  We show that the dispersion relation for gradient-drift and Farley-Buneman
instabilities within the approximation of the two-fluid MHD should contain the
terms which are traditionally supposed to be small. These terms are caused by
taking into account divergence of particles velocity and electron density
gradient along the magnetic field direction. It is shown that at heights below
115km the solution of the dispersion relation transforms into standard one,
except the situations, when the electron density gradient is parallel to
magnetic field or wave-vector. In these cases the traditionally neglected
summands to the growth rate of the irregularities becomes significant. The
additional terms depend on relative directions of electron density gradient,
magnetic field and mean velocities. This leads to the different instability
growth conditions at equatorial and high-latitude regions of the ionosphere.
The obtained results do not contradict with the experimental data.
