Formation of ray trajectories of HF radiowaves in artificially and naturally disturbed ionosphere according to radiotomography and IRI model data

We present the results of the radiotomographic imaging of the subauroral, mid-latitude and equatorial ionosphere under various space weather conditions as well as the spatial structure of artificial HF-induced ionospheric disturbances obtained in the experiments on the modification of the midlatitude ionosphere by powerful HF radiowaves carried out at the SURA heater. We show the comparison of obtained tomographic crossections with IRI 2012 model data and study the peculiarities of the formation of ray trajectories of HF radio waves in real and model ionosphere under the geometric optics of inhomogeneous anisotropic media approximation. We show significant differences in ray patterns of HF radiowaves even for undisturbed geomagnetic conditions especially in the regions of significant spatial gradients of electron density, i.e., in equatorial anomaly and main ionospheric trough regions. The ray trajectories may strongly differ by the size of the receivers radio shadow, the length of the one-hop trace, and other parameters. Besides, in some cases the IRI 2012 model is incapable of predicting the HF channeling up to the heights of the outer ionosphere and different focusing and caustic structures which are observed in the RT imaging data. For heating experiments we show the possibility of formation of narrow trough in electron density distribution, corresponding to the directivity pattern of HF heater, which can effectively serve as a waveguide for high-power HF radiowaves and channel them to the upper ionosphere heights.