Difficulties and uncertainties are discussed concerning the calibration of the response of a solar vector magnetograph (linear and circular polarization degrees) in terms of field strength H and angle of inclination to the line of sight Ψ using the theory of line formation in magnetic fields (Unno, 1956).

The results of an investigation of the magnetic-field structure in the chromosphere around filaments are briefly given in this paper. The hypothesis of the existence of a two-field system is suggested: an outer field which has the form of arches and supports the filament against gravity, and an inner one, directed along the filament.

During periods of good image quality the large-scale films of the Lockheed Solar Observatory obtained during 1966 and 1967 have proven to be especially useful for studying filaments and their relationship to the Hα fine structure observed in active regions. Structures with dimensions on the order of 1 sec of arc can frequently be resolved, as illustrated in Figure 1. The ‘solar vortices’ are clearly resolved into numerous individual fibrils. Bright Hα plage is resolved into a fine granular structure resembling the solar granulation. Filaments are usually seen as irregular dense collections of material which are occasionally resolved into finer striations which run parallel to the path of the filaments.

As an interplay of plasma and magnetic field the solar prominences represent a nice example of cosmical hydromagnetics. Due to stronger magnetic fields in active regions the prominence processes are more violent there than in non-active regions. Observing with a coronagraph or watching prominence movies one recognizes some characteristic processes in prominences of active regions:

1. (a) Condensation of Hα material in coronal space without any direct observational evidence of the material supply. It seems that the material had been ejected in the form of corpuscular radiation, captured, thermalized and cooled down to 104 °K, when it becomes observable in the Hα line.

2. (b) Dissolution of the Hα material. Knots, streamers of sprays, surges and puffs often undergo a reverse process: they dissolve and disappear when observed in Hα. Similar dissolution is typical for a great part of the material in eruptive prominences.

3. (c) Spiraling motion and detwisting of streamers are observed in surges, sprays, funnels and also in eruptive prominences. In the prominence movies of HAO Boulder, Sacramento Peak Observatory and Pic-du-Midi we have found many nice examples. In most cases (if not in all) the detwisting resulted in a marked simplification of the prominence structure.

4. (d) Emerging magnetic fields may be seen in form of small rising and expanding Hα loops (velocities about 50 km/sec). In this type of loops the material is dragged out of the chromosphere. A similar process with low velocities has been observed in 5303 Å movies of coronal condensations.

5. (e) Capture of a surge by transversal magnetic fields has been observed and photographed on October 7, 1956. Surge material has been captured in a complex loopy structure and then streamed sunwards along the loops. This event shows that capturing of plasma streamers by transversal magnetic fields does occur on the Sun and that the material supply for AS prominences (see item (a)) would thus be possible.

6. (f) Collisional processes occur in different forms. One has been just described in item (e). However, the well-known prominence of Lyot from 1937 shows that a surge hits a prominence without being (completely) captured. The prominence which has been hit decays and the surge material seems to be reflected (at least partly) downwards. Two colliding knots may bypass each other in complicated trajectories. Cases have been recorded of falling prominence knots being stopped and accelerated upwards, with measured acceleration more than 10 solar g.

7. (g) Oscillating prominences at the limb have been photographed. The oscillations followed shortly after a nearby flare-spray event. This may be the counterpart of the disk phenomenon discovered by astronomers of Lockheed Observatory.

The eruptive prominence of September 12, 1966, is described. Detwisting of two filaments is demonstrated and compared with similar phenomena and with tornado prominences. A short discussion of mechanisms of the origin and subsequent development of the phenomenon is given.

It is shown that proton flares and loop-prominence systems form in the same type of active regions. Quite often both these phenomena appear simultaneously, but there are also many cases, when only the proton flare or only the loop-prominence system fully develops. The proton-flare active regions are not randomly distributed on the solar disk, but they tend to occur in complexes of activity which stay on the solar surface for many months and even years. Attention is called to the peculiar clustering of proton-flare regions on the Southern hemisphere, where two sources of activity, at a longitudinal distance of about 180°, seemed to move on the solar disk between 1956 and 1962 opposite to the solar rotation, shifting in the longitude at about 70 heliographic degrees per 10 solar rotations.

A brief survey is given on the main characteristics of ‘bright points’ and of arch filaments in young bipolar spotgroups. Observations show that both features are closely associated, thus indicating that they have a common cause. It is suggested that the propagation of expanding and increasing active-region magnetic fields through the chromosphere might be that cause.

During 1965 two rockets were launched in order to obtain monochromatic pictures of the Sun in the Mg II line at 2802·7 Å. The Mg II filterheliograms have been compared with simultaneous Hα and Ca II K spectroheliograms. An important observation from both flights concerns the relative intensities of different active regions. In the Hα pictures there are old fairly faint active regions and newly formed more intense regions. In the Mg II pictures, however, the older regions appear more intense than the younger regions.

Different explanations for this intensity-reversal effect have been considered. The effect has also been looked for in broadband Ca II H filterheliograms, with negative results, however.

The ‘Cooperative Study of Solar Active Regions’ (or CSSAR) originated from a suggestion of E. R. Mustel at the IQSY General Assembly in Rome, 1963. Later on it was also endorsed by IAU Commission 10, and a special Working Group of this Commission, under chairmanship of the writer, had the duty to organize this project, an experiment in both solar physics and international cooperation.

The flare activity of all active regions having ten or more flares from August 1959 until December 1961 was investigated. We constructed flare-activity curves drawn with the aid of Kleczek's q-index. The characteristic magnetic situation on the Mt. Wilson synoptic charts of photospheric magnetic fields in which the flare-rich active regions developed is described. A flare activity rate ‘quantization’ was found. From the CSSAR magnetic observational material we studied the reorganization of the magnetic fields of active regions which correlated with sudden changes in the rate of flare activity.

