This paper summarises the key features of the Hipparcos satellite design, and presents an overview of the current project status including results of the evaluation of the first hardware items. A recent assessment of the achievable accuracy for the main mission and for Tycho is also presented.

The Input Catalogue Consortium (INCA) is in charge of selecting the 100 000 stars to be observed by Hipparcos. In this paper the organisation of the Consortium is described, and the technical requirements to be met by the Input Catalogue for optimal scientific return are summarised. The various tasks of this long collective work are then reviewed including the identification process of the proposed stars, the progressive construction of a list without redundancies, and the definition of the ‘survey’. The final selection of the stars which will be observed by Hipparcos is being made using a numerical simulation which takes into account the observing constraints of the satellite, and a parameter called the ‘pressure’ which is calculated for each star. Finally, the organisation of working groups set up to deal with special objects (multiple stars, variable stars, minor planets, stars in dense areas, and stars for linking the Hipparcos system to an extragalactic reference system) are described. New photometric and astrometric programmes undertaken for stars with unsatisfactory precision on magnitudes and/or coordinates, a major part of the Consortium’s work, will be presented elsewhere.

The contribution of earth-based photometry to the preparation of the Input Catalogue for the Hipparcos mission is summarised for the pre-launch, the mission itself and the post-launch phases. An evaluation of the needs for new data and a progress report are also presented.

The proper and efficient operation of the Hipparcos satellite requires positions within 1.5 arcsec at the 1989 epoch for the 100-120 000 programme stars. The first steps of the preparation have been the identification of the proposed stars in the well-known astrometric catalogues and the output of successive lists of ‘non-astrometric’ stars not belonging to these catalogues. Ground-based observations needed to supplement the presently available data, which are now under way, are also described.

The complete reduction of data from the ESA astrometry satellite Hipparcos, from some 1012 bits of photon counts and ancillary data to a catalogue of astrometric parameters and magnitudes for the 100 000 programme stars, will be independently undertaken by two scientific consortia, NDAC and FAST. This approach is motivated by the size and complexity of the reductions and to ensure the validity of the results. The end product will be a single, agreed-upon catalogue. This paper describes briefly the principles of reduction and the organisation and status within each consortium.

The Hipparcos star mapper will be used to determine magnitudes in two colours and the positions for about 500 000 stars. The typical accuracy for the mean values is about 0.03 mag and 0.03 arcsec respectively for a star of B = 11.0 mag and B-V = 0.7 mag. The photometric system and part of the data analysis are outlined.

The Tycho Input Catalogue will be used as a finding list to identify the stellar data within the stream of raw data received from the Hipparcos star mappers. It is based on an a priori list of stars, namely the Guide Star Catalogue being created for the Hubble Space Telescope at the Space Telescope Science Institute. The Guide Star Catalogue will contain positions and magnitudes for objects in the sky complete to at least 14 mag, and will contain about 20 million stars. The data for the sky complete to approximately 12.5 mag, extracted from the Guide Star Catalogue, along with some additional data from the SIMBAD data base of about 500 000 stars at the Strasbourg Stellar Data Centre, will form the Tycho Input Catalogue of approximately 2 million objects. As of the 1985 IAU General Assembly nearly 50 per cent of the plates for the Guide Star Catalogue had been measured, and 10 per cent of them were completely processed and catalogued.

The link of the Hipparcos Catalogue to an extragalactic reference system will be carried out on two time-scales: the first, coinciding with the duration of the mission, to provide data for immediate use by the data reduction consortia, and the second over a longer term to provide for subsequent reappraisal of the final Hipparcos catalogue. The first will be based on radio stars and on an indirect link to extragalactic objects by means of observations with the Hubble Space Telescope; for the second we hope for longer term programmes with the Hubble Space Telescope, together with a variety of ground-based links.

We describe the multifrequency radioheliograph of the Clark Lake Radio Observatory. Its use for studies of large scale structures of the corona at long radio wavelengths is discussed. We show that features corresponding to coronal streamers and coronal holes are readily apparent in the Clark Lake maps. We also present results on studies of microbursts at meter-Decameter wavelengths.

The observations obtained with high resolution spectrometers flown in the past solar maximum, in the years 1979-1981, have shown that the soft x-ray plasma during the impulsive phase of solar flares is characterized by upflows. as well as by isotropic flows, at velocities of a few hundred km s-1. Isotropic flows can be directly related to the primary energy release process. While, upflows are an indication of chromospheric material heated to coronal temperatures and evaporating along the magnetic fluxtubes; they are therefore related to the development of the coronal soft x-ray source in the flare region.

