Thresholds of modulational instability

摘要

It is emphasized that the simplifications in the description of the modulational instability such as the WKB approximation or the method based on the modified Zakharov's equations cannot be used to find the modulational instability thresholds. The correct expressions for the thresholds can be obtained on the basis of the exact method which assumes the separation of regular and random fields. The thresholds and the conditions of their existence are given for the cases of Langmuir and lower-hybrid turbulence. Theoretical magnitudes of the threshold are compared with the observed ones for the conditions of the FREJA experiment.