3-WAVE COUPLING IN THE AURORAL E-REGION

Analysis of al simple model for two-stream weak fluid turbulence sugge sts a straightforward mechanism for the excitation of plasma wave mode s whose phase velocities are roughly half the ion-acoustic speed. This slow wave is linearly stable but is excited through three-wave intera ction with two other waves, at least one of which is an unstable two-s tream wave. All three waves satisfy the linear dispersion relation, Ca reful consideration of the coupling coefficients and resonance conditi ons shows that the driven (linearly stable) modes have k-vectors that are perpendicular to the local magnetic field and are at some large an gle to the Hall drift direction. These waves have phase velocities in the neighborhood of half the ion-acoustic speed and they could be resp onsible for type-3 radar echoes observed at 50 MHz. This mechanism can not, however, generate fast waves moving at type-4 velocities.