Solar physics in Bulgaria is a young field of Bulgarian science. In 1958 Department of Astronomy becames an independent division of the Bulgarian Academy of Sciences (BAS). In this new section A. Bonov, M. Kalinkov, D. Raikova and their colleagues worked on the Sun’s problems and the solar activity.

In 1977 Vladimir Dermendjiev took up his duties as a research worker at the Department of Astronomy in BAS. In 1985 he founded a division “Sun” in the Department of Astronomy, now Institute of Astronomy. He laid the foundations of systematic and detailed researches of the Solar physics in our country. Furthermore, V. Dermendjiev was associated lecturer of the solar physics at the Astronomy Department in the faculty of Physics at Sofia University.
There are three main directions of solar research in “Sun” division at the Institute of Astronomy: 1) Processing of observational materials from the observatories in other countries using the Joyce Loebl microdensitometer in National Astronomical Observatory-Rozhen; 2) Magnetohydrodynamic (MHD) rendering, modeling and computer simulation of the solar activity phenomena; 3) White-light solar observations.
The processing of observational materials is a part of the energetic mutual collaborations between “Sun” division and: the Meudon Observatory in Paris and the Institute d’Astrophysique Spatiale in Orse, France; the Astronomical Observatory of Wroclaw University in Poland; the Astronomical Observatory in Ondйejov, Czech Republic; the Astronomical Institute in Tatranska Lomnica, Slovakia; the Astronomical Institute in Bucharest, Romania; the IZMIRAN in Troitsk, Russia; the Astronomical Observatory of Istanbul University in Turkey.    One of the main tendencies of Dermenjiev’s scientific researches was the MHD modeling of different activity phenomena. His researches in this direction laid the beginnings of the solar MHD and plasma physics in our country.
In 1992-1993 a solar station in the Rozhen National Astronomical Observatory in the Rodopi Mountains was built. It is situated on the altitude of 1750 m. This is a tower with 8-meter dome where a 15-centimeter photosphere telescope with H filter and spectrograph has been assembled. White-light solar observations carry out everyday in the people observatories in Smolian, Dimitrovgrad and Kardjali.

2.1. Solar activity

   At an early stage of existence of the Department of Astronomy systematic researches concerning solar physics were made by A. Bonov. Besides, some of the first solar physic researches were devoted to the behaviour of the green coronal line 5303 A, as well as of the monochromatic solar corona. When “Sun” division had been founded there were formed the directions for the systematic researches of the Sun.
The solar activity researches cover the several scientific problems.
– New indirect index of the solar activity. A new indirect index of solar activity named “Microzonality of luminescence of cave flowstones” has been suggested. A method of Laser Luminescent Microzonal Analysis for registration of this index has been developed. A method for transformation of densitograms of luminescence intensity into time series as well as a method for dating have been worked out (Shopov and Dermandjiev, 1990).
– Activity of the solar prominences (filaments). The secondary polar zone of the solar prominences has been established in distribution of the polar prominences, observed in the coronal station Lomnicky Shtit, Slovakia in the period 1976-1990 (Dermendjiev et al., 1994c). The prominence activity in the ascending phase of cycle 22 has been studied and it has been compared with one in the ascending phase of preceding cycle 21 (Dermendjiev, Stavrev and Andreeva, 1992). Time distribution of long-lived filaments from Meudon catalogues during the period of 1931-1987 have been researched (Duhlev, 1992). The behaviour of the prominence height in time by observational material from Meudon’s catalogues of the filaments in the period 1945-1965 has been studied (Dermendjiev, 1977). A similar research of the height has been made for the quiescent prominences, observed in the coronal station Lomnicky Shtit during the period 1976-1986 (Duchlev et al., 1989).
– Distribution and large-scale motions of the solar filaments. The periodicities of the north-south (N-S) asymmetry of long-lived filaments has been analyzed using Meudon catalogues for the period 1919-1989 (Duchlev and Dermendjiev, 1996; Duchlev, 2001). The large-scale motions of the solar filaments published in Meudon’s catalogues for the period 1931-1963 have been researched (Dermendjiev, Duchlev and Velkov, 1990).
– Physics of the solar prominences. The morphology and geometry of the quiescent prominences registered on Meudon’s H-alpha and K3 spectroheliograms during the period 1991-1992 have been researched (Duchlev, Mouradian and Dermendjiev, 1994). By observational material from the Institute of Astronomy in Wroclaw it was researched the structure, internal motions and oscillations of the quiescent prominences (Petrov, Dermendjiev and Rompolt, 1998; Dermendjiev et al., 1998; Rudawi and Madjarska, 1998; Dechev et al., 1999), as well as the morphology and dynamics of the eruptive prominence observed on June 20,1994 (Rudawi et al., 1998). Three instances of a sudden disappearance of a quiescent prominence have been researched (Madjarska et al., 1996; 1998). Some physical mechanisms that bring to sudden disappearance of prominences have been examined (Dermendjiev et al., 1995). The main physical characteristics of prominence plasma have been researched before and during the eruption according to SUMER, CDS and EIT/SOHO observations of quiescent and eruptive prominences (Madjarska et al., 1999).
– Faint H-alpha emission in the solar corona. The objects of the faint H-alpha coronal emission observed in Pic du Midi has been researched. The relationship of the faint H emission and the quiescent prominences, active regions and emission corona has been researched comprehensively (Dermendjiev et al., 1994a, b). A detailed analysis of this emission and associated prominences has been made (Duchlev, 2001b).

