Solar flare hard X-ray data obtained by Prognoz-9 spacecraft (Abrosimov et al 1988) in the energy range 10-200 keV are analysed. In examples of events which we consider here, high energy X-ray pulses appear earlier than low energy ones, which is contrary to many other events where the low energy X-ray peak emission takes place earlier. The variation of the spectral index was dynamical.

It is shown that stochastization of particles due to Alfven waves is accompanied by their effective acceleration in loop structures of solar magnetic fields.

We present an analysis of the relationship between coronal mass ejection (CME) events with solar flares and coronal holes. Out of 79 CME events whose location and span are known, 10 CME event circles contain solar flares. On the other hand out of 71 CME events (selected for the study with coronal holes) 40 CME event circles contain coronal holes. We have examined the above associations by calculating the Yule's coefficients of association and found that the CME events and solar flares are statistically not associated while CME events and coronal holes are statistically associated. This does not necessarily indicate that CMEs come from coronal holes as claimed by Hewish and Bravo (1986).

Part of the energy of low solar corona may be released and converted to plasma energy that may be the dominant source of solar flares. Recently, many physical mechanisms have been proposed to accelerate ions and electrons. In the magnetic mirror device of fusion research, ion cyclotron instability and the energetic ion losses have been discovered and the plasma becomes electron rich.

A slow evolving flaring loop has been observed by the UVSP, XRP and HXIS instruments on board SMM on June 10, 1980. Simultaneous radio observations from Toyokawa (Japan) are also available. The former instruments have an angular resolution ranging from 3“ to 30“ which allows the determination of the loop structure. The XRP:FCS rasters permit to analyze the flare only during three time intervals of about 10 minutes each.

The emission of solar flares at millimeter wavelengths is of great interest both in its own right and because it is generated by the energetic electrons which also emit gamma rays. Since high-resolution imaging at gamma-ray energies is not presently possible, millimeter observations can act as a substitute. Except for that class of flares known as gamma-ray flares the millimetric emission is optically thin. It can be used as a powerful diagnostic of the energy distribution of electrons in solar flares and its evolution, and of the magnetic field. We have carried out high-spatial-resolution millimeter observations of solar flares this year using the Berkeley-Illinois-Maryland Array (BIMA), and report on the preliminary results in this paper (Kundu et al 1990; White et al 1990). We also report some recent results obtained from multifrequency observations using the VLA (White et al 1990).

The ISEE-3 spacecraft has provided in situ observations of electron beams, plasma waves, and associated solar type III radio emission in the interplanetary medium near 1 AU. These observations show that electron beams are formed by the faster electrons arriving before the slower ones, following an impulsive injection at the Sun. The resulting bump-on-tail in the reduced one-dimensional distribution function, f(v||), is unstable to the growth of electrostatic electron plasma (Langmuir) waves. The Langmuir waves are observed to be highly impulsive in nature. The onset and temporal variations of the observed plasma waves are in good qualitative agreement with the wave growth expected from the evolution of measured f(v||). However, far higher Langmuir wave intensities are predicted than are detected. In addition, the lack of obvious plateauing of the bump-on-tail suggests that the waves have been removed from resonance with the beam electrons by some wave-wave interaction. Bursts of low frequency, 30–300 Hz (in the spacecraft frame) waves are often found coincident in time with the most intense spikes of the Langmuir waves. These low-frequency waves appear to be long-wavelength ion acoustic waves, with wave number approximately equal to the beam-resonant Langmuir wave number. The observations suggest several possible interpretations: modulational instability, electrostatic decay instability, and electromagnetic decay instability; but none of these are fully consistent with the observations. Microstructures, too short in duration to be resolved by present experiments, have been invoked as an explanation of the phenomenon. Experiments are currently being developed to study these processes using fast wave-particle correlation techniques.

This paper summarises the methods and main results of studying physical parameters and processes in different structures of the solar atmosphere from the chromosphere to the outer corona as based on the observations with a 600-meter radio telescope the RATAN-600 (1975-1989). Diagnostics of the solar plasma have been made using the spectral-polarization measurements in the wavelength range 0.8-32 cm with high spatial resolution. The most important results concern the structure and strength of the magnetic fields at different levels of the solar atmosphere and detection of localized regions of long-lasting (several days) energy release in the lower corona.

A short-time variability study of a solar active region simultaneously at 6 and 2 cm wavelengths was carried out using the VLA. The observations show interesting uncorrelated brightness temperature variation at the two wavelengths. The observed low brightness temperatures indicate that the emission is mainly originating from the chromosphere - corona transition region.

A transition region model with constant pressure and power-law temperature variation as a function of height has been assumed to analyse the data. The uncorrelated variation of the observed brightness temperature at the two wavelengths suggest different dominant emission mechanisms (bremsstahlung at 2cm and gyro-resonance at 6cm) operative at the two wavelengths. It is shown that an independent variation of a few percent in the magnetic field (900 ±45 G) and a factor of two variation in the density (2 to 4 × 1010 cm−3) over a time scale of few hours is required to explain the uncorrelated brightness temperature variations at the two wavelengths.

