SELF-FOCUSING INSTABILITY OF PLASMA-WAVES EXCITED BY POWERFUL HF RADIATION

The possibility to create small scale plasma density fluctuations at the initial stage of ionospheric modification by powerful HF radio waves is discussed. The thermal self-focusing instability of plasma waves excited in the reflection region of the pump is considered as a main source of striations. Ponderomotive effects are also taken into account resulting in a decrease of the scales of the instability orthogonal to the magnetic field and a strong increase of the growth rate. Striations produced by the self-focusing instability can be a source of initial fluctuations at the upper hybrid resonance level of the pump necessary to striation development at further stages of the ionospheric modification.