The long-term variations of critical frequency in the F2 region of ionosphere and solar activity / E. S. Kolesnik, R. K. Khaitov, A. A. Mitaev [et.al.] // Proceedings of SPIE. 2015. Vol. 9680 : 21st International Symposium Atmospheric and Ocean Optics: Atmospheric Physics, June 22-26, 2015, Tomsk, Russian Federation. P. 968077-1-968077-4. URL: http://vital.lib.tsu.ru/vital/access/manager/Repository/vtls:000524995