1. Quasi-periodic oscillations (2- and 5-day) in the minimum virtual height of the ionospheric F-region;Altadill;Compt. rend. Acad. bulg. Sci.,1994
2. Quasi-five- and ten-day oscillations in foF2 and their possible connection with oscillations at lower ionospheric heights;Altadill;Ann. Geofísica,1996
3. An estimation of the zonal wave number of the quasi-2-day oscillation in the ionospheric F-region;Apostolov;Bulg. Geophys. J.,1994
4. Characteristics of quasi-2-day oscillations in the foF2 at northern middle latitudes;Apostolov;J. Geophys. Res.,1995
5. Further spectral studies and investigations of quasiperiodic variations;Apostolov,1993