Coherent generation of the terrestrial kilometric radiation by nonlinear beatings between electrostatic waves

The propagation of electrostatic plasma waves in an iahomogeneous and magnetized plasma is studied analytically. These waves, which are • • dxiven unstable by auroral beams of electrons, are shown to suffer a further geometrical, amplification while they propagate towards resonances. Simultaneously their group velocities tend to be aligned with the geomagnetic field. Then it is shown that the electrostatic , energy tends to accumulate at or near wLH and wLTH, the local lower and upper hybrid frequencies. Due to this process, large amplitude electrostatic waves with very narrow spectra should be Observed near these frequencies at any place along the auroral field lines where intense beam driven instability takes place. These intense quasi-mono(-hroma tic electrostatic waves are then "shown to give rise by a'coherentt non-11near three wave process to an intense electromagnetic radiation. Depending upon the ratio wpe/Wce between the electron plasma frequency and the electron gyroI frequency the electromagnetic wave can be radiated in the ordinary mode (ht wUH ) or in the extraordinary mode(at 2w UH); As,far as the ratio wpe/WCe tends to be rather small it is shown that the most intense radiation should be observed at 2wUH in the extraordinary mode.