The current state of research into the mechanisms of attenuation of high-frequency radio waves in the ionosphere is presented. Comparison of gas-kinetic and empirical estimates of the effective frequency of collisions e between electrons and heavy plasma particles is a basis of consideration. The experimental evaluation has been obtained from measurements of the absorption of radio waves in different ways: by the A1 (ground-based vertical pulse sounding), by A2 (registration of the intensity of cosmic radio noise), for oblique propagation. It is shown that in the region E of the ionosphere only collisional e absorption is significant, which can be adequately assessed using the gas-kinetic model. It is also established that in the F-region an additional collisionless (nondissipative and dissipative) attenuation of the HF waves takes place besides the collisional absorption.
One of the non-dissipative attenuation mechanisms is the multiple small-angle scattering on the natural irregularities of electron density. It makes a significant contribution in two cases. During ground sounding of the ionosphere by a spherical source, scattering leads to the redistribution of the reflected radiation between the ray tubes, which is in the vertical direction gives the deficit of energy. For oblique propagation at the maximum usable frequency (MUF), scattered waves fall into the shadow zone, which significantly reduces the radiation intensity near the boundary of the dead zone.
Scattering has little effect on signal intensity in two cases. The first – an oblique sounding at frequencies below the MUF, because the exchange of scattered waves between adjacent ray tubes leads to a small contrast of the radiation. The second – when receiving signals from space sources at mid-latitudes, when the wave can be considered flat. In it the average intensity along the wave front re-mains constant.
It is identified two mechanisms of dissipative losses. One of them is based on measurements of cosmic sources of radiation and observed in high-latitude ionosphere and is explained by interaction of radio waves with the developed plasma turbulence. Another, the anomalous absorption of ordinary waves due to their statistical transformation into a slow extraordinary waves is observed only when vertical sounding.