M. Rybanský

Coronal index of solar activity VII, years 1988–1991

A brief description is given of the coronal index of solar activity (CI) derived from ground-based observations of the green coronal line 530.3 nm (FeXIV) and its computation. The final data of CI are presented in graphical form over the period 1988–1991. The maximum of CI coincided with the Wolf number in cycle 22, and no second maximum, sometimes seen two years after the first one, was observed in this cycle.

The Solar Corona 530.3 nm in the 20th cycle

The paper presents an analysis of the coronal data of emission line 530.3 nm. The analysis was carried out with data recorded at the coronal station of Lomnický Štít for the period 1965–1976. Up till now the observed time and latitudal changes of the shape of the corona as well as the existence of two maxima have been fully proved. The development of intensities has, in many cases, different characters in the southern and northern hemispheres. Apart from this, the third increase of intensities in the course of the cycle was observed nearly at all latitudes in the coincidence with sunspot numbers in 1974–1975 years. The maximum of intensities of this ‘impulse of activity’ was observed at latitudes around 60° in both hemispheres.

Prominences and the Green Corona Over the Solar Activity Cycle

Prominences, in contrast to other solar activity features, may appear at all heliographic latitudes. The position of zones where prominences are mainly concentrated depends on the cycle phase of solar activity. It is shown, for prominence observations made at Lomnický Stit over the period 1967–1996, how the position of prominence zones changes over a solar cycle, and how these zones could be connected with other solar activity features. Our results obtained could be an additional source to do a better prediction of solar activity. Time-latitudinal distribution is also shown for the green corona (Fe XIV, 530.3 nm). Distribution of the green coronal maxima shows that there are equator-migrating zones in the solar corona that migrate from latitudes of 45° (starting approximately 2–3 years after the cycle start) to higher latitudes 70°, and then turn (around the cycle maximum) towards the equator, reaching the equator in the next minimum (this duration lasts 18–19 years). Polar branches separate from these zones at the cycle minimum (2–3 years before above-mentioned zones) at latitudes of 50°, reaching the poles at the maximum of the present cycle. The picture becomes dim when more polar prominence zones are observed. Prominences show both the poleward and equatorward migration. Comparison between both solar activity features is also discussed.

Coronal line photometer

A new photoelectric photometer to determine intensities of the spectral lines of the solar corona is described. The measurements cover the range from 400 nm to 900 nm with a time resolution up to 0.04 s. Starting from 1 January, 1991 the new photometer is used for patrol observations of the green (λ530.3 nm) and the red (λ637.4 nm) coronal lines at the coronal station Lomnický Štít.

The Green Corona Index and Soft X-Ray Flux

We present the behaviour of the coronal index (CI) of solar activity over the period 1986-(May) 1996. These data are not only a good tool for studying the activity of the Sun as a star, but they also represent an invaluable source of information in our quest to understand the properties of the heliosphere as a whole. Having compared the variations of CI and of the solar 0.05–0.8 nm soft X-ray flux over the period 1986–1995 we did not find any significant correlation between the two quantities. This favours a scenario in which the sources of the soft X-ray flux are small-scale regions of the corona, and processes of both ionization and recombination do not occur in the same volume of the solar corona as for the green corona.

WHAT SHOULD THE COLOUR OF THE SOLAR CORONA BE

This paper determines what should be the difference between the colour of the solar disc centre and the integrated light of the corona at different heights. We define the colour as the ratio of the spectral intensity at λ = 640 nm and λ = 490 nm. The optical radiation of the corona is assumed to be caused by a contribution of photospheric light scattered on free electrons in K-corona and by a contribution of the photospheric light scattered on solid dust particles in F-corona. The limb-darkening law was taken from Pierce and Slaughter (1977). The distribution of electron density and brightness of the F-corona was taken from van de Hulst (1950). We indicate that the solar corona should be already close to the limb, more reddish than the centre of the solar disc, where reddening increases with the height due to the increase of the contribution of the F-corona.

The Indication of Neutral Hydrogen in the Solar Corona

This paper describes an experiment carried out during an eclipse to determine the colour of the solar corona. It contains the results of the solar corona colour measurements during the total eclipse of 1994 near the solar limb. The difference in the colour of the corona and the colour of the photosphere might indicate the presence of neutral matter. Moreover, it theoretically indicates the colour according to the present model of the corona. On the basis of the results, it seems that solar corona should be redder everywhere than the solar disc centre even near the limb. The experiment shows that the corona near the limb is bluer than the centre of the solar disc, and this effect decreases with the height in the corona. We compared the results of the experiment with those of the theoretical calculation of the colour. This comparison shows that the experimental results are in agreement with the theoretical, above the height of 3′. This fact probably proves the presence of neutral hydrogen in the corona.

Coronal index of solar activity VIII, years 1992–1994

The coronal index of solar activity over the period 1992–1994 is given. The data are a good tool to study solar activity, for the Sun as a star, in the solar corona over a solar cycle and its influence in the heliosphere.

The White-Light, Far-Red (600–700 nm) and Emission Coronae at the July 11, 1991 Eclipse

Preliminary results of the analysis of the white-light, emission (green and red), and far red (600–700 nm) corona during the July 11, 1991 eclipse are given. Even though the corona is of nearly-maximum type, four different principal coronal structures are seen, combined with faint, small-scale structures (loops, arches, cavities, voids or plasmoids). Scattered light is seen up to 10 R ⊙ in helmet streamers. The Ludendorff index of the corona shape turns out to be a+ b = -0.02, and the estimated brightness of J K = 1.47 x 10-6 B ⊙. Some aspects of multiwavelength observations are discussed.

Flares of radio type II and coronal emission in the course of cycle No 20

The butterfly diagram was constructed for the coronal emission of Fe XIV line (530.3 nm) in the course of cycle 1965–1976 complemented with the positions of the source flares with coronal shock waves.