2.2 TOTAL SOLAR ECLIPSES

   The Bulgarian scientists showed a strong interest in the total solar eclipse on August 11, 1999. The preparation for the eclipse and the eclipse observations were coordinated by the National Eclipse Committee with a chairman acad. D. Mishev and a secretary prof. V. Dermenjiev. The Committee’s main purpose was a selection and including into the National Eclipse Programme of detailed and various investigations of the Sun-Earth system in the frame of joint projects between the Institutes of Bulgarian Academy of Science, Sofia University and People Astronomical Observatories. Some foreign science and research teams took part in these programmes: the Astronomical Observatory of Wroclaw University, Poland; IZMIRAN and GAIS, Russia; Rutherford Appleton Laboratory, UK. The programmes for the total eclipse were presented and discussed during the three consecutive JOSO annual sessions of Working Group 7 (Dermendjiev and Mishev, 1999).
In accordance with the National programme for the total solar eclipse 11 science-research programmes have been implemented.
Five scientific programmes have been related to the corona:

• Registration of Rapid Changes in the Visible Wavelength of the Solar Corona;
• Registration of Low-Excitation Emission Lines of the Solar Corona;
• Registration of the Outer Boundary of the Dust-free Zone of the Solar Corona;
• Polarimetry Researches of the Solar Corona;
• Photographic Study of the Solar Corona.

Six of the programmes are related to the research of solar-terrestrial influences:

• – Spectral components and colourometric characteristic changes in the solar radiation;
• Investigation of the visible and infrared atmospheric radiation;
• Study of the ionosphere response;
• Research of the phenomenon “fleeting shadows”;
• Study of the atmosphere conditions and radiate characteristics of the Earth’s surface;
• Solar eclipse influence on some physiological factors.

The initial results have been reported in the Sixth National Conference on Solar-Terrestrial influence and The Annual JOSO’s meeting (Dermendjiev, 1999).2.3 THEORETICAL RESEARCHES

The theoretical researches are devoted by the following problems.

• The effects of magnetic energy increasing in an region of a quite strong magnetic field have been researched by means of 2D-hydromagnetic computer simulation for increasing of the magnetic energy in a homogeneous plasma medium (Dermendjiev and Buyukliev, 1987).
• The nature of elementary eruptive phenomena has been researched (Dermenjiev 1984). Part of Dermenjiev’s theoretical researches have been devoted to the physical mechanisms that bring up to the appearance and development of surge prominences (Dermendjiev, Buyukliev and Stavrev 1989).
• The MHD rendering of the prominence plasma weak leaking into the solar corona has been suggested (Rusin, Dermendjiev, Rybansky, Buyukliev, 1990).

