ON ORIGIN OF OUTSHIFTED PLASMA LINES DURING HF MODIFICATION EXPERIMENTS

Outshifted plasma lines, observed during injections of intense HF radi owaves into the ionosphere, are due to scattering on Langmuir waves wi th an unusually large spectral width shifted above the injected freque ncy. Incoherent scatter radar observations reveal that these waves som etimes are located at altitudes exceeding by several kilometers the al titude of the reflection layer of the ordinary HF wave, thus making al l explanation in terms of the so-called ''free mode'' problematic. We show that these observations may be consistently described by the crea tion, due to the ponderomotive force, of density depletions in the Air y pattern of the Z mode. The generation of Langmuir plasmons by accele rated suprathermal electrons inside the depletions causes the wide, sp ectral width of these plasma line features and their mean frequency sh ift.