In the past decade, impulsive hard X-ray bursts have been extensively observed (cf. Kane et al., 1980). These observations have increased our knowledge of the energy spectrum of the accelerated electrons and their temporal evolution. However, because of the lack of spatial resolution and direct plasma diagnostics, many important questions concerning the nature of the impulsive phase are still left unanswered. Since direct imaging of hard X-rays above 30 keV with high resolution of is still beyond our present technology, we have to use other indirect means to deduce the spatial structure of the hard X-ray source. With the recent launch of the Solar Maximum Mission (SMM) satellite, we are able to obtain correlated observations of the flare impulsive phase in hard X-ray and simultaneously in the UV lines of Si IV (1402 Å) and 0 IV (1401 Å). The Si IV/0 IV intensity ratio is density sensitive and therefore provides plasma diagnostics in the emission region. Analysis of the spatially resolved UV observations with the correlated hard X-ray observations allows us to study the spatial structure and physical conditions in the UV and hard X-ray sources (Cheng et al., 1981; 1982; 1984). Descriptions of the various solar instruments on SMM can be found in Solar Physics (vol. 65, pp 5-116). In this paper, I briefly summarize the important observational results and discuss their theoretical interpretation.

This paper gives a brief review of current observation, interpretation and theory of coronal transient mass ejections. Some recent new results are described.

Observations of interplanetary scintillation on about 900 sources made with the 3.6 hectare array at Cambridge have been used to map, and to track, 96 heliospheric transients during August 1978–September 1979. This sample included most of the interplanetary shocks recorded by near-Earth spacecraft in the same period.

Some corotating interaction regions were observed but the most frequent disturbances were “erupting streams” in which spherical shells of enhanced density were driven by the birth of high-speed solar wind streams which persisted for several days. Although these transients were of the type traditionally associated with solar flares or disappearing filaments they were not caused by such processes. Back-projection to the Sun indicated that the sources were always associated with coronal holes. It is concluded that the sources of the most energetic interplanetary transients are mid-latitude coronal holes which, near solar maximum, produce sporadic high-speed streams lasting several days, in contrast to the more stable flows typical of the declining phase of the solar cycle.

As is known from eclipse observations, the microwave emission of an active region consists of three main components: floccular, inter-spot (halo) and spot components which differ in intensity, the degree of polarization, and in structure and sizes /1/. A possibility of identifying the finer spatial and temporal structure in the active region (AR) emission has existed since RATAN, the VLA and WSRT became operational. The construction of the SSRT permitted the initiation of a systematic study of spatio-temporal properties of the development of active regions /2, 3/.

The majority of the properties in the AR development are reflected in detail and rapidly in the microwave emission characteristics /4, 6/.

We conclude that coronal holes exhibit differential rotation and they do not follow a specific polarity rule. Further, proximity of holes to type II burst rich active regions (ARs) indicates that shock waves propagate almost along the magenetic field of the holes.

Two dimensional maps obtained with the Nançay Radioheliograph at 169 MHz from 1980 are used to describe and identify the features of the quiet sun at meter wavelengths.

The Nançay Multifrequency Radioheliograph is an extension of the previous Mark III instrument described in Solar Radioheliograph Group (1983). The table I summarizes the characteristics of the instrument.

The complete set of MHD equations is solved by numerical calculation. The aim is to study the energy storage mechanism of a solar flare by shearing the magnetic field. Results show that the magnetic energy is stored in local regions and is large enough to explain a big solar flare.

The Working Group on the Rotation of the Earth was established in 1978 and developed a programme of international collaboration to Monitor Earth-Rotation and Intercompare the Techniques of observation and analysis (MERIT). The MERIT Short Campaign was held in 1980 to test and develop the organisational arrangements required during the MERIT Main Campaign in 1983-4. The Working Group on the Terrestrial Reference System was established in 1980 to prepare a proposal for the establishment and maintenance of a new Conventional Terrestrial Reference System (COTES) that would be based on the new techniques of space geodesy. The Working Groups collaborated closely and organised two intensive campaigns in 1984 and 1985 that were aimed primarily at determining the relationships between the reference systems of the six different techniques that were used to determine earth-rotation parameters. Observational data were obtained from 35 countries; analyses and intercomparisons of the results were carried out in 7 countries. The Working Groups reviewed the results at the Third MERIT Workshop and recommended that a new International Earth Rotation Service be set up in 1988 and that it be based on the use of very-long-baseline radio interferometry and both satellite and lunar laser ranging.

A Joint Meeting of Commissions 19 and 31 was held during the IAU General Assembly at Delhi to consider the recommendations for a new international earth-rotation service put forward by the IAU/IUGG Joint Working Groups on the rotation of the Earth and the conventional terrestrial reference system. Wilkins gave summaries of the MERIT programme of activities to monitor earth rotation and intercompare the techniques of observation and analysis and of the COTES programme to establish the basis of a new conventional terrestrial reference system. He reviewed the recommendations of the Groups, and then introduced a draft resolution of the Commissions. An amendment on the continuation of the use of the technique of optical astrometry was accepted and the resolution was then adopted without objection. Four papers on work related to the MERIT/COTES programmes were then presented. Paquet discussed the agreement in the results by different techniques for polar motion and universal time. Preuss presented a paper by Campbell and Schuh on short-period variations in earth-rotation determined by VLBI. Dickey discussed the intercomparisons between the earth-orientation parameters obtained by different techniques and then reviewed the close correlation between the length of the day and the angular momentum of the atmosphere. Finally, Vicente and Verbeiren presented new techniques for processing time and polar motion series.