Recent results in the search for strong Langmuir turbulence effects during ionospheric modification experiments performed at the Arecibo Observatory are presented. Indirect evidence of Langmuir wave collapse is obtained through the observation of theoretically predicted ''caviton-type'' enhanced plasma waves spectra using the 430 MHz incoherent radar at Arecibo. A typical spectrum consists of a ''free-mode'' peak with a frequency that is significantly higher than the heater frequency, and a broad ''caviton continuum'' with frequencies below the heater frequency. Free modes are freely propagating Langmuir waves radiated by collapsing cavitons during collapse. The generation and dynamics of these ''free modes'' will be discussed. Asymmetries between the frequency shifts and strengths of the upshifted and downshifted free-mode lines and their dependence on the time delay following the onset of heating are explained in terms of the radiation of free Langmuir modes by cavitons and the subsequent propagation of free modes down or up the density gradient. Experimental results are compared with theoretical predictions. Results on the transition of ''caviton-type'' plasma line spectra to the commonly observed ''decay-type'' spectra will also be presented. ¿ American Geophysical Union 1992