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 , 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 , 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).

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