The study of a moving type-IV burst shows that acceleration of electrons can happen in the high corona, and a mechanism is proposed for such an acceleration. The low-frequency cut-off of radio spectrum is interpreted as due to the effect of the coronal plasma upon the synchrotron emission, and this leads to an accurate determination of the intensity of the magnetic field at an altitude of 1 R⊙. Then it is possible to compute the energy and the density of the relativistic electrons.

It has been shown that moving type-IV bursts at decameter waves are infrequent events compared with the moving type-IV bursts at meter waves. They belong preferably to sunspot groups of relatively homogeneous magnetic structure. Moving type-IV bursts at decameter waves have a very low tendency to repeat. This statement indicates that the magnetic structures producing them have only a short lifetime. The short duration of these bursts cannot be interpreted by radiation damping of high energetic electrons. Furthermore it can be shown that the storm continuum at decameter waves is related preferably to sunspot groups of complex magnetic structure. Probably stationary type-IV bursts at decameter waves are more representative for the acceleration of high-energetic particles than the moving type-IV bursts at decameter waves.

To study the nature of the local sources of solar radio emission connected with active regions, it is important to investigate the structure of such sources and their emission spectra. These problems are being investigated in detail by a group of workers of the Radio Astronomy Department of the Pulkovo Observatory led by G.B. Gelfreikh. The Pulkovo large radio telescope used for the observations allows them to investigate the solar radio sources at the whole cm-wavelength range with a high resolution reaching 40 sec of arc at the 2-cm wave. Observations are taken at 2·0, 3·2, 4·4, 6·6, and 9-cm waves. The author observes at the 2-cm wave.

A high-resolution 24-element, E–W interferometer situated at Kalyan near Bombay has been used to study the sources of slowly varying component and of bursts at a frequency of 612 Mc/s. This interferometer has a half-power beamwidth of 2·8 min of arc, and the fan beams are located about a degree apart in the sky. Thus, a strip scan is obtained roughly every 4 min as the Sun drifts through the beams. About 20 slowly varying sources and 11 bursts were observed during the period June 1965 to February 1967 at 612 Mc/s.

The second Advanced Technology Satellite (ATS-II) carries a radio-astronomy experiment designed to perform radio-noise measurements in the range 0·5 to 3·0 MHz from above the terrestrial ionosphere. The spacecraft was launched into an 11000-km apogee, 180-km perigee orbit on April 6, 1967, and since there was considerable solar activity in the ensuing months it is possible to present a preliminary description of the kind of solar observations this experiment will provide.

The new big 22-m radio-telescope of Crimean Observatory shown on Figure 1 appeared to be of very good quality: for radio emission at λ = 8 mm the antenna beam is 1′.6, the efficiency ≅0·4, and the error of automatic pointing (guiding) does not exceed ±20″. A Dicke type radiometer was used as receiver possessing a sensitivity ≃2°K at the time constant τ = 1 sec, and pass-band ~30 MHz. The error in the contrast determination does not exceed ±0·5% of the level nearly corresponding to that of the quiet Sun. The scanning of the solar image is made in right ascension with velocity 0·5 sec in sec of time, the distance between successive scans in δ being 1′.5. An example of a single scan is shown in Figure 2, where the scan through Jupiter (for the determination of antenna beam) is also presented (for description see Moiseev, 1968).

The slowly varying component (SVC) of solar radio emission and burst activity associated with different types of spotgroups from the past solar minimum have been investigated statistically.

This work analyses some correlations or associations between certain features of solar radio bursts at 600 Mc/s and of associated chromospheric flares respectively. Subject of the analysis are 149 radio events observed at the Humain station and 125 associated flares observed at Uccle during the years 1957–61. It is found that the radioelectric energy of the bursts is associated with the eruptive area and the Hα intensity of the flares, whereas the duration of the bursts is associated with the Hα broadening. For bursts connected with a second phase, the eruptive area and the importance of the magnetic field have a bearing on the characteristics of the radio event.

We consider the variation of the synchrotron-flux density of relativistic electrons with the power law spectrum when the region of generation of this emission initially experiences a homogeneous compression and then an expansion. Ionization losses have been taken into account. The velocities of compression and expansion have been taken as constant. It is shown that in the cases of compression or expansion the flux density at a given frequency changes as

S(t) ~ S0Kγ(t)

where S0 = flux density before the compression, γ = index of the power law spectrum, K(t) = (H(t))/(H0 (t = 0)), and H is the magnetic-field strength. In the case of compression K(t) > 1·0 and in the case of expansion K(t)< 1·0.

The results obtained are applied to an explanation of the increasing and decreasing parts of impulsive bursts of the centimeter range. Such a description of the impulsive bursts has allowed us to estimate both the parameters of the radiating region and the parameters of the differential energetic spectrum of relativistic electrons.

One of the largest radio bursts on record took place on May 23, 1967. Peak flux densities ranged between 23000 flux units (10−22 wm−2 Hz−1) at 8800 MHz to about 370000 units at 606 MHz. In addition to the high-accuracy measurements of the peak flux densities at 606, 1415, 2695, 4995, and 8800 MHz, sweep frequency observations from 19–39 MHz show Type-IV emission with Type-II bursts occurring during the Type-IV continuum. The associated flare was clearly visible in white light. In reviewing and compiling microwave-radio data recorded during earlier white-light flares, it was found that all but one of eight events listed by Svestka (1966) had high microwave flux densities associated with them. In comparing radio-burst intensity with optical flare importance for the series of three flares between 1809 and 2150 UT on May 23, there is only moderate agreement. The first radio burst was small; the third was by far the largest, while the second flare had the highest optical classification. The flux densities of the third burst may have been the highest ever recorded in the decimeter portion of the radio spectrum and amongst the largest four in the 8800 MHz region. It is suggested that the details of the development of the flare might be followed on radio flux-density plots to determine detailed correlation with particle events in space and with terrestrial effects.

The concept active region, previously called ‘centre of activity’, was introduced by D'Azambuja as “the totality of all visible phenomena accompanying the birth of sunspots”. This idea was first placed in the centre of attention during the Convegno Volta in 1952 when Kiepenheuer put forward his paper: ‘Was ist ein Aktivitätszentrum auf der Sonne?’. At that time our knowledge of an active region could still be condensed in a five-page paper. During the present symposium we have listened to 89 communications, all dealing with certain aspects of the active region. Particular attention was given to the role played by supergranulation and by detailed magnetic fields; this was possible since we now have precise observations with good spatial and time resolution of the magnetic field, and very detailed information on the related optical structures.