THEORY OF CONVECTIVE SATURATION OF LANGMUIR-WAVES DURING IONOSPHERIC MODIFICATION OF A BARIUM CLOUD

In recent experiments (Djuth, F. T., Sulzer, M. P., Elder, J. H. and G roves, K. M. (1995) Journal of Geophysical Research, 100, 17,347), a p arametric decay instability was excited by an ordinary-wave HF pump du ring an ionospheric chemical release from a rocket over Arecibo, PR, w hich created an artificial 'barium ionosphere,' with peak plasma frequ ency above the pump frequency, and a density gradient with a (short) 5 km scale length. Simultaneous incoherent scattering measurements reve aled a strong initial asymmetry in the amplitudes of almost vertically upgoing versus downgoing measured plasma waves. We can account for th is asymmetry in terms of linear convective saturation of parametricall y unstable plasma waves propagating over a range of altitudes along ge ometric optics ray paths. Qualitative features of the frequency spectr um of the measured downgoing wave are in agreement with this model, al though the theoretically predicted spectrum is narrower than observed. The observed altitude localization of the enhanced spectrum to a few range cells is consistent with the theory. (C) 1997 Elsevier Science L td.