Gradient drift instability in the equatorial electrojet is susceptible to the presence of a high-power radio wave. When the latter has a turning point near the region of instability, interaction becomes an efficient four-wave process involving four electrostatic waves. The high-frequency waves exert a low-frequency pressure gradient force on the electrons, destabilizing further the unstable mode. The effect is important when v02/vd2≲ vi&ohgr;0Δ/ve&ohgr;pi2, where v0 and vd are electron oscillatory and dc drifts, ve and ve are ion electron collision frequency, and Δ is a frequency mismatch. ¿ American Geophysical Union 1989