  In the paper within the approximation of the two-fluid magnetohydrodynamics
and geometrooptical approximation the dispersion relation was found for
ionacoustic instabilities of the ionospheric plasma at 80-200km altitudes in
three-dimensional weakly irregular ionosphere. Low freqeuncy solution was
found. The difference between obtained and standard solution becomes
significant at altitudes above 140 km. As the analysis shown in this case the
solution grows with time. The conditions for existence of such solution are the
presence of co-directed electron density gradients and electron drifts and
perpendicularity of line-of-sight to the magnetic field. The necessary
conditions regularly exist at the magnetic equator. Detailed analysis has shown
that this solution corresponds to well-known 150km equatorial echo and explains
some of its statistical characteristics observed experimentally.
