S K A L N A T t ~ P L E S O A S T R O N O M I C A L I N S T I T U T E

(Report f rom Solar Institute)

J. S'~'KORA

Received 11 January, 1968)

1. Introduction

The Skalnat6 Pleso Astronomical Institute is a comparatively small institute. It was established in 1943. At the beginning it was composed of only one building with two

cupolas and one displacable roof. The offices and flats of the first workers were also

in the same building. The solar section came into being only in 1960, when three

sections were established: the stellar section, the solar section and the section for

interplanetary matter. At that time some further flats and an administration building

Fig. 1. Skalnat6 Pleso Astronomical Observatory and Mt. Lomnick3) gtit corona station.

were obtained in Tatransk~i Lomnica (at the foot of Mt. Lomnick3~ gtit). At the same time the solar section amplified its effort for the completing of the corona station on Mt. Lomnick~ gtlt. This station was finished in 1962, and the first regular coronal observations began in 1964. At present there are four astronomers and six observers

in the solar section.

Solar Physics 4 (1968) 122-125; �9 D. Reidel Publishing Company, Dordrecht-Holland

SKALNATE PLESO ASTRONOMICAL INSTITUTE 123

2. Location and Situation

The Astronomical Institute Skalnat6 Pleso (Rocky lake) is situated on the East slope

of Mt. Lomnick3~ gtit - the second highest mountain in Czechoslovakia - in the High Tatra mountains. Coordinates: longitude 20~ latitude 49~ altitude 1783 m above sea level. At this level there is no forest.

The corona station is situated on the top of Mt. Lomnick3~ gtit. Coordinates: longitude 20~ latitude 49~ altitude 2634m above sea level. Transport

between the corona station and Tatransk~i Lomnica village is made by a funicular

which has a midway station at Skalnat6 Pleso.

The observational conditions are best early after sunrise. After 3-4 hours the

conditions deteriorate owing to the increasing air turbulence. As to season of the year the best observational conditions and most of the observational days are in

Fig. 2. The C. Zeiss coronagraph on Mt. Lomnick3~ gtit.

124 J. SYKORA

autumn, partly also in March. There are on average 220 days per year when the sun shines for at least 15 rain, during which visual or possibly photographic observation of the photosphere are made, and there are about 75 days per year with coronal weather at Mr. Lomnick~ ~tit.

3. Instruments

The refractor at Skalnat6 Pleso has an objective of 15cm aperture and of 304cm focal length. It is used for securing drawings and photographs of the sun's photosphere. The solar diameter on the drawings is 25 cm. The photographs have diameters of 8 em or 14cm. These observations are made from 1943 and 1958 respectively.

On Mt. Lomnick~ ~tit a new C. Zeiss coronagraph with a 20-cm objective and 300cm focal length was recently installed. The coronal spectrograph is adopted to fit to the eyepiece-end of the coronagraph, so that the image of the sun (37ram in diam- eter) produced by an intermediary system is projected directly on the slit. The diffraction reflection grating has 600 grooves per millimetre, and the linear dispersion is 15 A/mm in the 2nd order of the spectrum. This equipment is mainly used for the corona service, according to the international program. We use the photographic method for measuring the intensity of the corona. When particularly active and interesting regions are crossing the sun's limb, the corona observations are made at mutual distances of 0(5 or at 1 ~ around the limb.

By exchanging the spectrograph for a Ha Solc filter, with 3.1 A pass-band, it is possible to observe prominences visually or photographically.

During the next 3 years a second coronagraph will be installed on Mt. Lomnick~ ~tit. For the spectrograph a linear dispersion of 2 A per millimetre will be chosen. In this year a ~olc filter with 2 A pass-band for the study of the green corona structure is expected to be available.

4. Research Program

As a source of the research program both the observational material of our institute and the material of other observatories are used. Recently the following problems were investigated.

A. S O L A R ACTIVITY

The asymmetry of the solar corona: The measurement show, that the North-South asymmetry exists and that it corresponds to that of other phenomena on the sun. But the East-West asymmetry of the corona can be explained by imperfections and differences between the observational methods (PAJDU~A~OVA, 1966).

The East-West asymmetry of the sunspots: From rich material it was shown that there is no positive East-West asymmetry of the sunspots (PAJDU~AKOVA, 1968).

Interdependence of sunspot proper-motions and chromospheric flares: The longitu- dinal acceleration A L and the latitudinal acceleration AB were determined for 717 sunspots. An interdependence was found to exist between the acceleration values and the occurrence of flares in a 10 ~ zone above the spots (ANTALOVA, 1965).

SKALNATt~ PLESO ASTRONOMICAL INSTITUTE 125

The photospheric situation connected with the development of flares accompanied by

the type-IV radio bursts: For 148 type-IV bursts flares the configuration of the corre- sponding sunspot groups was determined. They may be divided into seven types. From the characteristic features of the seven configurations flares with importance of 3 or above may be forecast (ANTALOVA, 1967).

Distances of filament feet: A close relation of filament feet to the supergranular structure has been found. (S'~KORA, 1968).

B. PHYSICS OF THE CORONA

The method of quantum defects was used to calculate the collision strength for exci- tation of the state 2peP3/2 in the isoelectronic sequence BI (L~xA, 19,66). The energies of the 12 lowest levels and the ionization limit for ions Sxu, Clxni, Axlv, K x v and, in so far as there are no experimental values, also for lower ions were determined by extrapolation along the B I isoelectronic sequence (LEXA, 1967). Further the excitation equilibrium of Axxv and the coronal abundance of argon are studied.

5. Publications

The institute has its own publication appearing irregularly: Contributions of the

Astronomical Observatory Skalnat~ Pleso, but in this publication the papers of all three sections are published. Most of the papers on solar physics are. published in the Bulletin o f the Astronomieal Institutes of Czechoslovakia. The coronal and photospheric observations are published in the Quarterly Bulletin. The institute took part in the

international activities IGY, IQSY and PFP.

References

ANTALOVA, A. : 1965, Bull. Astron. Inst. Czech. 16, 32. ANTALOVA, A." 1967, Bull. Astron. Inst. Czech. 18, 61. LEXA, J.: 1966, Bull. Astron. Inst. Czech. 17, 1. LEXA, J. : 1967, Bull. Astron. Inst. Czech. 18, 269. PAJDU~AKOV~,, L. : 1966, Contributions of the Astronomical Observatory Skalnat~ Pleso 3, 6. PAJDU~KOVA, L. : 1968, Contributions of the Astronomical Observatory Skalnatd Pleso (in press). S~KORA, J. : 1968, Bull. Astron. Inst. Czech. (in press).