From the material gathered during the ‘Cooperative Study of Solar Active Regions’, we studied the flare locations in AR magnetic structure, and flare relations to changes in the magnetic fields and spot configurations. Besides a confirmation of previous results, we find that flares are often associated with two features of the spot configuration evolving in opposite senses, one growing, the other declining.

For each plage of the CSSAR period the daily projected areas have been determined and plotted versus time. Also the correction for foreshortening has been taken into account.

Daily fluctuations, of the plage corrected areas, certainly greater than the errors, have been found. Also area fluctuations of shorter periods have been observed.

The shapes of K2,3,2 Ca plages and longitudinal magnetic fields are compared for CSSAR active regions during their first transit on the solar disk.

The bipolar magnetic regions follow the Hale polarity law.

Often the region of inversion of the magnetic field corresponds to a gap in the Ca plage structure.

Bright patches of plages may coincide with magnetic inclusions, magnetic hills, and occasionally also with regions of inversion of the magnetic field. The outline of Ca plages follow well the isogauss of 20–40 oersted.

Histograms of the distances of individual magnetic field intensity peaks do not only correspond to the geometry of the supergranular network but also seem to indicate a difference in the organization of these peaks between the leading and following polarities.

A study of the development in the solar corona of active centers born during the CSSAR period leads to the following remarks: the ‘enhancement’ of coronal emissions seems to take place first within a localized ‘core’ close to the plage, then to extend to a much larger ‘halo’; the core from which all the radiations under study originate does not last much more than the spots, whereas the ‘halo’ is characterized by the major importance of the emission at 5303 Å, and lasts as long as the K 3 plages. These features can be explained by the assumption that the enhancement is inhomogeneous during the first part of its life, then becomes homogeneous after the core has disappeared.

The ‘K-Coronameters’ in use since 1956 at Pic-du-Midi and Meudon Observatories for the study of the white-light corona were replaced in 1963 by new improved instruments.

The coronal activity was continuously recorded from 1964 to 1967. The electron density in the low corona and its variation with height are summarized by curves and maps.

A study is given of the corona streamers above the proton flares area in July and September 1966.

A current-free approximation to the coronal magnetic field is calculated from measured photospheric magnetic fields (Mt. Wilson) and traced by computer. The calculated field structure is then compared to a white-light photograph of the November 12, 1966 eclipse.

A photograph of the white-light corona from a rocket on May 9, 1967 showed many streamers, all straight and nearly radial, extending across the field of view, 3 to 9R⊙. At the North there were three spectacular streamers, making angles with the polar axis of 21° E, 20° W and 27° N. Projected inward, they were radial and passed through the probable positions of the principal plages on the back side.

The coronal plasma, which is the cause of geomagnetic storms, can impinge on the Earth only when some coronal formation is pointing towards the Earth. If no such formation is directed towards the Earth, a period of geomagnetic calm follows.

It was found that certain coronal formations, governed clearly by the appropriate local magnetic fields, correspond to the individual stages of development of active centres. Above the sunspot groups, there occur either conical rarifications or cylindrical, condensed fluxes dependent on a relatively non-variable, or unstable magnetic field. The filaments which outlive the sunspot period, provided they are located in floccular fields, are appropriate to streamers of helmet shape. The final stage of the active region, characterized by the presence of filaments only (called ‘free filaments’ by the authors), does not change the arrangement of the corona appreciably, i.e. the normal shape, or so-called minimum type. On the basis of the mentioned relations, it is possible to determine the shape and direction of so far currently unobservable coronal structures from chromospheric situations.

The mentioned facts enable a unified interpretation of the geomagnetic activity during the whole of the solar cycle to be made. The differences given for some types of storms (sporadic and recurrent, sudden and gradual commencement), may be explained by the various arrangements, occurrences or absences of local magnetic fields. The paper presents examples of chromospheric situations which were used, in some cases, at the Geophysical Institute in Prague for forecasting geomagnetic activity.

The relation of solar active regions to the large-scale sector structure of the interplanetary field is discussed. In the winter of 1963–64 (observed by the satellite IMP-1) the plage density was greatest in the leading portion of the sectors and lesser in the trailing portion of the sectors. The boundaries of the sectors (places at which the direction of the interplanetary magnetic field changed from toward the Sun to away from the Sun, or vice versa) were remarkably free of plages. The very fact that since the first observations in 1962 the average interplanetary field has almost always had the property of being either toward the Sun or away from the Sun (along the Archimedean spiral angle) continuously for several days must be considered in the discussion of large-scale evolution of active regions. Using the observed interplanetary magnetic field at 1 AU and a set of reasonable assumptions the magnetic configuration in the ecliptic from 0·4 AU to 1·2 AU has been reconstructed. In at least one case a pattern emerges which appears to be related to the evolution of an active region from an early stage in which the magnetic lines closely couple the preceding and following halves of the region to a later stage in which the two halves of the region are more widely separated.

The coronal features associated with solar active regions can be observed by recording images of the Sun at extreme ultraviolet (XUV) wavelengths. Pinhole cameras have been flown on stabilized sun-pointing ‘Skylark’ rockets to obtain broad-waveband XUV solar images. These images show localised emission from high-temperature regions located in the corona above calcium-plage areas. An improved design of pinhole camera, which uses a plane-diffraction grating to give increased spectral resolution, has recorded spectroheliograms in several intense solar lines including He II (304 Å), Fe IX–XI (180 Å), and Si X–XII (50 Å). Estimates are made of the size and brightness of the coronal emission region associated with a developing calcium-plage area.