The nature of flare activity on dMc stars (red dwarfs with strong chromospheric IIα emission lines) has been the subject of many studies. Some years ago Lacy et al. (1976) demonstrated a relationship (see also Doyle et al., 1986) between mean flare power and quiescent luminosity, in the photometric U-band. This study was extended, independently, by Skumanich (1985, 1986) and Doyle and Butler (1985) to show that the time averaged U-band power-loss due to flaring is linearly related to a star's quiescent X-ray luminosity. Skumanich also showed an inverse relationship between a star's flaring-rate and its quiescent X-ray luminosity.

We present an overview of recent observations of stellar X-ray flares obtained with the EXOSAT Observatory. We discuss a few examples of flares from M dwarf flare stars, from RS CVn and Algol-type binaries, from single late-type stars (including a G0 dwarf and an A-type visual binary), and from pre-main-sequence objects. We also draw some general conclusions from the pieliminary analysis of the EXOSAT data sample.

Simultaneous optical spectroscopy and X-ray monitoring of stellar and solar flares shows that a well-defined linear correlation exists between the integrated Hγ and soft X-ray flux that extends over four orders of magnitude. The existence of this relationship implies a direct proportionality between the emission from the cooler and denser regions (T ≈ 104K) responsible for Balmer lines and the emission from the hot plasma (T ≈ 107K) responsible for soft X-rays. The consequences are considered for (a) several models which have been proposed for solar flares, and (b) the suggestion that the Balmer emission results from irradiation by soft X-rays.

An overview of the many topics discussed at IAU Colloquium No. 104 is presented as an introduction to the Proceedings. Suggested areas for future research emerging from the conference are summarized.

We study the effect of chromospheric bombardment by an electron beam during solar flares. Using a semi-empirical flare model, we investigate energy balance at temperature minimum level and in the upper photosphere. We show that non-thermal hydrogen ionization (i.e., due to the electrons of the beam) leads to an increase of chromospheric hydrogen continuum emission, H− population, and absorption of photo-spheric and chromospheric continuum radiation. So, the upper photosphere is radiatively heated by chromospheric continuum radiation produced by the beam. The effect of hydrogen ionization is an enhanced white-light emission both at chromospheric and photospheric level, due to Paschen and H− continua emission, respectively. We then obtain white-light contrasts compatible with observations, obviously showing the link between white-light flares and atmospheric bombardment by electron beams.

EXOSAT observed the flare star EV Lacertae for 17 hr over 2 days in October 1985. Two flaring episodes were recorded. During a significant fraction of these observations, IUE, photometric and spectroscopic coverage was available. A 2-hour long period of X-ray flaring was observed during which there was no U-band activity and almost no chromospheric activity. On the other hand, two ΔU ~ 1° m° 5 optical flares produced normal chromospheric enhancements, but only a weak X-ray response. We suggest that these and a few other observations of stellar flares may imply the occasional existence of magnetically isolated regions in M-dwarf atmospheres.

The Danish experiment WATCH (Wide Angle Telescope for Cosmic Hard X-rays) is to be flown on board the Soviet satellite GRANAT in middle of 1989. The performence characteristics of the WATCH instrument is described. It is estimated that WATCH can detect about 100 solar hard X-ray bursts per day. WATCH can also detect about 40 energetic stellar soft X-ray flares, similar to the fast transient X-ray emissions detected by the Ariel V satellite.

New perspectives in solar diagnosis have been opened in recent years with the advent of high-resolution soft X-ray spectroscopy for plasmas forming at temperatures above 107 K. The spectra obtained with the soft X-ray spectrometers flown during the last solar maximum on the major space missions dedicated to flares have allowed detailed studies of the hydrodynamic response of coronal loops to impulsive energy deposition and of the formation of the high-temperature plasma as a consequence of such dynamic effects. These studies are possible since high-resolution spectrometers give an accurate measure of both line intensities and profiles in important spectral regions, covering the emission of highly ionized heavy ions, which allow a direct determination of most of the crucial plasma parameters in the flare region. In response to the impulsive energy release in the flare region, while the intensity of soft X-ray lines increases, line profiles show large non-thermal broadenings and strong blue-asymmetries.

There have been important contributions in the understanding of the formation of the flare high-temperature plasma, as an effect of the hydrodynamic response of the solar atmosphere to impulsive chromospheric heating. On the other hand, the attempts to investigate the primary energy release and transport, on the basis of the soft X-ray spectral data, have not yet been entirely successful. Significant differences in the emitted spectra are expected at the very onset of flares for different energy deposition and transport processes, but the sensitivity of the present experiments is still insufficient to detect with good statistics the early stage of flares and, therefore, to allow a reliable discrimination. It is expected that future experiments with higher sensitivity will be of great importance for relating with less ambiguity the observed flare evolution in soft X-rays to the primary energy deposition in the flaring coronal loops.

