Records of solar and lunar eclipses in the period 700 BC to AD 1600 originating from the ancient and medieval civilisations of Babylon, China, Europe and the Arab world are amassed and critically appraised for their usefulness in answering questions about the long-term variability of the Earth’s rate of rotation. Results from previous analyses of lunar occultations in the period AD 1600-1955.5 and from high-precision data in AD 1955.5-1990 are included in the dataset considered in this paper.

By invitation of Edoardo Proverbio, the President of Commission 31 (Time), this report attempts to summarize “new possibilities in time-frequency standards”. Especially because recent work in this highly specialized field is so diverse and so complex, I appreciate the support of our colleague, Dr. G. Busca, who will survey the extensive progress being made east of the Atlantic. I confine this paper to work that has appeared in the open literature from the United States and Canada.

Since much of this work is proprietary, and since it is difficult to assign credit fairly, I will not attempt to give either a comprehensive or selected bibliography of references in this paper. There are three primary symposia where such work is published: (1) the annual IEEE International Frequency Control Symposia; (2) the more recently organized annual European Frequency and Time Forums; and (3) the (United States) Annual Precise Time and Time Interval (PTTI) Applications and Planning Meetings, organized jointly by the Department of Defense and NASA. The best general review of recent work on frequency standards, which appeared just before our last IAU General Assembly and is not yet outdated, is the special issue of the Proceedings of the IEEE, volume 79, no 7 (July 1991), and especially the papers “An Introduction to Frequency Standards” by Lindon L. Lewis, “Atomic Ion Frequency Standards” by Wayne M. Itano, and “Laser- Cooled Neutral Atom Frequency Standards” by Steven L. Rolston and William D. Phillips.

Since early eighties, the introduction of new methods for distant time transfer has led to a major improvement in the precision, accuracy, and coverage of world-wide time metrology. All of these methods are based on the use of satellites. Communication between ground stations and satellites is effected through microwave or optical signals according to a one-way or two-way mode.

There is one astrophysical system, where the sites of a star’s mass loss can be localised and observed in detail, and where the behaviour of the resulting stellar wind in the star’s environment and around orbiting obstacles can be investigated in situ: it is the Sun, the heliosphere and the surroundings of planets — among the latter most prominently the terrestrial magnetosphere. Indeed, within a year or so a fleet of satellites equipped with sophisticated remote-sensing and in-situ instruments will make this astronomical paradigm, or more precisely, the solar-terrestrial system accessible to intensive, multi-disciplinary study.

Four identical CLUSTER spacecraft, orbiting the Earth within the magnetosphere, the surrounding space and the particularly interesting plasma boundary layers will perform a three-dimensional in-situ study of plasma-heating, particle-acceleration and other small-scale plasma processes (Schmidt and Goldstein,1988). A number of other missions — some of them already in orbit, like GEOTAIL and WIND, some to be launched within one or two years, like INTERBALL and POLAR — will provide information about the Earth’s magnetosphere and the solar wind on larger spatial scales. These missions are described in a Brochure issued jointly by the European Space Agency, NASA, the Japanese Institute of Space and Astronomical Science and the Rssian Space Agency, which can be obtained from A. Pedersen at the above address.

This review presents a discussion of radiometrie measurements of solar total irradiance variation, ΔS, on time scales of days, to the 15-year extent of present measurements. It does not cover variations in spectral irradiance, particularly the relatively large solar UV and EUV flux variations of great importance in studies of trends in stratospheric ozone, and of upper atmospheric heating variations.

Analysis of the pyrheliometry of ΔS carried out from the Solar Maximum Mission, Nimbus-7 and ERB S satellites has shown that a) the sun brightens by 0.05 - 0.1% around activity maximum and; b) it dims by up to approximately 0.2% when large sunspot groups cross the disc. Empirical models of ΔS based on areas and photometric contrasts of sunspots, and on proxies of facular area, are remarkably successful in matching the observed record of ΔS, on time scales between days and the full 15- year record available since 1978. These models indicate that the opposite contributions of sunspots and faculae in active regions roughly balance over time scales of years, and the net solar brightening observed around peak activity is caused by increased emission from the photospheric magnetic network.

