The discovery of the afterglows of gamma-ray bursts (GRB) at X-ray, optical and radio wavelengths heralds a new era in the investigation of these objects. This provides a quantum jump in the type of observational information available, providing much more severe constraints on models, and probably represents the light at the end of the tunnel for this long-standing puzzle of astrophysics.

X-ray variability is a distinguishing property of Active Galactic Nuclei (AGN), and the energetics and time scales of the emission dictate that the X-rays must originate very close to the central engine. In this review I discuss two basic topics from AGN variability research. The first is the correlation of the variability time scale with the X-ray luminosity, and the second is the structure of the X-ray light curve. In each case, I first review the old results that have been known for approximately the last 10 years and then I discuss very new results which may force us to modify our ideas about the origin of AGN X-ray variability. Note that I am discussing the variability of non-blazar type AGN.

Active Galactic Nuclei (AGN) are thought to be powered by accretion onto a massive black hole. Understanding how gravitational energy freed by the infall of matter into the black hole is transferred to the radiating regions of the nucleus is one of the main challenges of AGN research. The question is made very complex by the presence of several cooling mechanisms with very diverse physical properties: We observe synchrotron radiation, thermal emission from hot dust, possibly thermal optical and ultraviolet emission and Comptonization processes in the X-ray domain. For each component the radiation is a signature of the cooling process rather than of the heating process. It is our hope that by observing the links and correlations between the emission of the different components we will be able to understand how they are interrelated and how they get their energy supply from the accretion process.

During the last years it has become evident that blazar class of AGN emit a lot of energy in the gamma-ray regime. It is generally thought that the non-thermal emission from blazars, observed from radio to GeV/TeV 7-rays, is radiation of very energetic particles via both synchrotron and Compton processes. This underlines the importance of high-energy emission for models of the blazar class of active galactic nuclei. Most of the gamma-bright AGN are variable, and flares with time scales as short as a day have been observed. This suggests that the site of gamma-ray emission is very compact and situated in a fluid which moves relativistically at a small angle towards the observer. It is generally assumed that the emission originates from the jet of these objects.

This paper is devoted to a discussion of the rapid variability observed in γ-ray blazars. We do this in the framework of a widely accepted scenario, according to which the blazar emission arises from a jet, i.e. a continuous flow of relativistic fluid, with perturbations occasionally superimposed. The jet is assumed to have a bulk Lorentz factor Γ ≫ 1, and length z and radius r such that r ã z/Γ.

The discovery of strong, variable gamma-ray sources identified with blazars by CGRO has motivated many theoretical investigations concerning the emission from relativistic jets, and several models of gamma-ray blazars have recently been developed (e.g., Dermer & Schlickeiser 1993; Bloom & Marscher 1993; Manneheim 1993; Sikora, et al. 1994; Blandford &; Levinson 1995). However, most of these efforts have been devoted to examine spectral properties of blazars using steady-state models, which are most suitable for exploring quiescent states. Although some progress in identifying the emission mechanisms in different types of blazars, and constraining the content and dynamics of relativistic jets on small scales has been made, it has became widely recognized that quantitative analysis of variability data is crucial for advancing our understanding of the nature of relativistic jets further. Despite recent observational efforts (e.g., Reich et al. 1993; Maraschi et al. 1994; Wagner 1996; Buckley et al. 1996; Takahashi et al. 1996; Mattox et al. 1997) to characterize the transient emission in blazars, only very few attempts have been made (see e.g., Romanova & Lovelace 1997) to model variability data using time dependent models.

Almost all nondegenerate stars have chromospheres and coronae. These hot outer layers are produced by mechanical heating. The heating mechanisms of chromospheres and coronae, classified as hydrodynamic and magnetic mechanisms, are reviewed here. Both types of mechanisms can be further subdivided on basis of the fluctuation frequency into acoustic and pulsational waves for hydrodynamic and into AC- and DC-mechanisms for magnetic heating. Intense heating is usually associated with the formation of very small spatial scales, which are difficult to observe. Yet, global stellar observations, because of the dependence of the mechanical energy generation on the basic stellar parameters (Teff, gravity, rotation, metallicity) can be extremely important to identify the dominant heating mechanisms.

The last few years have seen the development of solar wind models characterized by high coronal proton temperatures, low electron temperatures and low densities in the inner corona. In addition the emphasis on “additional” acceleration via Alfvén waves has been muted and been replaced by models where the acceleration is essentially complete within 1OR0. This work has, in part, been spurred by Ulysses observations of the high speed wind and by the expectations of the observations to be made by the various instruments on the SOHO satellite. In this paper we show how these new models differ from earlier ones.

Dependencies of solar wind parameters as a function of latitude are presented and discussed, based on measurements of the Ulysses mission during times of declining or minimum solar activity. The heliosphere is found to be structured into a band of slow solar wind at low latitudes and two large high-speed streams from the polar coronal holes. Only small trends of solar wind parameters with heliolatitude are found within these streams. A small but significant temperature difference is seen between the south and the north polar coronal hole.

The solar wind is an ionized gas which, as a consequence of a hot solar corona and a low fluid pressure in the interstellar space, continuously emanates from the Sun into space to define a region known as the heliosphere. Since the electrical conductivity of the solar wind is very high, diffusion of the magnetic field through the plasma is not taken into account. In this picture (the frozen-in approximation) one imagines that the solar magnetic field is dragged into the heliospheric space by the radially outflowing solar wind. The structure of the solar wind is therefore intimately related to the structure of the solar corona and the solar magnetic field. The solar wind plasma itself is composed of protons, electrons, alpha particles, and a minor fraction of heavy ions.

