Electron collision frequency and HF waves attenuation in the ionosphere

[1] The results of the studies of the effective electron collision frequency and HF wave attenuation in the midlatitude ionosphere carried out in the Rostov-on-Don University during the past 35 years are presented. It is found that (besides the collision absorption) the attenuation is determined also by additional dissipative and nondissipative mechanisms. The absorption is reliably determined using the gas-kinetic model of the effective electron collision frequency. The additional dissipation mechanism (anomalous absorption of ordinary waves) may occur because of their statistical transformation into slow extraordinary waves at the scattering at small-scale fluctuations of the electron concentration. The only cause of the nondissipative attenuation is the small-angle multiple forward scattering at largescale irregularities of the ionospheric plasma. At signal propagation in the vicinity of the maximum usable frequency (MUF) the scattering provides an input into the attenuation comparable to the collision absorption. At the illumination of the ionosphere by a point source from the Earth surface, the multiple scattering leads to such distribution of the field at which the emission reflected under small angles to the vertical has an energy deficit. Presence of addition (to the collision) absorption mechanisms of HF waves attenuation leads to overestimated (as compared to the gas-kinetic ones) estimates of the electron collision frequency. The determination of the physical nature of the attenuation made it possible to develop correct methods of modeling of HF radio waves propagation in the three-dimensional inhomogeneous magnetically active ionospheric plasma and test these methods in a vast series of experiments.