On origin of outshifted plasma lines during HF modification experiments

Outshifted plasma lines, observed during injections of intense HF radiowaves into the ionosphere, are due to scattering on Langmuir waves with an unusually large spectral width shifted above the injected frequency. Incoherent scatter radar observations reveal that these waves sometimes are located at altitudes exceeding by several kilometers the altitude of the reflection layer of the ordinary HF wave, thus making an explanation in terms of the so‐called “free mode” problematic. We show that these observations may be consistently described by the creation, due to the ponderomotive force, of density depletions in the Airy pattern of the Z mode. The generation of Langmuir plasmons by accelerated suprathermal electrons inside the depletions causes the wide spectral width of these plasma line features and their mean frequency shift.