The question of the boundaries of the heliosphere is considered. The termination heliospheric shock should exist because the solar wind plasma flowing supersonically away from the Sun must make a transition to a subsonic flow. The heliopause is at the outermost extend of the solar wind. Beyond the heliopause lies the (very local) interstellar wind. Intensity of radio emissions at 2 to 3 kHz detected by the Voyager plasma wave instrument in the outer heliosphere can be explained provided that the electron beams generating Langmuir waves exist in the post-shock plasma due to secondary shocks in the compressed solar wind beyond the termination shock. The field strengths of Langmuir waves required to generate the second harmonic emissions are 50 – 100 μ V m-1. Alternatively, the emissions are generated in the vicinity of the heliopause. The Voyager 1 and 2 are proceeding toward a likely source region for Langmuir wave and these waves may be observed in situ in the near future.

The magnetism of the Sun manifests itself in innumerable ways, many of which constitute what is referred to as solar magnetic activity, while others are counted among the phenomena of the quiet Sun. After a brief overview of the structure of the solar magnetic field, a few examples of its manifestations are pointed out.

The combined operations of the Large Angle Spectrometric Coronagraph (LASCO) and the Extreme Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), launched in December 1995, have provided an unprecedented opportunity for observing essentially all coronal phenomena that are not hidden behind the disk of the Sun. Consequently, observations with these instrument are providing information on coronal mass ejections (CMEs) from their initiation through their development over 30 ⊙. They reveal a coronal that never reaches a steady state. The corona is the site of continuous, time-dependent outflows, both within the coronal holes and the high speed streams and in the streamer belts and their mid-latitude sources. The spatial scales of these outflows range from 10s of arc-seconds through about a solar radius in large CMEs.

The acceleration of electrons and charged nuclei to high energies is a phenomenon occuring at many sites throughout the universe, including the galaxy, pulsars, quasars, and around black holes. In the heliosphere, large solar flares and the often associated coronal mass ejections (CMEs) are the most energetic natural particle accelerators, occasionally accelerating protons to GeV and electrons to tens of MeV energies. The observation of these particles offers the unique opportunity to study fundamental processes in astrophysics. Particles that escape into interplanetary space can be observed in situ with particle detectors on spacecraft. In particular, particle spectra can be diagnostic of flare acceleration processes. On the other hand, energetic processes on the sun can be studied indirectly, via observations of the electromagnetic emissions (radio, X-ray, gamma-ray) produced by the particles in their interactions with the solar atmosphere. The purpose of this article is to give a brief overview on current models on particle acceleration and the present status of observations of solar energetic particles.

The solar corona is not easily observable from the ground; only with the coronagraphs located in high mountains or at infrequent and short occasions of total eclipses. And in any case these observations are limited to the corona beyond the solar limb. Therefore, it is natural that our understanding of the physics of the solar corona has advanced dramatically by sustained observations from space-based platforms, like Skylab in the 1970’s. These several years are particularly notable in this respect, because of a wide variety of data from Yohkoh, Ulysses, and SOHO.

The second session of the Joint Discussion 19 (Physics of the Sun and Heliosphere in the Era of Space Probes; Scientific Highlights of SOHO, ULYSSES and YOHKOH) was devoted to the Evolution and Termination of the 3-D Solar Wind. As was the case with the other two sessions, it consisted of three invited talks, three oral contributions, a number of posters and a general discussion.

Excellent review papers have been presented in this session by S. Solanki (Manifestations of solar magnetic fields), K. Dere (Coronal Mass Ejections and interplanetary ejecta) and W. Droege (Particle acceleration by waves and fields) and the relevant texts can be found in this volume.

The Joint Discussion benefited from three contributed papers: X-Ray/Radio network flares of the quiet Sun by A.O. Benz, S. Krucker, L.W. Acton, and T.S. Bastian (presented by A.O. Benz) Radio observations of coronal X-ray jets by M.R. Kundu

Astronomy research and teaching has been actively carried out in the Department of Physics and Astronomy at the University of Nigeria, Nsukka for the past 20 years and before that, in the Department of Mathematics. Economic problems, lack of recent journals, text books and computing facilities, and very poor communications have made it increasingly difficult for these programmes to continue. Added to these problems there is often unrest in the Universities resulting in frequent closures and almost constant uncertainty about payment of the already poor salaries. It is a wonder that anyone could be even thinking of astronomy.

Recently, important efforts have been made to organize and consolidate the Assembly of Central American Astronomers (AAAC), an organization created to contribute to the development of astronomy and astrophysics in Central America, with the help of international cooperation; the Central American Courses in Astronomy and Astrophysics, have been hosted each year by a different national university in Central America (1995: Universidad Nacional Autonoma de Honduras, 1996: Universidad de El Salvador, 1997: Universidad de San Carlos de Guatemala, and 1998: Universidad de Panama). These courses aimed to provide an exchange of knowledge and experience among university staff and students interested in continuing studies in Astronomy and Astrophysics. Regional Observational Campaigns have been organized to train young astronomers in the use of astronomical equipment and observational techniques. It seems that the broad development of astronomy and astrophysics in Central American as a whole, will be possible only when nuclei of astronomers in each of the countries concerned begin to develop many more activities, countries As part of the III Central American Course on Astronomy and Astrophysics (III-CURCAA, April 1997, Guatemala), in a forum about Education and Research in Astronomy in Central America, several important conclusions were stated. In this paper we present the six most relevant conclusions discussed there.