On the excitation of large aspect angle Farley-Buneman echoes via three-wave coupling: A dynamical system model

Linear and quasi-linear theories of Farley-Buneman waves predict that coher ent scatter can only be observed at aspect angles of about 1 degrees or les s. Coherent echoes at large aspect angles, with values greater than the lin ear upper bound, axe not very well understood theoretically. Radar observat ions have revealed the existence of such plasma irregularities at aspect an gles as large as 10 degrees and larger (Millstone Hill and European Incoher ent Scatter). In this paper we suggest a possible mechanism capable to exci te Farley-Buneman waves at large aspect angles, which are otherwise damped according to the known conventional models. The proposed model is based on a nonlinear mode-coupling mechanism channeling energy between waves at smal l aspect angles (linearly unstable) and ones at large aspect angles (linear ly stable and damped). We studied the nonlinear coupling of Farley-Buneman waves with significant parallel components. We are able to show that the in teraction of the three modes can be very selective but very efficient at th e same time. It is also found that the finite aspect angle effects have a p rofound impact on the saturation levels of Farley-Buneman turbulence in the auroral region, a subject of further study indeed.