Some examples are presented of fast variations of the structure and the intensity of coronal condensations. For the observations the lines λ 5303 and λ 5694 have been used. Most of the observed phenomena are flare-connected, but all of them are related to some kind of Hα-activity. Sudden brightening of the emission lines may occur after the disappearance of a filament. The events have a duration of up to several hours and consist of hot clouds of high density that are slowly rising from the upper chromosphere into the lower corona.

The reader is kindly asked to consider this report for some sort of greatly unbalanced equilibrium between the pressure exercised by the steadily increasing flow of papers discussing the vastly extending subject of solar activity, and the equally steady pressure exercised by the editor who insists on having this report twice as short as three years ago. Therefore, some selection of the subjects and papers had to be made, a fairly severe one, since only about one third of all published papers could have been mentioned in the references, and I am well aware that almost everyone who will read the report, could suggest some improvement of the selection scheme. Nevertheless, I believe that the report as it stands, can fulfill its basic purpose, i.e. to give to the readers a general picture on the progress, which has been achieved in the field of solar activity research since the Prague meeting and on some of the research activities which are in progress at the present time.

It is a great pleasure to acknowledge the enormous help I have received from the authors of the individual sections of the Draft Report, as well as from many other members of the Commission, who have very kindly provided me, or the co-authors, with useful information on the recent progress of their work. It is necessary to add that some highly valuable comments made by V. Bumba, J. T. Jefferies, C. Sawyer, E. v. P. Smith, B. Valníček and J. M. Wilcox, have also been included in other chapters than those prepared by themselves.

Beginning in summer 1957, W. N. Christiansen and D. S. Mathewson [1] have regularly obtained two-dimensional radioheliograms at λ = 21 cm with a resolving power of 3 minutes of arc. The authors have already noticed the close connection between the regions of radio emission on the one side and the fields of spots and faculae on the other side. Considering, however, that the radio emission at the wavelength involved emerges from heights of 20,000 to 50,000, or even up to 100,000 km above the photosphere, i.e. from the inner corona, it seems to be more suitable to compare the radioheliograms with the optical emission of the corona than with the photospheric and chromospheric phenomena. Yet, as the coronal observations can be carried out at the solar limb only, it is difficult to get optical pictures of the corona in front of the sun's disk. Such a picture has to be built up from the daily limb-observations covering the period from 7 days before to 7 days after the date in question; e.g. the coronal intensities shown along the central meridian are measured 7 days before at the E-limb or 7 days later at the W-limb. Since the corona may change greatly within a few days—especially during the present high solar activity—the reliability of an optical corona-picture diminishes from the limb to the central meridian. In addition, a further uncertainty has to be considered in constructing coronal maps, in so far as no station possesses complete coronal observations; therefore, observations of different stations have to be used, which are very difficult to reduce to each other. The main difficulty of such a reduction arises from the fact that the single stations carry out their observations at different distances from the sun's limb ranging from 20,000 to 45,000 km. The coronagrams discussed in the following are based on the intensities of the green coronal line 5303 Å as published in the Quarterly Bulletin on Solar Activity [2].

The so-called enhanced radiation appears at all frequencies in connexion with sunspot-regions. For λ < 25 cm. it may be interpreted satisfactorily as thermal radiation of the coronal condensations (Waldmeier, 1955) [1]. For λ > 50 cm. the mechanism responsible for the production of this emission is still unknown. The enhanced radiation from 62 to 200 Mc./s. shows a strong concentration in the radial direction and, therefore, becomes observable only when the source of radiation is close to the centre of the sun's disk. For sin θ = 0·3, i.e. at an angular distance of θ = 17° from the centre of the solar disk, the intensity has dropped to 50 % of that for θ = 0° and for sin θ = 0·5 to about 25 %. For sin θ = 0·8 the radiation intensity is certainly < 10%. This property has recently been investigated again by Machin and O'Brien (1954) [2] as well as by H. Müller[3], using the entire material of the years 1947–53 published in the Quarterly Bulletin on Solar Activity. According to Müller the limb darkening is very similar for the frequencies 62 to 200 Mc./s., the radiation intensity having decreased at sin θ = 0·29 to half of its value for sin θ = 0. A diminution to 10% may be found for 62 Mc./s. at sin θ = 0·54, for 80 Mc./s. at sin θ = 0·59, for 98 Mc./s. at sin θ = 0·62, for 175 Mc./s. at sin θ = 0·72 and for 200 Mc./s. at sin θ = 0·77. For 600 and 1200 Mc./s. the limb darkening is much weaker, the intensity decreasing to 50% at sin θ = 0·57 and 0·75 respectively.

Introduction. Ten years have passed since the last Congress and so much has been published since then in the field of photospheric and allied phenomena that this can only be a summary and not an exhaustive report.

Periodicity of Sunspot Activity. The long period already established by R. Wolf in 1862, has again been investigated by W. Gleissberg (Publ. Istanbul Obs. No. 14, 1941; No. 15, 1941; Z. Ap. 21, 347, 1942; Ap. J. 96, 234, 1942), with the result that the period amounts to approximately 70 years.