The magnetic structuring of the solar corona can be seen during a solar eclipse. The X-ray corona has been photographed during recent satelite missions and balloon flights and it is made up of a few main building blocks including (i) coronal loops and (ii) X-ray bright points. These appear to be connected with interacting loops and magnetic cancellation. Much cooler structures, called, quiescent prominences, are also visible in the corona and these are supported against gravity in a stable magnetic configuration.

In the talk, some of the new findings about the magnetodynamical phenomen a in the inner corona obtained by the satellite Yohkoh were reviewed. Here we give a very brief summary of it. (The long list of references has been omitted due to the limitation of space here.)

One of the most remarkable findings by Yohkoh is that the corona is quite a dynamic entity. Such a dynamicity was recognized in its full form by the wide-dynamic range, high cadence imaging from Yohkoh-Soft X-ray Telescope (SXT) with the use of the video-movie presentation. It turned out that the background corona formed quite a dynamic system with the active regions (Uchida 1992, Tsuneta and Lemen 1992), and many examples of interesting dynamic interplay between them were found. It may well be said that the corona is not an “atmosphere” of the usual sense, but is an entity highly electromagnetic in character.

Five different categories of magnetic energy conversion will be discussed, and their manifestations in the ionosphere-magnetosphere and chromosphere - corona will be compared. They are: (1) Ohmic dissipation of d.c. currents, (2) Damping of Alfvén waves, (3) Magnetic pumping, (4) Reconnection, and (5) Electrostatic acceleration parallel B. Manifestations are: (1) Ionospheric heating by Peder sen currents and generation of chromospheric faculae and plages by dissipation of field-aligned currents; (2) Generation of spicules by collisional damping of Alfvén-waves and Alfvén-wave damping in the corona by resonant absorption; (3) Drift resonance acceleration of radiation belt particles and stochastic acceleration of high-energy flare particles by Alfvén waves; (4) Reconnection at the magnetopause and in the tail, plasmoid formation and a wide variety of configuration changes and mhd instabilities in the corona for which reconnection plays a decisive role; (5) Acceleration by field-aligned potential drops as origin of auroral arcs and of > 10 MeV electron and ion beams in solar flares. Some of these mechanisms, e.g. plages, spicules, nanoflares and field-aligned electrostatic accelerations will be selected for more detailed discussion.

Spectroscopic diagnostics play an important role in determining the electron density and temperature structure, element abundances and dynamic nature of different features in the solar atmosphere. To ensure that these techniques are accurate it is necessary to interface solar analysis programs with the best available atomic data calculations. Recent progress in this work is presented and applications to planned space observations are discussed. In particular, the CDS and Sumer instruments, on the Solar Heliospheric Observatory, will make extensive use of spectroscopic diagnostics to study the heating processes for the solar corona and the acceleration mechanisms for the solar wind.

The solar corona is thought to be sustained by waves, currents, turbulence or by velocity filtration. For efficient wave heating of the corona, only the Alfven waves seem to survive the effects of steepening and shock dissipation in the chromosphere (Zirker, 1993, Solar Phys. 148,43) and these can be dissipated in the corona by mode conversion or phase mixing (Priest, 1991 in XIV Consultation on Solar Physics, Karpacz). Enhanced line width of 530.3 nm coronal line seen within closed structures (Singh et al., 1982, J. Astrophys. Astron. 3,248), association of enhanced line width of HeI 1083 nm line with enhanced equivalent width (Venkatakrishnan et al., 1992, Solar Phys. 138,107), and gradients seen in the MgX 60.9 and 62.5 nm coronal line width (Hassler, et al., 1990, Astrophys. J. 348, L77), are possibly some examples of the observed signals of wave heating. Current sheets, produced in a variety of ways (Priest and Forbes, 1989, Solar Phys. 43,177; Parker, 1979, Cosmical Magnetic Fields, Ox. Univ. Press), can dissipate and provide heat. The properties of current sheets can be inferred from fill factors, emission measures (Cargill, 1994, in J.L. Burch and J.H. Waite, Jr. (eds.) Solar System Plasma Physics: Resolution of Processes in Space and Time, AGU Monograph), hard xrays (Lin et al., 1984, Astrophys. J. 283,421), and radio bursts (Benz, 1986, Solar Phys. 104,99). The association of large scale currents with enhanced transition region (deLoach et al., 1984, Solar Phys. 91,235.) and regions of enhanced magnetic shear with brighter corona (Moore et al., 1994, Proc. Kofu Symp) are of some possible interest in this context. Self consistent calculations of the turbulent cascade of energy from the scales of photospheric motions down into dissipative scales (Heyvaerts and Priest, 1992, Astrophys. J. 390,297) predict the width of coronal lines as a function of the properties of the forcing flows. Velocity filtration caused by free streaming effects off a non maxwellian boundary distribution of particles may well result in a plasma having coronal properties (Scudder, 1992a, Astrophys. J. 398,299; 1992b, Astrophys.J.11 398,319). The observable signals are the variation of line shapes with altitude.

