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Analysis of a simple model for two-stream weak fluid turbulence suggests a straightforward mechanism for the excitation of plasma wave modes whose phase velocities are roughly half the ion-acoustic speed. This slow wave is linearly stable but is excited through three-wave interaction with two other waves, at least one of which is an unstable two-stream wave. All three waves satisfy the linear dispersion relation. Careful consideration of the coupling coefficients and resonance conditions shows that the driven (linearly stable) modes have k-vectors that are perpendicular to the local magnetic field and are at some large angle to the Hall drift direction. These waves have phase velocities in the neighborhood of half the ion-acoustic speed and they could be responsible for type-3 radar echoes observed at 50 MHz. This mechanism cannot, however, generate fast waves moving at type-4 velocities.
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