observational evidence offered by photometric and spectroscopic data of magnetic cycles in lower main sequence stars, has confirmed the general expectation that the same basic dynamo mechanism operates in the Sun and main sequence in stars with outer convective envelopes.

Unfortunately, no clear correlation has been found, up to date, with stellar parameters as mass, rotation rate and age, even if irregular activity and shorter cycle periods seem to be characteristic of stars more massive than the Sun, while hyperactive fast rotating components of binary systems like RS CVn’s and BY Dra’s show a tendency for cycles as long as several decades.

Although dynamo theory has probably captured die essential physics of the convection-rotation interaction giving rise to stellar magnetic activity, as evidenced, for instance, by the correlation between proxy activity, indicators and the Rossby parameter related to the dynamo number, the reliability of the present theoretical background should be measured by its capacity of interpreting and predicting characteristics and periodicities (or aperiodicities) of stellar cycles. This should be done in the framework of the nonlinear approach, which, in principle can describe multimodal dynamo behaviour with a variety of time scales.

The fundamentals of the theory must be tested, however, in the closest astrophysical laboratory, our Sun. Serious problems to a dynamo mechanism operating in the convection zone have been posed by most recent helioseismological results, which, on the other hand, do not rule out the possibility of dynamo action in the transition layer between the convective and die radiative zones, which is suggested independently by the global solar cycle features. Indeed, assuming the correct sign of helicity in the transition layer, the helioseismological data on the radial gradient of angular velocity support both equatorward propagation of dynamo waves at lower latitudes and poleward propagation at higher latitudes, which is evidenced by different tracers of the solar cycle.

The phenomena of solar activity are connected with a general magnetic field of-the Sun which is due to a dynamo process essentially determined by the α-effect and the differential rotation in the convection zone. A few observational facts are summarized which are important for modelling this process. The basic ideas of the solar dynamo theory, with emphasis on the mean-field approach, are explained, and a critical review of the dynamo models investigated so far is given. Although several models reflect a number of essential features of the solar magnetic cycle there are many open questions. Part of them result from lack of knowledge of the structure of the convective motions and the differential rotation. Other questions concern, for example, details of the connection of the α-effect and related effects with the convective motions, or the way in which the behaviour of the dynamo is influenced by the back-reaction of the magnetic field on the motions.

We discuss the observational data that are available to illustrate the rotational history of the Sun, and those data that are particularly pertinent to the interiors of the Sun and stars.

The possible mechanisms of angular momentum transport in convectively stable regions of a star are reviewed, with emphasis on transport by magnetic torques. The strength and configuration of the field in such layers is quite uncertain, because it is not known if the field can reach a dynamically stable configuration. A lower limit to the field strength is obtained by assuming that the field is always dynamically unstable, and decaying at the (rotation modified) dynamical time scale. The present field in the sun would then be of the order 1G, with poloidal and toroidal components of similar strength. The differential rotation in the core, if due only to the solar wind torque, would be very small for this field strength, and instead would more likely be governed by magnetic coulpling to the differential rotation of the convection zone. If small scale hydrodynamic transport mechanisms are present, their properties would also be influenced by a field of this strength.

This review focuses on the transport of matter and angular momentum in the radiative zones of stellar interiors. The two main causes of such transport are the convective overshooting in the vicinity of convection zones, and the slow motions (meridional circulation and turbulence) due to the rotation of the star. In addition, momentum can be transfered through waves (generated by the motions above) and through magnetic stresses. The characteristics of those processes are examined, with special emphasis on turbulent diffusion.

Observations of the surface abundances of lithium, beryllium and helium-3 in the Sun and in solar-type stars of different ages should be interpreted in a coherent way. The abundance of lithium at the surface of a star decreases slowly with age; for stars of the same age it decreases with mass and a dependence on the rotation velocity is suggested. The solar surface lithium is depleted by a factor of 100 relative to the cosmic abundance while an He-3 enrichment of 15% at the solar surface during evolution is suggested.

Observations favour the hypothesis of a slow transport process at work between the outer convective zone and the radiative interior of these stars. Orders of magnitude of the transport coefficient as well as its dependence upon the physical parameters can be inferred from surface abundances of light elements, but at the moment we are far from producing a completely consistent modelization.