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.

By action of the Executive Committee of IAU, the report for 1970 is restricted to half the length of the 1967 draft report; the bibliography and the abstracting of work already published are essentially eliminated. The report is no longer to serve primarily as a review of accomplishments between General Assemblies but rather as a report of work in progress. These changes were adopted by the Executive Committee in order to reduce the cost of publishing the reports.

Several commission members have voiced the opinion that the extensive bibliographies in past reports have been the most valuable contributions of the reports and they have objected to the elimination of the bibliography. Also, material from some observatories and from some national groups that have been submitted for inclusion in this draft report have contained only abstracts of work already published. The rules adopted by the Executive Committee do not permit the report to abstract published works except in evaluating general progress in solar astronomy. We express our regret to these commission members and to these institutions that we have not been able to include the abstracts and bibliography in the present report.

Of a select list of about 40 Fraunhofer lines distributed between λλ 3083-7122, central intensities corrected for scattered light, ghosts and instrumental profile, has been obtained for 18 lines. Uncorrected values of central intensity for a number of other lines are available. A complete list of the results of all observers will be given at the General Assembly in Brighton.

The working Group on Solar Eclipses regrets Dr J. Houtgast’s resignation as Chairman of the Group. However, Dr Houtgast will maintain his membership and continue to give his experience to the group.

The total eclipse of September 22,1968 was actively observed by many groups in spite of its short duration. Reports on the observations are expected to be given during the 1970IAU General Assembly.

A great deal of work has been done everywhere in view of the total eclipse of March 7, 1970.

Experiments on Lamb-dip stabilization of the 6328 Å He-Ne laser line made at NBS, NPL and PTB and reported in a joint paper (10) show that the wavelength from different lasers may differ by as much as one part in 107. Similar work on a C02 laser line at 10.6 μm and a xenon line at 3.5 μm is reported from NPL, the 1.15 μm line in a pure Ne 20 laser has been investigated at JILA (5), and several lines in a xenon ion laser are being studied at PTB. However, the method that holds the greatest promise of accuracy and stability is the use of saturated absorption to lock a laser wavelength to some molecular transition having a sharply defined and stable wavelength. Thus the 6328 Å He-Ne laser line can be fixed on a line in the molecule 127I2 (6, 7), and the 3.3913 μm line of a He-Ne laser has been connected to a line of CH4, promising a unique precision and stability according to experiments made at JILA (5) and NRC. This technique will evidently yield excellent substitutes for the Kr 86 line, and may, when the time is ripe, lead to the adoption of an improved primary standard.

A very great deal of work has been done during the last three years on the determination of atomic transition probabilities, and complete coverage here is impossible. Wiese and his staff at the National Bureau of Standards, Washington, have continued to collect all numerical results, and they can supply information as to what is available for any particular atom. They have published (1) a critical compilation of atomic transition probabilities for the atoms sodium to calcium (inclusive) in essentially the same way as an earlier compilation covered hydrogen to neon. At the time of writing a complete new bibliography is also in preparation (2), which will be published as NBS Special Publication 320 and which is complete up to June 1969. Review articles published include that of Layzer and Garstang (3) on theoretical allowed and forbidden transition probabilities, and the two volume conference report (4) covers beam foil spectroscopy (including lifetime measurements) in some detail. A fairly complete review of forbidden line transition probabilities was given by Garstang (5), where references to many original papers may be found. In the following we shall confine ourselves to mentioning a few areas of particular interest; detailed references can be traced through the bibliographies, reviews and abstracting journals.

Two review articles have been published (1,2).

1.1. Collision broadening by neutral atoms. The continued study of widths and shifts produced by neutral atom impacts at low densities has shown that the Van der Waals interaction ( — C6r−6) as used, for example, in (3), is an inadequate representation of the interatomic potential. The classical theory for a Leonard-Jones potential (C12r−12 — C6r−6) has been developed (4) and subjected to experimental test (5). Further improvements result from introducing a term C8r−8 in the potential (6) and from allowance for quasi-static effects (7). Applications to broadening by H—atom impact are discussed in (8).

A current Bibliography on The Analyses of Optical Atomic Spectra is in print (43). It appears in four sections, three of which cover the same elements as the respective volumes on Atomic Energy Levels. Relevant literature references from the dates of publication of these Volumes are continued for each spectrum to the present, as follows:

The last Section contains references to rare-earth spectra:

The present report will be limited to later work.

The laboratory study of molecules of interest to astrophysicists has been progressing at a steady pace. A complete bibliography for the year September 1968 to September 1969, for instance, would show work on 68 molecules of possible interest; the diatomic and simpler polyatomic molecules. During this period 38 papers appeared on H2 alone, notably on its polarizability and the character of its spectrum near the ionization limit. Twenty-one papers on CO included studies in the vacuum UV, its Franck-Condon factors, chemiluminescence and IR absorptances. In the interests of saving space in this report, the detailed discussion of published work on each molecule of interest will be deleted. Instead, attention might be called to the fact that a complete card file bibliography is maintained by Phillips at Berkeley, and inquiries are welcome. In addition, a bi-monthly Newsletter is distributed from Berkeley in collaboration with S. P. Davis of the Physics Department.