(Solar Phys.). High spectral, temporal and spatial resolution observations were obtained with the 60-channel Utrecht solar radio spectrograph (160–320 MHz) and the 169 MHz Nançay solar radioheliograph. From a large number of type III bursts the average height was found to be 0.37 solar radius above the photosphere, corresponding to approximately the Newkirk streamer density, if the bursts are emitted at the harmonic of the local plasma frequency. No center-to-limb variation, nor east-west asymmetry was observed. All double bursts, double humped bursts, precursor-type III had exactly the same position and general shape for both members of the pair. From this it was concluded that fundamental-harmonic pairs are very rare at frequencies above 160 MHz (Mercier and Rosenberg, 1974).

(Solar Phys.). We identify from daily K-corona observations the coronal regions located above type III burst sources. The radio observations were made with the east–west log-periodic array at Clark Lake Observatory. East–west positions of 260 isolated type III bursts during March and April 1971 have been measured between 20 and 65 MHz (disregarding storms of type III bursts). The K-corona observations are made with the HAO K-coronameter at Mauna Loa, Hawaii. During 1971, the heights of observation were 3.6′, 5′, 9′ and 13′ above the limb.

(Solar Phys.). The positional analysis of solar bursts at meter and decameter wavelengths during the period July 31–August 7, 1972 is presented. The observations were taken with two arrays – a log periodic array of 16 elements situated on an E–W base line of 3.3 km and portions of the new Clark Lake array in the form of a Tee (an E–W arm of 32 log spiral antennas and a N–S arm of 16 similar antennas). The new array operates over the frequency range 10–120 MHz and has angular resolutions of approximately 3.′5 at 100 MHz and 8.′5 at 40 MHz in the E–W direction.

(Astrophys. Letters). The type V burst has been defined as a wideband continuum which sometimes appears for a minute or so following a type III burst (Wild et al., 1959b). It is now generally accepted that type III bursts arise from plasma waves set up by electrons escaping with velocity ~c/3 along open magnetic field lines (Wild et al., 1959a; Stewart, 1965); the most widely accepted explanation of type V continua is that they arise from plasma waves set up by electrons of similar velocity which have become trapped in a coronal magnetic loop (Weiss and Stewart, 1975). On this hypothesis the plasma waves are set up by two opposing electron streams in the trapping region, and from this consideration Zheleznyakov and Zaitsev (1968) have concluded that type V emission should be predominantly at the second harmonic of the local plasma frequency. In this paper we describe and discuss some two-dimensional observations of source positions of type III–V events which were obtained at 80 MHz on the Culgoora radioheliograph.

A 169 MHz radiometer fed from a Sun-pointed 8 dB antenna was flown on the Soviet probe Mars-3 in 1971–1972. This experiment called Stereo-1 is part of a cooperation program between the U.S.S.R. and France.

Radioheliographic observations of type III bursts occurring by pairs in a repetitive fashion were made with the high time resolution E–W Nançay Radio heliograph (169 MHz). The data were available in digitized form with a rate of 25 E–W profiles per second. The study is based on two days of observations: for the first one (January 14, 1971) the radio source was at 16′W and for the second one (June 29, 1971) at only 4′W.

(Proc. Astron. Soc. Australia). The Hα-flare on 1968 August 23d23h45m was followed by a short-lived type II burst. The radio spectrograph record obtained at Culgoora is shown in Figure 1. The type II burst was of split-band structure with unusual spectral features in each component of the split band. The most remarkable one is an absorption feature between 23h52m02s and 23h52m10s: suppression of the bright type II emission along an ‘inverted U’ (between 120 and 85 MHz).

(Solar Phys.). During 1969 and 1970 groups of generalized fast drift bursts were observed on 21 days with a high resolution radio spectrograph at Oslo Solar Observatory. Totally 48 groups were detected in the frequency band 310–340 MHz. In the great majority of the cases the groups were accompanied by metre wave type III bursts at lower frequencies.

