ELECTRON DENSITY AT F1-LAYER HEIGHTS IN THE LAST SOLAR MINIMUM (2007-2009)
We present the results of the analysis of annual variations in daily electron density (N) at heights 140-160 km for the last solar minimum (2007-2009) obtained from digisonde measurements at the ionospheric station Irkutsk (52 °N, 104 °E). New coefficients of the known semi-empirical model (SEM) describing the connection between N and thermospheric characteristics are calculated to identify regularities of these variations. We have revealed that a characteristic feature of the annual N variations during the solar minimum is a change in their phase by 180° in a relatively narrow altitude interval (170-180 km). These results and the new SEM coefficients are original and important for atmospheric and ionospheric physics.