It was shown by Zheleznyakov and Zlotnik (1980a, b) that in complex configurations of solar magnetic fields (in hot loops above the active centres, in neutral current sheets in the preflare phase, in hot X-ray kernels in the initial flare phase) a system of cyclotron lines in the spectrum of microwave radiation is likely to be formed. Such a line was obtained by Willson (1985) in the VLA observations at harmonics of the electron gyrofrequency. This communication interprets these observations on the basis of an active region model in which thermal cyclotron radiation is produced by hot plasma filling the magnetic tube in the corona above a group of spots. In this model the frequency of the recorded 1658 MHz line corresponds to the third harmonic of electron gyrofrequency, which yields the magnetic field (196 + 4) G along the magnetic tube axis. The linewidth Af/f ∼ 0.1 is determined by the 10% inhomogeneity of the magnetic field over the cross-section of the tube; the line profile indicates the kinetic temperature distribution of electrons over the tube cross-section with the maximum value 4 x 106 K. Analysis shows that study of cyclotron lines can serve as an efficient tool for diagnostics of magnetic fields and plasma in the solar active regions and flares.

A long term multifrequency campaign on the T Tauri star RU Lupi has been carried out in the X-Ray, UV optical and IR spectral regions with ASTRON and IUE satellites and ESO 1.5 m+IDS, 1.4 m CAT, 0.5 m UBVRI and 1 m IR telescopes, respectively.

We present two flare-like events occurred on April 17, 1984 and June 30, 1986. The first one was detected only in the UV, clue to lack the of simultaneous observations in other spectral regions. The second one was observed in UV, optical and IR regions showing a maximum roughly in the U band A comparison of the whole energy distribution of this event with that of a quiescent state observed on June 27 shows a flux enhancement of (89 ± 2)%. A detailed analysis of UV continuum and lines, namely N V, C I, CII, CIV, Si II and Si IV, shows that ihe surfaces fluxes of RU Lupi are always larger than those observed on typical flare stars and on the Sun by a factor of roughly 2 and 3 orders of magnitude, respectively. This fact allows us to conclude that RU Lupi activity cannot be explained even invoking a complete coverage by solar-like plages. On the contrary a patchy distribution of the emitting regions could explain the observed behaviour of this active star.

Photographic surveys completed during the past 30 years yielded data of about 600 flare stars in these fields. Although their average flare number is very low 17 of the stars produced 10… 120 flare ups. In order to investigate the possible long-term activity changes of these objects a new method - which is described below -was developed. Making use of it significant flare frequency variations were found at two out of three ‘active’ flare stars.

The results of the intensive observations of the Pleiades flare stars for more than 20 years (≈ 3000 h effective observational time) were compiled and published by G.Haro and collaborators in a catalogue (Haro et al.,1982) including the flare stars discovered in the Pleiades region up to 1981. The catalogue contains the data far 519 flare stars.

In the frame of the international programme for the study of flare stars in stellar aggregates the Department of Astronomy and the National Astronomical Observatory have carried out patrol observations in the Pleiades since 1979. Parallel to the observations, work on the creation of machine-readable versions of the published flare stars catalogues has begun. As a firatstep, a machine—readable version of the Tonantzintla catalogue (TC of the Pleiades flare stars has been prepared (Tsvetkov et al., 1987).

In the frames of the programme of nonstable star investigations in the Department, of Astronomy and National Astronomical Observatory the investigations of flare stars- stars of spectral type dK - dM whose nonstabi1ity appears first of all through the quick brightness increases take an essential place. The programme of systematic observations was initiated in 1979 with setting into operation of the telescopes of the Rozhen National Observatory.

We have been observing in the Sco-Oph dark cloud region for 12 years. We have only reduced a small part of the plates. Nevertheless, 12 new flare stars and more than 40 new variables stars have been found. Besides, we have discovered a new subgroup of RR stars in the-globular cluster M4.

The size, location, and flux of four solar soft X-ray sources, including a flare, were derived from the disk-integrating GOES X-Ray Sensors during a solar eclipse. The fluxes were used to derive the temperatures and emission measures of these sources. If an incorrect amount of flux is attributed to the flare source, then the evolution of these derived plasma properties seems unphysical.