Mechanisms for explaining the various forms of particles and radiation observed during the flash phase of solar flares are reviewed under the working hypothesis that the flash phase is the time in which electrons and to a lesser degree protons are accelerated in less than one second. A succession of such accelerations is allowed to explain longer lasting or quasi-periodic phenomena. Mechanisms capable of such acceleration are reviewed and it is concluded that first-order Fermi acceleration in a reconnecting current sheet is the most likely basic process. Such acceleration, however, gives rise to a rather narrow distribution of particle velocities along a given field line which is unstable to the production of electron plasma and ion-acoustic waves. This plasma turbulence can heat the plasma to produce soft X-rays and filter the initially narrow velocity distribution to produce a power law energy distribution. Electrons travelling inward from the acceleration region produce hard X-rays by bremsstrahlung and microwave bursts by gyro-synchrotron emission. Whereas the interpretation of X-ray spectra is relatively straightforward, the interpretation of microwave spectra is difficult because the source at low frequencies can be made optically thick by several different mechanisms.

Electrons travelling further inward presumably thermalize and produce impulsive EUV and Hα emission. The theory for these emissions, although amenable to present techniques in radiative transfer, has not been worked out. Electrons travelling outward give rise to type III radio bursts by excitation of electron plasma waves and the electrons observed at the Earth. Study of the interaction of a stream of electrons with the ambient plasma shows that the electron spectra observed at the Earth do not necessarily reflect their spectrum at the acceleration region since they interact via plasma waves as well as through Coulomb collisions. The mechanisms for the conversion of plasma waves into radiation and the propagation of the radiation from its source to the observer are reviewed.

This paper is an extension of an earlier paper by Zaitsev et al. (1972).

(Solar Phys.). The widely accepted emission mechanisms for type III bursts involve at least two stages. The first stage is the generation of Langmuir waves by the inferred stream of electrons. Emission at the fundamental frequency arises when these waves are scattered by thermal ions. Emission at the second harmonic arises when two Langmuir waves coalesce; however, the coalescence is possible only after an intermediate stage in which the distribution of Langmuir waves evolves towards isotropy due to scattering by thermal ions.

Generation of third harmonic plasma emission by coalescence of second harmonic radiation with a plasma wave is more favourable than by direct coalescence of three plasma waves. The predicted ratio of the intensities at the third (III) and second (II) harmonics is IIIIω/IIIω ~ ωLWlL/Nmc3, where ωL is the plasma frequency, Wl is the energy density in plasma waves, L the linear dimensions of the source and N the ambient electron number density. The intensity at the second harmonic is where LN(∼1010 cm at ωL ∼ 2π × 108 s−1) is the typical dimension of coronal inhomogeneities.

(Solar Phys.). The fine structure in solar type IV radio bursts was studied using the 169 MHz Nançay radioheliograph and the 60 channel radiospectrograph at Utrecht (160–320 MHz). The observed fine structure includes pulsating structure, zebra patterns (parallel drifting bands) and intermediate drift bursts. All are considered as modulation of high frequency radiation by low frequency oscillations or as the result of up conversion of low frequency oscillations to higher frequencies (Rosenberg, 1973).

(Proc. Astron. Soc. Australia). A theory is developed to account for the observed properties of type I storm bursts in terms of plasma radiation – that is, electromagnetic radiation at the electron plasma frequency resulting from the non-linear scattering of electron plasma waves on plasma ions. Now the average brightness temperature of a type I source is greater than 109 K, or even higher if, because of coronal scattering, the apparent source size is larger than the true source size. For brightness temperatures as high as 109 K the non-linear scattering must be of the induced kind in which electromagnetic radiation below the frequency of the electron plasma waves is amplified. For such radiation to be strongly circularly polarized in the o-mode, as observed in type I bursts, requires that the amplification be more effective in the o-mode than in the x-mode (Figure 1). This is found to be so for plasma waves excited by electrons travelling parallel to the magnetic field. The electric field of the plasma waves is then also parallel to the magnetic field. The non-linear scattering is more efficient for that magnetoionic mode which has the greater component of electric field in the same direction. This mode is the o-mode.