By researchs of the faint coronal H-alpha emission, observed in the near vicinity of the prominences, a physical process of the instability in the coronal plasma has been examined. In this process the participation of the local magnetic field traced by prominence is of great importance (Dermendjiev et al., 1994a, Dermendjiev, Mouradian and Leroy, 1994). 

Solar research in Bulgaria is carried out in following other institutions.

• Section of Solar-Terrestrial Physics of the Solar-Terrestrial Influences Laboratory (STIL)
• Section of Ionosphere Plasma Physics of the Institute of Geophysics (IG)
• Plasma Astrophysics Group of the Faculty of Physics, Sofia University (SU)

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• Dermendjiev, V. N., Buyukliev, G., Stavrev, K.:1989, Astrophys. Investigations 5, 118.
• Dermendjiev, V. N., Detchev, M., Petrov, N. and Rompolt, B.: 1998, in A. Antalova, H. Balthassar and A. Kucera (eds) JOSO Annual Report., p.122.
• Dermendjiev, V. N., Duchlev, P. I., Mouradian, Z., Leroy, J.-L.: 1994a, in V. Rusin, P. Heinzel & J.-C. Vial (eds.), IAU Colloq. 144 “Solar Coronal structure”, VEDA Publ. Co Bratislava, p. 339.
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• Dermendjiev, V. N., Mouradian, Z., Nenovski, P., Madjarska, M. S.: 1995, JOSO Annual Report 1994, p. 179.
• Dermendjiev, V. N., Stavrev, K. Y., Andreeva, P.: 1992, Solar Phys. 138, 415.
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• Duchlev, P. I. and Dermendjiev, V. N.: 1996, Solar Phys. 168, 205.
• Duchlev, P., Dermendjiev, V., Velkov, K. and Rusin, V.: 1989, Bull. Astrn. Inst. Czechosl. 40, 109.
• Duchlev, P., Mouradian, Z., Dermendjiev, V. N.: 1994, in V. Rusin, P. Heinzel & J.-C. Vial (eds.), IAU Colloq. 144, “Solar Coronal structure”, VEDA Publ. Co. Bratislava, p. 387.
• Duhlev, P. I.: 1992, Solar Phys. 140, 195.
• Madjarska M. S., Vial, J.-C., Bochialini, K. and Dermendjiev, V. N.: 1999, Proc. 8th SoHO Workshop “Plasma Diagnostics of a Solar Prominence observed on 18 June 1997 by EIT, SUMER and CDS/SoHO”, ESA SP-446, p. 467
• Madjarska, M.S., Dermendjiev,V.N., Mouradian,Z., Nenovski,P.: 1996, Astroph. .Lett.& Comm., 34, 113. Petrov, N., Dermendjiev, V. N., Rompolt, B.: 1998, in A. Antalova and A. Kucera (eds) JOSO Ann. Report, p.145.
• Rudawi, P., Dermendjiev, V. N., Madjarska M. S., Rompolt B., Tomchak M.: 1998, Compt. Ren. l’Acad. Bulgare des Sci., 51, No 5-6, 5.
• Rudawy, P. and Madjarska, M. S.: 1998, in D. Webb, D. Rust and B. Schmieder (eds) New Perspectives on Solar Prominences ASP Conf. Series, vol. 150., p.63.
• Rusin, V., Dermendjiev, V. N., Rybansky, M., Buyukliev, G.: 1990, in E. Priest and V. Krishan (eds.), IAU Symp. 142, “Basic Plasma Process on the Sun”, D. Riedel P.C., p. 405.
• Shopov, Y., Dermendjiev, V. N.: 1990, Compt Rendue de L’Acad. Bulg. des Sci., 43, 7, 9.