LOW-FREQUENCY WAVES ASSOCIATED WITH LANGMUIR-WAVES IN SOLAR-WIND

The Ulysses spacecraft has detected several events of low-frequency el ectromagnetic waves in association with Langmuir waves in the solar wi nd. The high time resolution observations show that the Langmuir waves are very intense and occur as broad peaks superposed by collapsing mi llisecond spikes. The low-frequency waves are identified as electromag netic lower hybrid waves. The observed energy densities of these waves often exceed the strong turbulence thresholds. It is shown that none of the parametric decay instabilities involving Langmuir and low-frequ ency waves are energetically favorable to explain the present observat ions. The low-frequency waves are proposed to arise from currents asso ciated with gradients in the electron beam originating at sites where Langmuir waves scatter the beam electrons.