The new digitalized polarispectrometer IKARUS (Perrenoud, 1979) at ETH, Zürich gave the possibility for a more detailed investigation of the solar type III spectral evolution.

The data on type V solar radio bursts obtained at IZMIRAN with the 45-90 MHz radiospectrograph are analyzed. A great variety and complexity in the dynamic spectra of these events is found. A number of categories of bursts with different emission characteristics of the leading and following edges are distinguished. A number of types of fine structure were found in the dynamic spectra of many bursts. Type V bursts, for which the radio emission at the fundamental and the second harmonic is clearly observed are analyzed.

Crannell et al. (1978) have reported an observed correlation between the time profiles and flux densities of impulsive hard x-ray and microwave solar bursts. We report here on a significant correlation between the fluxes of extended bursts of hard x-rays and microwaves. The significance of our observations follows from the suggestion of Wild, Smerd and Weiss (1963) that the extended bursts are evidence for a second phase acceleration process in the corona. We show that the observed characteristics of these extended microwave bursts (viz. rather a flat spectrum below a turnover frequency which is independent of intensity) can be explained by gyrosynchrotron radiation from the same population of energetic (E ~ 100 keV) electrons as those emitting thin-target x-ray bremsstrahlung.

One of the most exciting plasma physics investigations of the recent years has been connected with the understanding of a new strong turbulent plasma state excited by propagating electron beams. This new state is initiated on the linear level by parametric instabilities (OTS, modulational, etc.) and results in a very dynamic state composed of collective clusters of modes called solitons, cavitons, spikons, etc. Introduction of these concepts to the classic beam plasma interaction problem has rendered quasilinear and weak turbulence theories inapplicable over most of the interesting parameter range, and helped explain many paradoxes connected with the propagation of beams in the laboratory and space. Following a brief review of these non-linear notions, we demonstrate how their application to type III solar radiobursts has revolutionized our understanding of their propagation, radio-emission and scaling properties and has guided the in situ observations towards a more complete understanding. A particular burst (May 16, 1971) is analyzed in detail and compared with numerical predictions.

By the use of semi-analytical method, modeling of three kinds of type III solar radio bursts have been made. Many basic problems about the type III bursts and associated solar electrons have been solved showing some striking or unexpected results. If the fundamental radio emissions should be really observed as the normal type III bursts, the emission mechanism would not be the currently accepted one, i.e. the scattering of plasma waves by ions.

To investigate by numerical analysis the transition towards the asymptotic regime postulated by Ryutov and Sagdeev (1970) in their study of the expansion of a hot electron cloud through a plasma, a physically consistent system of one-dimensional quasi-linear equations has been solved on a Cyber 7600. The results obtained for various conditions of injection were all found in qualitative agreement with the simple analytic description of Ryutov and Sagdeev, in spite of the serious deficiencies of their analysis. Although the aims and techniques of the present study are quite distinct from published numerical works by Takakura and Shibahashi (1976) and Magelssen (1976), all the results point towards the same conclusion: plasmon emission, due to advective instability at the front of the electronic disturbance, is followed by strong reabsorption through a reverse advective process at the rear. In the present note, only the case of a localized hot electronic distribution (initially Maxwellian with Te = 108 K) is considered. The effect of plasmon-plasmon scattering, which can also be included in the programmes, is mentioned in the conclusion.

It is hypothesized that the observed clumping of plasma waves in type III sources in the solar wind is due to suppression of the linear stream-plasma interaction by density inhomogeneities of scale size comparable to the characteristic amplification length. Criteria are given for when such suppression should be important. The magnitude and scaling of density inhomogeneities in the 50–200 km range near 0.5 AU is estimated from interplanetary scintillation data. This information is used to construct model sources in which plasma waves are traced and amplified with random inhomogeneities to test the hypothesis. Significant clumping occurs for inhomogeneity scales of 50 and 100 km with inhomogeneity expectation values of 4.8×10−3 and 6.0×10−4, respectively, but not for 200 km scales. Further research is suggested to determine more completely the effects of density inhomogeneities in type III sources. Further details can be found in Smith, D.F., Sime, D. 1979, Astrophys. J., in press.

We present simultaneous energetic electron and solar radio observations from the ISEE−3 spacecraft of several solar type III radio bursts. The UC Berkeley energetic particle experiment measures from 2 to ~ 103 keV with good energy and pitch angle resolution while the Meudon/GSFC radio experiment tracks type III radio bursts at 24 frequencies in the range 30 kHz—2 MHz.

We report on the positional and polarization characteristics of Type III bursts in the range 24–220 MHz as measured by the Culgoora radioheliograph, spectrograph and spectropolarimeter. Our study includes 997 bursts which are of two classes: fundamental-harmonic (F-H) pairs and “structureless” bursts with no visible F-H structure. In a paper published elsewhere (Dulk and Suzuki, 1979) we give a detailed description and include observations of source sizes, heights and brightness temperatures. Here we concentrate on the polarization of the bursts and the variation of polarization from centre to limb. The observed centre-to-limb decrease in polarization approximately follows a cosine law. This decrease is not as predicted by simple theory but is consistent with other observations which imply that open field lines from an active region diverge strongly. The observed o-mode polarization of harmonic radiation implies that the wave vectors of Langmuir waves are always parallel, within about 20°, to the magnetic field, while the constancy of H polarization with frequency implies that the ratio fB/fP, the Alfvén speed vA and the plasma beta are constant with height on the open field lines above an active region. Finally, we infer that some factor, in addition to the magnetic field strength, controls the polarization of F radiation.

