Results of global monitoring of sporadic plasma layers in the lower ionosphere obtained from analysis of radio signals in satellite-to-satellite communication link

Method of global monitoring of sporadic plasma layers in the lower ionosphere was developed. Investigations
were carried out by the use of analysis of the amplitude and phase components of radio holograms obtained
during the radio occultation missions CHAMP, FORMOSAT-3. A stable connection between the arrival of
shock wave of solar wind, precipitation of energetic particles from the radiation belts, increase of intensity of
small-scale ionospheric irregularities, and appearance of strong sporadic layers during night time in the lower
ionosphere was established. Sporadic amplitude scintillation observed in RO experiments contain important
information concerning the seasonal, geographical, and temporal distributions of the ionospheric disturbances
and depend on solar activity. The geographical and seasonal distributions of sporadic layers in the lower iono-
sphere as a function of solar activity for 2002-2008 were obtained. The general number of RO events with strong
amplitude variations can be used as an indicator of the ionospheric activity. We found that during 2001-2008 the
daily averaged S4 index measured during CHAMP mission depends essentially on solar activity. The maximum
occurred in January 2002, minimum was observed in summer 2008. Different temporal behavior of S4 index was
detected for polar (with latitude greater than 55 degrees) and low latitude (moderate and equatorial) regions.
For polar regions the S4 index was slowly decreasing with solar activity. In low latitude areas the S4 index
was sharply oscillating, depending on the solar ultraviolet emission variations. The geographical distribution
of S4 index variations indicated different origin of ionospheric plasma disturbances in polar and low latitude
areas. Origin of the plasma disturbances in the polar areas may be connected with the influence of solar wind,
the ultraviolet emission of the Sun may be the main cause of ionospheric irregularities in low latitude zone.
Analysis revealed global oscillations of S4 index with the periods of 5-7 months. Analysis of these oscillations
may provide additional connection with solar activity. Therefore, the S4 index of RO signal is important radio
physical indicator of solar activity.