Parametric decay of an electromagnetic pump wave in a two-dimensionally inhomogeneous plasma

The decay instability of an ordinary hf electromagnetic pump wave into quas itransversal upper hybrid (UH) and lower hybrid (LH) waves in the ionospher e is discussed. The longitudinal and transversal inhomogeneity of the mediu m is taken into account. It is shown that, due to the decay instability wit hin weak plasma density enhancements, a rather broad spectrum of UH oscilla tions, similar to the spectrum of the downshifted maximum (DM) in stimulate d electromagnetic emission (SEE) spectra, is formed. Estimates of the frequ ency range and characteristic k numbers of the DM spectrum are given. (C) 1 999 American Institute of Physics. [S1070-664X(99)03501-6].