HF radar observations of irregularities in the daytime equatorial F region

Abstract

During the International Equatorial Electrojet Year (1993–1994), irregularities have been observed with a multi-frequency zenithal HF radar in the whole F region from 130 up to 300 km of the daytime equatorial ionosphere. They appear as amplitude fluctuations with time scales increasing from few seconds in the lower F1 region up to tens of seconds in the upper F2 region. Fluctuations are specially strong in the F1 region, their amplitude decreases with increasing altitudes. F region irregularities are kilometric, they are observed simultaneously with E region type II irregularities, associated with gradient drift processes, but there is no coherency between irregularities in the E and F regions. Doppler spectra and echolocation indicate oblique scattering from east–west direction. Measurements show the existence of an altitude gradient in the horizontal velocities, varying from 150 m/s at 142 km to 190 m/s at 236 km in the F region. Some mechanisms, already proposed to explain equatorial irregularities are discussed as possible mechanisms at the origin of the observed irregularities, they are however concluded unsuitable. The irregularity origin remains then not understood, suggesting that the electrojet structure could be more complex than expected.