The nonlinear interaction between two parallel high-frequency electromagnetic waves can generate either an electron plasma waves or an extraordinary wave, depending upon the angle of propagation with the external magnetic field (either ϑ?0 or ϑ≠0). When propagating in the inhomogenous ionosphere the extraordinary mode couples to the plasma wave and the results obtained in both cases turn out to be identical. The plasma wave can be detected from the ground through the enhancement of the amplitude of the plasma lines associated with the scattered spectrum of a third electromagnetic probing wave. The amplitude of the electrostatic plasma wave, and of the scattered plasma lines, are calculated. The rendezvous conditions (in time and in space) between the plasma wave pulse and the probing pulse are evaluated and discussed for the Eiscat sounder.