The suggestion is made that the excessive time delays between the two components of f-h type III pairs might be partly accounted for by an initial production of the second harmonic only, followed later by the production of both components. The source is assumed to be inside coronal streamers. Theoretical arguments lend some support to the suggestion of a delayed start of the fundamental.

Type III solar radio bursts are known to be excited by solar electron clouds travelling outwards through the solar corona and interplanetary medium. According to the “plasma hypothesis”, electron plasma oscillations are created by the passing beam, which are in turn converted into electromagnetic waves.

It is well known that pulsations with quasi periodicities of the order of 1 s occur during type IV emissions. Recent observations of the Nançay Radioheliograph operating at 169 MHz have revealed the existence of modulations on a longer time scale and brought new information on the faster one.

From observations with the Culgoora spectropolarimeter operating in the frequency range 24–200 MHz, we find that Type V bursts are often polarized in the opposite sense from the preceding Type III bursts. We present here the results of a statistical study of how frequently this occurs and the relationship of such polarization reversals to the position of the source on the disk of the Sun. We then examine two possible reasons for the reversal of polarization. In a paper published elsewhere (Dulk et al., 1979) we give a more detailed description and include observations of source sizes, positions and brightness temperatures of Type III-V bursts.

The study of fine structures in continuum radio emission from the sun is important to probe the physics of radio sources and of flares in particular. In this light the existing fine structure theories are reviewed and an application to a radio flare is given.

When studying the propagation of accelerated electrons outwards in the corona, we have shown that the perpendicular momentum of the electrons remaining after the type I process is transformed into parallel momentum during the propagation along the decreasing magnetic field, and that type III emission can occur when the parallel velocity component reaches a critical value. With this model we explain in particular the low frequency cut-off of type I emission, the characteristics of the type III bursts near their starting frequency and the transition between type III- and type I-like decameter emission observed in few cases.

Plasma wave electric field measurements with the solar orbiting Helios spacecraft have shown that intense electron plasma oscillations occur in association with type III solar radio bursts, thereby confirming a well known mechanism for generating solar radio emissions first proposed by Ginzburg and Zheleznyakov in 1958. In this paper we review the principal characteristics of these plasma oscillations and compare the observed plasma oscillation intensities with recent measurements of the emissivity of type III radio bursts. The observed emissivities are shown to be in good agreement with two current models for the conversion of electrostatic plasma oscillations to electromagnetic radiation.

The Planetary Radio Astronomy receivers aboard the Voyager spacecraft have been recording solar bursts of Types II (Boischot et al. 1930) and III (Riddle 1979) since launch in August and September 1977. The receivers themselves have been described in detail elsewhere (Lang and Peltzer 1977, Warwick et al. 1977). This study restricts itself to data from the 69 channels uniformly spaced between 20.4 and 1326.0 kHz. Each of these channels can be sampled once each six seconds, and the signal to noise ratio on each sample is about 0.8 dB. The sense of circular polarization to which a receiver responds alternates with each sample.

Space observations of solar radio bursts have provided the following information:

– From a single spacecraft:

Measurements within the burst source or close to it: fundamental and harmonic type III radio emission, the corresponding plasma waves and spectra of the exciting electrons.

– From a spacecraft and the earth or from two spacecrafts:

A better evaluation of the influence of the ionosphere on some ground-based observations.

Measurements of the beaming of the emission which yield constraints on the radiation mechanism and/or the role of coronal propagation in determining the source size and directivity (type I and III's).

Measurements of the differential time delay which yield for type III:

At short (m- and dam-) wavelengths, some evidence of group delays,

At long (hm- and km-) wavelengths one coordinate of the source.

Complete (3-dimensional) localization of the source at long wavelengths and therefore maps of the heliosphere magnetic field and electron density as well as the source size and, in the future, its polarization.

The results of these observations and their interpretation are reviewed and discussed.

High resolution spatio-temporal observations of noise storms in the meter wavelength range show the existence of many center-limb effects. We show that these effects cannot be interpreted without appealing to scattering effects which must take place close to the primary source of electromagnetic radiation. This contradicts STEREO-1 observations of very directional radiation which require that the scattering takes place far from the primary source, in order to be consistent with the observed source sizes. Using other results of the STEREO-1 Experiment, we suggest a model for the noise storm region in which the Type I bursts are emitted by gyromagnetic radiation of electron packets. The emission is very directional and oriented in the direction of the local magnetic field. The various center-limb effects and the spatio-temporal behavior can be explained using strong scattering effects in a fibrous medium.

Three dimensional “snapshots” of the large scale solar magnetic field topology as well as the solar wind electron density distribution from about 0.1 to 1 AU are obtained by tracking traveling solar radio bursts at hectometer and kilometer wavelengths with instruments aborad the ISEE-3 satellite and the HELIOS-2 solar probe. Both instruments observe in the frequency range from 30 kHz to 1 MHz and both are equipped with dipole antennas located in the vehicle spin plane. ISEE-3 also has a dipole along the spin axis and the signals from the two ISEE-3 antennas are combined to give the azimuth and elevation angles of the radio source. Triangulation between HELIOS-2 and ISEE-3 provides the additional observation necessary to uniquely determine the position of the radio source in space at each observing frequency. The techniques will be outlined, and illustrated by an example of the three dimensional field geometry and electron density distribution determined by the observations.