The number of pages allocated to the commission report has been very limited and certainly not sufficient to cover in any exhaustive manner the wide range of topics relevant to cosmology and to provide also extensive bibliographies. Because of the vast amount of material to be covered, the report is based on a number of contributions from different colleagues who have been asked to highlight the main trends in the triennium (mid 1984 - mid 1987), together with a list of references sufficiently comprehensive to serve as a guideline for further reading. Unfortunately, two of the expected contributions did not reach me in time for inclusion in the report, and consequently topics such as the large scale structure and streaming motions, the clusters of galaxies and the counts of extragalactic radio sources are not included. However, it is my understanding that a large portion, if not all, of these topics will be covered in the reports of Commissions 28 and 40, and if true, this will at least avoid unnecessary overlaps. It should also be mentioned here that several proceedings of very recent IAU conferences provide excellent, updated and exhaustive reviews of the research work relevant to cosmology.

In recent years, the research on the very early universe has shown quite remarkable developments. As is well known, this development was brought about by the introduction of the Grand Unified Theories (GUTs) into cosmology. These theories have not only enabled us to trace the evolution of the Universe back to the very early stage at temperatures of 1016 GeV or higher, but also introduced various new aspects into cosmology, such as baryogenesis, phase transitions and topological defects (monopoles, etc.). In particular, inflation, which grew out of the study of GUT phase transition, is the most important and fascinating outcome.

Primordial nucleosynthesis which is responsible for the formation of the lightest elements (D, 3He, 4HE and 7Li) might be as important as the overall expansion of the Universe and the cosmic background radiation to prove the occurrence of a dense and hot phase for the Unvierse about 15 billion years ago. As recalled in many reviews (e.g. refs. 1, 2) the standard Big Bang nucleosynthesis leads to two important conclusions regarding (i) a limitation of the baryonic density such that the corresponding cosmological parameter ΩB ≤ 0.1; (ii) a limitation of the number of neutrino flavours to 3-4 consistent with the results concerning the widths of the Z0 and W± particles3.

a) Spectrum. A collaboration between US and Italian groups performed accurate observations at five wavelengths. The experiment was particularly conceived to achieve the highest possible relative accuracy, allowing an effective search for spectral distortions. The Berkeley and the Milano groups further improved the spectral coverage.

At the beginning of this review period a number of arguments were put forward against the neutrino model which became popular in 1980-1983: too high a rate of the structure evolution at the non-linear stage and the same difficulty in the galaxy formation. As a consequence, many other schemes of the structure origin have been elaborated: models with “cold” particles, with unstable missing mass, etc. In these models the missing mass is in the form of weakly interacting particles (axion, photino, gravitino, etc.), or of usual particles (e.g., neutrino) but with properties that are out of the ordinary (e.g. instability). However, the standard neutrino model cannot yet be regarded as rejected, the more so in view of the recent data on the large-scale peculiar velocities.

For many years now it has been known that the number vs. magnitude counts of quasars is the prima facie evidence of a cosmological evolution of this class of objects. Down to an apparent magnitude B - 19 the number of quasars increases by almost a factor 8 per magnitude Interval, compared to a factor 4 obtained in an Euclidean universe filled with a uniform distribution of sources, and correspondingly less for the classical Friedmann models of Gen. Relativity due to the red-shift effects. This strong evolution has been recently questioned because of several biases which may artificially steepen the slope of the counts in the optical surveys. Deep surveys of quasars selected via multi-colour techniques down to B - 23 have confirmed the long standing inference that the number count relationship must flatten beyond B - 20. As a consequence, pure density evolution models, where only the number density of quasars increases with the redshift z, leaving unchanged the shape of the local luminosity function, are ruled out, since too many quasars are predicted at faint magnitudes compared to the dramatic flattening of the counts (see, e.g., ref. 4).

The european X-ray observatory (EXOSAT), which was launched in 1983 and which finished operations in April 1986, has brought a rich harvest of results in the period 1984-1987, surveyed here. The EXOSAT payload consisted of three sets of instruments: two low energy imaging telescopes (LE:E<2 KeV), a medium-energy experiment (ME:E=l-50KeV) and a gas scintillation proportional counter (GSPC:E=2-20KeV). Over most of the energy range covered, EXOSAT was not more sensitive than its predecessor, the american EINSTEIN satellite. But the EINSTEIN satellite is far from having exhausted the treasures of the X-ray sky. And EXOSAT, thanks to its elliptical 90-hour orbit, had the extra advantage of being able to make long, continuous observations of interesting objects, lasting up to 72 hours. Thus, EXOSAT was very well suited for variability studies, and many of its most important findings are in this area. EXOSAT observations sample a vide range of astrophysical sources: X-ray binaries, cataclysmic variables and active stars; supernova remnants and the interstellar medium; active galactic nuclei, and clusters of galaxies. Among the highlights, let us mention:

The area of interest to the Commission includes:

1. 1. Solar wind composition and dynamics;

2. 2. Solar Interaction of solar wind with extended interplanetary sources of plasma and gases of non-solar origin;

3. 3. SolarStructure and dynamics of the three-dimensional heliosphere;

4. 4. SolarInteraction of heliosphere with the local interstellar medium.

The following reports summarize recent developments in the aforementioned fields.

It has been recently recognized that the presence of ions heavier than hydrogen determines to a large extent the dynamics of the expanding solar corona. In the following I shall give a brief account of the most recent results related to minor ions (i.e. He and ions heavier than helium).

To model and quantify the comet-like interactions with the atmosphere of Venus the extent and time-variability of the suprathermal exospheric coronas need to be established. An extensive suprathermal O corona hypothesized to arise via dissociative recombination of the dominant ion was confirmed by the Orbiter’s UV spectrometer. Uncertainties remain, firstly because the O-corona’s extension beyond the observed limit of 1500 km altitude depends on the poorly-known partition between dissociation channels, secondly because the ionosphere varies substantially with solar UV inputs, and thirdly because the ionosphere and therefore O-corona decrease strongly from dayside to nightside (Kliore et al., Adv. Space Res. 5(11), 1985).

At the XIXth General Assembly our Commission held two meetings during the course of which the following papers were presented:

The past three years have seen not only a growth in the activities of our commission, but an extension of its activities into important areas which have heretofore motivated too little activity. Of particular interest have been the many activities directed towards elucidating the question of the abundance of extrasolar planetary systems. There have been a number of observations showing the presence of disks of dust around nearby stars, disks which fit the idea that stars are often formed with an accompanying disk of dust which may in many or perhaps all cases produce a planetary system. Infra-red evidence for dust disks exists for something like twenty stars. The disk of Beta Pictoris has even been clearly imaged at optical wavelengths, showing without a doubt that such disk structures exist. One very impressive detection of an apparent brown dwarf object has also been made; should this be confirmed by other observations, it would be clear evidence for the existence of planet-like bodies in the systems of other stars.

Since the General Assembly at New Delhi in November 1985, the Working Group held two meetings within six weeks of each other; most members of the Working Group and several members of Task Groups were able to attend at least one of these meetings. The thirteenth meeting of the Working Group was held at Toulouse, France on June 30 to July 2,1986; the fourteenth meeting was held at Moscow, USSR, on August 10,1986; the fifteenth meeting took place from August 13 to 15 in Soviet Armenia.