<i>Letter to the Editor</i> UHF electromagnetic emission stimulated by HF pumping of the ionosphere

UHF electromagnetic emission (with a frequency near 600 MHz) from the F-region of the ionosphere pumped by an HF powerful radio wave is revealed. Possible mechanisms of the emission excitation, such as plasma mode con-version, scattering or Earth thermal noise emission off the plasma density irregularities, bremsstrahlung and excitation of high Rydberg states of the neutral particles by the accelerated electrons are discussed.Key words. Ionosphere (active experiments; wave-particle interactions) – Solar physics, astrophysics, and astronomy (radio emissions)

A Discussion on the early days of ionospheric research and the theory of electric and magnetic waves in the ionosphere and magnetosphere - Alteration of the ionosphere by man-made waves

In recent years powerful radio wave transmissions beamed at the ionosphere at frequencies somewhat below the penetration frequency of the F2 layer produced the following, mostly unexpected, spectacular effects. (a) Artificial spread F seen on ionograms and implying the presence of large scale field-alined irregularities in the ionospheric plasma density. (b)Very strong additional absorption of probing waves reflected by the F2 layer. (c) Field-alined ‘on frequency’ scattering of u.h.f. waves. (d) Scattering of u.h.f. waves by Langmuir waves which are believed to be parametrically excited. (e) The 630 nm airglow is artificially enhanced by a modifying wave of ordinary polarization but it is reduced in intensity by a modifying wave of extraordinary polarization. (f) The shape of the F2 layer is modified. The results of the observations are described and their tentative interpretation in terms of different parametric instabilities is outlined.