LANGMUIR TURBULENCE AND IONOSPHERIC MODIFICATION

Experimental results, obtained with the Heating facility and the EISCAT UHF and VHF radars at Tromso, Norway, have been used to test existing Langmuir turbulence theories and to provide further observational facts to be taken into account in a new theoretical formulation of Langmuir turbulence. It is found that none of the available theories is able to describe the diversity of observational facts. The dominance of propagating Langmuir and ion acoustic waves in the observed spectra, and the fact that these waves closely follow their linear dispersion relations, lend support to a weak turbulence description. On the other hand, the observation that the number of cascade steps is limited to two, independent of input power, strongly contradicts such an approach. Unmistakable indications for the occurrence of cavitron collapse are not seen in the experimental results at Tromso. The low-frequency turbulence is found to consist of subsonic and sonic contributions, showing widely independent properties. The sonic turbulence is seen to evolve from the natural, thermally excited, ion acoustic background.