The heliosphere could be divided in three major acceleration Laboratories, the solar surface (Laboratory 1), the interplanetary medium (Laboratory 2) and Earth and Planetary magnetospheres (Laboratory 3). Our understanding of the acceleration process depends strongly on the nature of the drivers and the energy dissipation process. The energy gain by a particle with velocity where is the variation of the electric field in space and time. All three Laboratories mentioned above share a common characteristic, the drivers and the energy dissipation processes are closely connected to fully developed MHD turbulence. We can show that our understanding of particle acceleration depends strongly on the interaction of particles with fields resulting from fully developed MHD turbulence.

The composition of the solar wind (SW) is not a true sample of the solar composition, but it is fractionated with respect to the solar photosphere. This fractionation follows the well-known first ionization potential (FIP) pattern: When plotting the SW abundances with respect to the solar abundances as a function of FIP, a step function is obtained (Fig. 1). The step at ∼ 10eV has a height of 3-5 in the slow SW, but this is reduced to 1.5-2 in the fast streams, which originate in the coronal holes. The data given in Fig. 1 are collected and discussed in von Steiger & Geiss (1994), including the “FIP exceptions”, Kr and Xe.

The process leading to the observed overabundance of the low-FIPs has been located to operate by atom-ion separation across magnetic field lines in the chromosphere (Geiss, 1982), because this is the only region of the solar atmosphere where a significant fraction of the gas is neutral. The fractionated abundances observed in the S W are thus important tracers for processes and conditions at this site.

An issue of increasing importance in solar physics and climatology is solar variability (Sofia and Fox, 1994). It seems that most solar type stars show some magnetic variability at the surface (Baliunas and Jastrow, 1990). To understand the underlying processes and to model them is a big challengein solar physics.

When satellites began to observe the sun outside the disturbing atmosphere it soon became clear that the solar irradiance is also variable and seems to be related to the 11- year Schwabe cycle (Willson, et al., 1986), (Foukal, this volume). This result is of great importance because the sun is the engine which drives the climate system on earth. Although the variations are small (0.1-0.2%) there is clear evidence from observational data of solar type stars (Baliunas and Jastrow, 1990) and from theoretical considerations as well that much larger fluctuations over longer time scales potentially could occur.

This raises the question what role the sun plays in today’s climate change and possibly in the near future. There are basically two approaches to address this question. 1. The solar irradiance and the climate have to be monitored continuously with high precision in order to detect changes and to understand how they are caused. 2. Since there are reasons to believe that the sun also exhibits long-term changes which cannot be detected during short periods of direct observations, one has to investigate the past by looking for connections between climate and solar variability.

To reconstruct past climate changes is comparatively simple. There is a large amount of climatic observations available for the last 2-3 centuries. The climatic conditions of earlier times can be reconstructed quite reliably based on isotopic ratios, pollen assemblages, and many other parameters measured in natural archives.

