Ionospheric disturbances in years of solar activity minima and their effect on HF-radio wave propagation

Data about the oblique sounding on the paths Magadan-Irkutsk, Norilsk-Irkutsk and the vertical sounding at the stations located in the north-east region of Russia were used to analyze ionospheric disturbances occurred in September 2005 and during the geoactive period in December 2006. It was found that wave disturbances with 2-4-hour period can be recorded during main phases of magnetic storms. These disturbances generate layer maximum height variations up to 40-100 km and critical frequency variations up to 1,5-2 MHz, which can considerably change values of maximum observable frequencies (MOF) of the ionospheric radio channel on the paths under study. Such wave disturbances may arise from generation of acoustic gravitational waves (AGW) in the auroral zone and their propagation to the equatorial latitudes.