of electron density gradient direction on gradient-drift instabilities in the E-layer of the ionosphere

We show that the dispersion relation for gradient-drift and FarleyBuneman instabilities within the approximation of the two-uid 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 eld 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 eld or wave-vector. In these cases the traditionally neglected summands to the growth rate of the irregularities becomes signicant. The additional terms depend on relative directions of electron density gradient, magnetic eld and mean velocities. This leads to the dierent instability growth conditions at equatorial and high-latitude regions of the ionosphere. The obtained results do not contradict with the experimental data.