Since the early 1960s the surface of the Sun has been know to be in a regular pulsating motion with periods of about 5 minutes. While at the beginning various explanations were offered, only in the 1970s it was recognized that these so-called solar oscillations are manifestations of global motions of the Sun about its equilibrium. Helioseismology is the name of the branch of astrophysics that deals with deciphering these data, that cover the whole range of spherical harmonics from l = 0 (radial) to very high angular order (above l = 1000). Thanks to observational data of superb quality (each of the oscillation frequencies is measured accurately to better than one part in ten thousand), our knowledge of the Sun has leap-frogged in the last 20 years. For instance, we now know the run of temperature inside the Sun, or have good information about the internal solar rotation. In the solar neutrino problem the data from solar oscillations have become a compulsory testing stone for any model proposed to explain the discrepancy between observed and theoretically predicted solar neutrinos.

The field of observational helioseismology began in 1960 when Leighton, Noyes, and Simon (1962) discovered the so-called solar “5-minute” oscillations at the Mt. Wilson Observatory. However, it was not until roughly two decades ago, when these 5-minute oscillations were demonstrated to be sound, or acoustic waves, which were trapped within the solar interior (Deubner, 1975), that observational helioseismology began to grow in earnest. In the nearly two decades which have elapsed since its infancy, observational helioseismology has been the source of many surprising new results concerning the solar interior - far more results than there is space to list here. These past two decades have also been a time of great intellectual ferment which has led to the development of new instrumental tools and new analysis techniques at an astounding rate, with many of these tools and techniques not even being contemplated two decades ago.

In this section I concentrate on the spherically symmetric aspects of solar structure, corresponding to “classical” stellar evolution models. Such models are characterized by a number of simplifying assumptions, as well as by the physical properties of matter in the star, conveniently labeled “micro-physics”. The latter include descriptions of the equation of state, the opacity and the nuclear reaction rates; in addition, molecular diffusion, included in several recent calculations, should be considered as part of the micro-physics. The assumptions in the standard calculations, simplifying what might be called the macro-physics, include the neglect of effects of rotation and magnetic fields (implicit in the assumption of spherical symmetry), as well as the assumption that material mixing occurs only in convectively unstable regions, or possibly as a result of molecular diffusion and settling; also, convective energy transport is treated crudely through some form of mixing-length approximation and the contribution to hydrostatic balance from the turbulent motion in the convection zone, usually called turbulent pressure, is ignored.

The method of frequency inversion reveals that within the quoted observational errors, it is possible to achieve a precision of ∼ 10−3 in the sound speed determination through most of the Sun’s interior. Only for r < 0.05R⊙ is the precision ∼ 10−2. The accuracy of the density and pressure determinations is only slightly worse. Such restrictions impose significant constraints on the microscopic physical data, i.e. opacities, nuclear reaction cross-sections, and diffusion coefficients as well as on the solar age. The helioseismic age is consistent with that from meteorites.

Recently released low-l solar oscillation data from the BISON network combined with BBSO data yield the most up-to-date solar seismic model of the Sun’s interior. For the core, the solar seismic model from the new data are consistent with the best, current standard solar models. An astrophysical solution to the solar neutrino problem fades away.

The present internal dynamics and magnetism of the Sun have been determined by the initial conditions in the pre-main sequence age, by the angular momentum loss and its redistribution in the interior, and the interaction of motion with magnetic field.

The history of the Sun rotation is traced back by observing the present rotation of stars with the same mass as the Sun at earlier evolutionary stages. The present angular momentum of the Sun, as deduced from its internal rotational behavior derived from helioseismological data, appears to be a small percentage of the original one contained in similar mass stars (T Tauri and α Persei). It is not easy to reconcile the sharp decrease in the surface angular velocity, which follows the α Persei phase, with the subsequent soft decrease, taking place after Pleiades phase, unless some very effective mechanism transfers angular momentum from inner to outer regions, where is lost in the solar wind. Such a mechanism is probably magnetic in origin, since purely hydrodynamic instabilities fail to transfer angular momentum at a rate sufficient to determine the presently observed flat radial gradient of the internal angular velocity.