Ionospheric irregularities due to powerful HF radio transmissions

It was known for some time that sufficiently powerful HF radio transmissions produce a great variety of ionospheric irregularities. Ionospheric modification facilities with effective radiated powers of the order of 100 MW directed toward the ionosphere were used for the study of such irregularities since 1970. Such man-made irregularities were employed to establish experimental scatter communications links. Very powerful short wave broadcast transmitters must also produce man-made irregularities which affect the ionospheric propagation of short waves. All aspects of the physical phenomena which play a role in the production of ionospheric irregularities by powerful HF transmissions, are discussed. These include thermal self-focusing of radio waves, formation of short-scale field-aligned irregularities by a thermal parametric instability in which the scattering of the HF pump wave by the irregularities into Langmuir waves plays an important role, and those parametric instabilities in which the ponderomotive force dominates over thermal forces. The latter two parametric instabilities can lead to the acceleration of electrons to energies of tens of electron volts. Such accelerated electrons can produce artificial airglow and also additional ionization which under certain conditions could be significant. In their strongly nonlinear stage parametric instabilities can lead to the formation of localized electron density depletions (cavitons) maintained by the ponderomotive force of Langmuir waves trapped in them.