Wave collapse at the lower-hybrid resonance

The modulational instability and collapse of waves in the vicinity of the lower‐hybrid resonance including both magnetosonic and lower‐hybrid waves are investigated by analytical and numerical methods. The mechanism leading to the modulational instability is the nonlinear coupling of lower‐hybrid waves with the much lower‐frequency quasineutral density perturbations via the ponderomotive force. The result is a filamentation of the high‐frequency field producing elongated, cigar‐shaped nonlinear wave packets aligned along the magnetic field with the plasma expelled outside (cavities). The analytical self‐similar solutions describing cavity collapse are obtained and compared with the results of numerical simulation for both two‐ and three‐dimensional cavity geometries. It is shown that in three‐dimensional solutions the transverse, with respect to the magnetic field, contraction remains prevailing. The possibility of ion acceleration as the result of the lower‐hybrid collapse is discussed and detailed comparison is made with the observations of the phenomena in the auroral ionosphere.