RESUMO

Observation results are presented on time variations in the Doppler spectra and amplitudes of ionosphere-reflected radio waves at 3.2 and 4.2 MHz. Diurnal behavior shown by these variations during autumnal equinox days is analyzed. Quasiperiodic and aperiodic processes have been detected which can be associated with the motion of the solar terminator and periodic heating of the ionospheric plasma by powerful HF radio emission from the "Sura" heater. Periods and amplitudes of the quasiperiodic processes correspond to those characteristic of the internal gravity waves. Switching the powerful emission on/off was accompanied by amplification of the quasiperiodic processes and increase in electron density. The latter effect is most likely associated with precipitation of high-energy electrons from the magnetosphere at midlatitudes.