NON-LINEAR LANDAU DAMPING IN THE IONOSPHERE

A model is presented to explain the nonresonant waves which give rise to the diffuse resonance observed near 3/2 f//H by the Alouette and Isis topside sounders, where f//H is the ambient electron cyclotron frequency. These waves are the result of plasma wave instabilities driven by anisotropic electron velocity distributions initiated by the high-power (400 W) short-duration (100 mu s) sounder pulse. In a strictly linear analysis these instability-driven waves will decay owing to Landau damping on a time scale much shorter than the observed time duration of the diffuse resonance. Calculations of the nonlinear wave-particle coupling coefficients, however, indicate that the diffuse resonance wave can be maintained by nonlinear Landau damping of the sounder-stimulated 2f//H wave. This 2f//H wave is always observed with a time duration longer than that of the diffuse resonance wave, a finding which indicates that it is available as an energy source for the proposed nonlinear mechanism. The time duration of the diffuse resonance is determined by the transit time of the instability-generated and nonlinearly maintained diffuse resonance wave from the remote short-lived hot region back to the antenna. The model is consistent with the Alouette/Isis observations and clearly demonstrates the existence of nonlinear wave-particle interactions in the ionosphere.