We review some recent studies of mass ejections from the Sun using 2-D imaging observations of the Clark Lake multifrequency radioheliograph. Radio signatures of both fast and slow coronal mass ejections (CMEs) have been observed using the Clark Lake radioheliograph. Using temporal and positional analysis of moving type IV and type II bursts, and white light CMEs we find that the type II's and CMEs need not have a direct cause and effect relationship. Instead, the type II seems to be generated by a “decoupled shock”, probably due to an associated flare. The moving type IV burst requires nonthermal particles trapped in magnetic structures associated with the CME. Since nonthermal particles can be generated independent of the speed of CMEs, moving type IV bursts need not be associated only with fast CMEs. Specific examples are presented to support these views.

VLA observations at 20-cm wavelength specify the brightness temperature and magnetic structure of plasma constrained within coronal loops in solar active regions. Comparisons with simultaneous SMM observations at soft X-ray wavelengths lead to measurements of physical parameters like electron density, electron temperature and magnetic field strength. Such comparisons also indicate coronal loops can be detected at either radio or X-ray wavelengths while remaining invisible in the other spectral domain, and that the dominant radiation mechanisms can be thermal bremsstrahlung or thermal gyroresonance radiation. VLA observations at the longer 90-cm wavelength reveal the thermal emission of a hot transition sheath enveloping a cooler, underlying Hα filament seen in absorption. The 20-cm VLA observations indicate that the precursor, impulsive and post-flare components of solar flares originate in spatially separated and resolved sources.

We trace type III radio bursts from the sun until they can be observed as in situ electrons at the ISEE-3 spacecraft. Our study extends over the period of operation of the electron experiment on ISEE-3 from August 1978 to November 1979. Our observations include data from solar flares, kilometric type III burst spectra from ISEE-3, and in situ measurements of low energy electrons from ISEE-3. By carefully restricting the data sets involved, we find a peak of 20° width in the number of flares associated with in situ electrons near 60° west solar longitude. This peak shows that the electron beams of type III bursts are not spread over a great range of longitudes as earlier studies indicate, but are fairly limited in spatial extent. However, even in that longitude range only 40% of type III-flare events are associated with in situ electrons.

We observed the continuum emission from the radio sun when there is no burst activity at λ = 8.7 m with the large decameter wave radio telescope at Gauribidanur (Latitude 13° 36‘ 12“ N and 77° 27‘ 07“ E) with a resolution of 26'/40'. A compound grating interferometer with one dimensional resolution of 3' is also used. These observations are made during August 1983 and June 1986. The brightness temperature at the center of the sun varied from 0.2 106 K to 0.8 106 K during these periods on time scales of several hours to a day. Since the sun is absolutely quiet during these periods we believe that the radiation is purely thermal in nature. In this case the observed brightness temperature variations imply large scale density variations by more than a factor of three if the corona is optically thin at these wavelengths. Alternatively if the corona is optically thick the observations imply real electron temperature variations with or without accompanying density variations.

A possible contribution of whistlers to fine structure production due to the coalescence process with plasma waves 1 + w → t at frequencies ω1 ± ωw = ωt is estimated. A whistler ray tracing in the solar corona with the twice Newkirk density model and with a dipole magnetic field using a standard numerical integration of the Haselgrove two-dimensional equations is performed.

The large behind -limb (W 140, 0900 UT) flare on February, 16, 1984 was followed by a complex of radio bursts - type II, III, IV and V bursts.

We study Parametric Decay Instabilities(PDI) using the kinetic description, in the homogeneous and unmagnetized plasmas. These instabilities cause anomalous absorption of the incident electromagnetic (e.m)radiation. The maximum plasma temperatures reached are functions of luminosity of the non-thermal radio radiation and the plasma parameters.

A statistical analysis of Clark Lake microbursts was performed using a data base consisting of ~ 100 bursts. The results are: (1) Microburst sources are well correlated with coronal streamers and active regions. (2) The peak brightness temperature (T^) of the microbursts is in the range 6 × 105 K to 6 × 107 K (see Fig. 1; see also Kundu et al, 1986; White et al, 1986; Gopalswamy et al. 1987) whereas it is quite high for normal type III bursts (107 < Tb < 1012 K - Dulk and Suzuki, 1980). (3) The source size varies over a wide range of 5.5 − 10.5 arc min at 73.8 MHz, 8.5 − 13.5 arc min at 57.5 MHz, 10.5 − 16.5 arc mins at 50 MHz and 12.5 − 22.5 arc min at 38.5 MHz. (4) The drift rate lies mainly in the range 4.5 to 6.5 MHz s−1, with an average value of ~ 5.5 MHz s−1.

During September 1988 (International Solar Month) we observed the Sun with the Very Large Array on 4 days in the period Sep. 11-17. The VLA was in its most compact configuration, which is ideal for studying large-scale coronal structures. Here we summarize some preliminary results of the observations at 0.333 and 1.5 GHz. Despite the presence of numerous active regions the Sun was actually very quiet, with no flares during our observing, and this allowed us to make high-dynamic-range maps.

We discuss observations of a highly-circularly polarized multiply-impulsive microwave burst detected by the Very Large Array and the Phoenix Digital Radio Spectrometer. The VLA was used to resolve the burst in two dimensions, while PHOENIX provided high time resolution information about its spectral properties. During part of the burst, positive frequency drifts were detected, suggesting inwardly propagating beams of electrons emitting Type III-like radiation.