(Astrophys. Space Sci.). It is characteristic of the radiation from a particle of mass m bearing a charge e moving with ultrarelativistic velocity βc in a magnetic field of induction B0 that the bulk of the emission is confined to a small cone of directions within O(ξ) of the direction of motion τ which makes a constant angle α with B0, and that for any direction n at an angle θ with B0 such emission is observed as harmonics of the fundamental frequency fB0ξ/a, where fB0 is the nonrelativistic cyclotron frequency and

The passage of shock waves and ejected matter through the solar corona can produce type II and type IV radio bursts. This paper reviews the observations of these types of bursts and their interpretation, with particular emphasis on recent work.

Plasma ejections and waves in the solar corona are almost exclusively flare associated phenomena. Ejections of relatively cool and dense plasma are frequently observed in Hα whereas observations in coronal light (visible, EUV- and X-radiation) are still rather scarce. Occurrence of coronal waves is so far best known from their effects on the Hα chromosphere and, of course, from the production of radio bursts. Only in relatively few cases have observations been made in coronal lines and in coronal continuum by ground based as well as by satellite borne equipment. We may expect, however, that the white light coronagraph and the X-ray telescopes on board of the Skylab will detect quite a number of events in front of the solar disk and high in the solar corona and will considerably increase and improve our imperfect knowledge and understanding of coronal ejections and waves as it is presented in this review.

(Solar Phys.). A very bright coronal streamer was observed on December 13, 1971 by the Naval Research Laboratory's coronagraph on board of OSO-7. The next day, the streamer had changed it's brightness and configuration considerably. Three subsequent coronagraph images, taken on December 14 at 0407, 0418 and 0430 UT show a large plasma cloud moving outward from the Sun between 3 and 10 solar radii. They also show distinct smaller clouds moving outward with projected velocities between 950 and 1100 km s−1. Traced back in time to the lower solar corona, these clouds coincide with discrete type II radiobursts observed from Culgoora between 0241 and 0256 UT. Each single cloud shows it's signature in the radio recording between 100 and 20 MHz. The drift velocity of the radio bursts can be determined to be 1600 km s−1 using Newkirk's coronal streamer model. Assuming, that the plasma clouds are ejected from an active region 30° behind the east limb vertically, their true velocities close to the surface of the Sun would be approximately 1400 km s−1, which is in good agreement with the drift velocities determined from the type II bursts, considering all uncertainties. Therefore, the type II burst disturbance moves with the same velocity as the driving material.

(Solar Phys.). The association of flare sprays, distortions of the overlying coronal structures and moving type IV radio bursts is a reasonable one and is well accepted despite the paucity of observational evidence. On 12 August 1972 there occurred a flare spray observed optically by both flare patrol (National Oceanographic and Atmospheric Administration) and coronagraph (High Altitude Observatory) instruments. A subsequent moving type IV radio burst was recorded on two swept frequency interferometers (Universities of Colorado and Maryland). In addition distinct changes in the K-coronal brightness at 1.6 R⊙ were measured (High Altitude Observatory K coronameter). These observations combine to form one of the most complete sequences of measurements yet recorded covering the range from the chromosphere to about 6 R⊙. The separate measurements are discussed and we show that they can be combined to form a relatively simple physical picture of the whole event.

(Solar Phys.). On 1973 January 11, a flare near the west limb of the Sun caused a coronal disturbance which was observed with a unique variety of instruments. Radio observations of a type II and a moving type IV burst were obtained by the CSIRO Division of Radiophysics at Culgoora, Australia; white-light observations of a large, moving cloud were made by the U.S. Naval Research Laboratory coronagraph on OSO-7; K-corona observations of a decrease in coronal density were made by the High Altitude Observatory at Mauna Loa, Hawaii and Hα observations of a flare spray were made by the Institute for Astronomy, University of Hawaii at Haleakala (and also by H.A.O.).