Radio emission from dMe flare stars has both a flaring and a quiescent component. When we compare stellar radio emission with the Sun, however, we find that the apparent brightness temperature of the quiescent component often exceeds the temperature of non-thermal solar radio flares, and so it is likely that stellar quiescent emission also comes from non-thermal electrons. The duration of stellar quiescent emission is much longer than solar non-thermal emission. Obvious questions to ask are, what is the source of the non-thermal electrons, where do they reside, and how can non-thermal emission last so long? Here we briefly review the observations of quiescent emission, argue that the emitting regions are small, show that such small regions can still account for the observed fluxes, and discuss the source of electrons.

We present observational data on stellar flares from a range of wavelength regimes, many of which were obtained simultaneously. Physical parameters of these flares are derived and discussed in the framework of the general solar flare model. It is found that flares on dMe stars are solar-like, except in mean energy. The parameters of flares on RS CVn stars are more extreme, however, and may require new models for their interpretation.

We describe high-sensitivity VLA observations of rapidly varying radio emission (‘flares’) from two stars of very different types, one of which (λ And) is a Long-Period RS CVn system, and the other (HR 5942) is a magnetic Bp star. In both cases, however, the physical mechanism producing the radio emission is most likely to be gyrosynchrotron radiation from mildly relativistic, power-law electrons.

Only during the previous solar cycle have systematic observations begun to be made with the sensitivity and time resolution, and the continuous coverage required to catch the impulsive phase and measure the rapid variations present in many wavelength ranges. Observations in X-rays, gamma-rays, UV, Ha, and radio wavelengths all reveal rapid variations during the impulsive phase and have contributed to our understanding of the different phenomena involved. Results have been obtained from several spacecraft, from rocket and balloon flights, and from ground-based observations. These are reviewed in the context of a simple single loop flare model with a view to showing what results are consistent with this model and what the major problems are in our understanding of the impulsive phase. New instrumentation planned for observations during the present Cycle 22 will provide a concerted attack on the impulsive phase as part of the Max'91 program.

Ultraviolet continua observed in IUE spectra of dMc stars in a flaring state are compared with those in solar Hares. There is evidence that, as may be the case with solar flares, stellar-flare continua in particularly the λ < 1683 Å region are due to free-bound transitions in neutral silicon excited by ultraviolet lino emission.

For the period September 1978 to December 1982 we have identified 55 solar flare particle events for which our instruments on board the ISEE-3 (ICE) spacecraft detected electrons above 10 MeV. Combining our data with those from the ULEWAT spectrometer (MPI Garching and University of Maryland) electron spectra in the range from 0.1 to 100 MeV were obtained. The observed spectral shapes can be divided into two classes. The spectra of the one class can be fit by a single power law in rigidity over the entire observed range. The spectra of the other class deviate from a power law, instead exhibiting a steepening at low rigidities and a flattening at high rigidities. Events with power-law spectra are associated with impulsive (< 1 hr duration) soft X-ray emission, whereas events with hardening spectra are associated with long-duration (> 1 hr) soft X-ray emission. The characteristics of long-duration events are consistent with diffusive shock acceleration taking place high in the corona. Electron spectra of short-duration flares are well reproduced by the distribution functions derived from a model assuming simultaneous second-order Fermi acceleration and Coulomb losses operating in closed flare loops.

We review the somewhat questionable concept of an isolated flare loop and the various physical mechanisms believed to be responsible, to some degree, for energy transport within the loop structure. Observational evidence suggests a predominant role for high-energy electrons as an energy transport mechanism, and we explore the consequences of such a scenario in some detail, focusing on radiation signatures in the soft X-ray, hard X-ray, and EUV wavebands, as observed by recent satellite observatories. We find that the predictions of flare loop models are in fact in excellent agreement with these observations, reinforcing both the notion of the loop as a fundamental component of solar flares and the belief that electron acceleration is an integral part of the flare energy release process.

Photometric and spectroscopic observations of a very large flare on AD Leo are presented. A self consistent model of a flare corona, transition region and chromosphere is developed; in particular the chromospheric temperature distributions resulting from X-ray and EUV irradiation by coronae of various temperatures are determined. The predicted line fluxes in Hγ are compared to the observed line fluxes to find the coronal temperature as a function of time during the flare. This run of temperature with time is then compared with the predictions of an independent theoretical flare model based on a dynamic scaling law (see paper by Fisher and Hawley, these proceedings).

Stellar spectral diagnostics are of utmost importance to test fundamental concepts of flare physics such as particle beam versus suprathermal heating, atmospheric response, mass motions, microflaring, statistics and recurrence of flares, flare activity and stellar interior. We review some of these diagnostics (from photometry, optical, and ultraviolet spectroscopy at medium- and high-spectral resolution, X-ray, and radio observations). Specific diagnostics from line and continuum fluxes, density sensitive lines, broadening and velocity field effects and the comparison with semi-empirical models are also described.

Some results on stellar flares obtained from previous multi-wavelength observing campaigns are presented. Future satellite missions and ground-based observatories, with new techniques for obtaining high spectral and temporal resolution, are discussed in light of their possible contribution to our understanding of solar and stellar flares.

We report on the first successful coordinated observations of stellar flares carried out on March 28, 1984 simultaneously over a wide range of wavelengths, from UV to microwaves, using the IUE satellite, three ESO telescopes at La Silla (Chile) and the VLA at Socorro (NM, USA).