We undertake a statistical analysis of the soft X-ray (3.5 – 5.5 keV) profiles of solar flares as observed with the Hard X-ray Imaging Spectrometer on the SMM. The durations, maximum intensities and intensity profiles of the flares are examined. The properties of the “typical” solar flare are discussed. The distributions of the measured parameters with respect to one another reveal some interesting results. In common with past studies, we conclude that there is no evidence to suggest that more than one type of event is being viewed, despite a desire evident in the literature to place events into distinct groups. We also conclude that commonly held views about the relatonships between flare duration and intensity, profile asymmetries and intensity etc.. are in error. For more details of the flare events, the selection of data and the method of analysis, the reader is referred to Pearce and Harrison (1988).

The eruption of a large flare on the east limb of the Sun was observed by the X-Ray Polychromator (XRP) on board the Solar Maximum Mission (SMM) on 19 May 1984. The XRP Flat Crystal Spectrometer (FCS) made polychromatic soft X-ray images during the preflare, flare and postflare phases. The XRP Bent Crystal Spectrometer (BCS) provided information on the temperature and dynamics of the hot (Te > 8 x 106K) coronal plasma from spectra integrated spatially over the whole region.

In two major solar flares, we have found evidence for hard X-ray footpoint emission using the X-ray imager on Hinotori. Possible factors are discussed as to why we only rarely detect footpoint emissions, which may not always take the form of double sources, depending on magnetic configurations in flares.

Neutrons are sometimes released from solar flares accompanied by generation of high energy protons and other nuclei and some of them occasionally reach the earth before they decay radioactively. Together with the satellite observations on gamma ray and neutron emissions associated with such flares, the ground-based observations of these neutrons give us a clue to understand the possible nuclear interactions of those protons and nuclei with gases ambient in the solar atmosphere. A review is given on the Japan-China collaborative program on the ground-based observations of these neutrons for the coming solar maximum, though there still remain obstacles to be resolved.

We study the statistical relationship between optical flares and type III radio bursts, using modern and extensive computer files. Results emerge along two main lines, concerning the physical mechanism of ejection of energetic particles, and the magnetic field geometry respectively.

First, we find that type III probability of occurrence increases strongly with the brightness of a flare and its proximity to a sunspot, and with accompanying prominence activity. This suggests that Bornmann's class I and III events correspond to distinct physical phenomena, particle acceleration and magnetic expansion respectively, both working simultaneously in class II events, which are the most favorable to the ejection of energetic particles out of flaring sites.

We study the dynamics of an IIα absorbing structure associated with a type III radioburst. We use a “Differential Cloud Model” to analyse the observed line profiles, in order to determine radial velocity and microturbulence (or shear) as functions of time.

We assume that the material is flowing inside magnetic flux tubes, with a given velocity distribution. We analyse the motions in a phase-diagram (position x,velocity V). The evolution of velocity and microturbulcnce is compared to the observations. The model accounts qualitatively for the evolution of velocity, and qualitatively for the decrease of microturbulence.

We present recent results from meter-decameter imaging of several classes of solar radio bursts: Preflare activity in the form of type III bursts, correlated type IIIs from distant sources, and type II and moving type IV bursts associated with flares and CMEs.

Some observational results and statistical characteristics of solar millimeter wave bursts observed by employing the 35 GHz, solar interferometer in Nagoya are briefly described.

A gradual rise and fall flare, with a duration of about one hour was observed on June 10, 1980 in the radio (Toyokawa and VLA), optical (Bing Bear) and XUV (SMM satellite) ranges of wavelengths. The flare developed as a large loop connecting two regions of opposite polarity in a pre-exisisting active region. A model of the differential emission measure of the loop observed at three different stage of the flare is deduced from the analysis of the XUV images in C IV (1549 A), O VIII (18.97 A), Ne IX (13.45 A), Mg XI (9.17 A) and Si XIII (6.65 A) emission lines. The differential emission measure as a function of temperature is controlled by the conductive flux via the temperature gradient; the evaluation of the divergence of the conductive flux is used in the energy balance to have information on the power deposition function.