Annual Variations of the Critical Frequency foF2 at the Equatorial Ionization Anomaly Station during the Two Last Solar Minima

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   The paper presents the investigations of foF2 critical frequency annual
   variations at equatorial stations to detect the causes which affect the
   ionosphere during solar minima. Electron density behavior in equatorial
   regions during solar minima is the subject for attentive investigation due
   to the peculiarities of the latest deep solar minimum. There are essential
   differences in the results obtained from the global TEC (total electron
   content) maps according to GPS measurements during the latest two solar
   minima. These differences are explained by the change of the Sun UV
   radiation and the low level of this radiation during the latest solar
   minimum. We considered annual Dst-variations and FoF2 critical frequency
   variations at Vanimo and Huancayo stations. The results of the
   investigation showed that one of the reasons for different electron
   density in the ionosphere during solar minima is geomagnetic activity,
   which change from one minimum to another.