Exploring the ionospheric structures by radio tomographic methods under different space weather conditions

The satellite radio tomography (RT) of the ionosphere is an efficient method for electron density imaging in the different geographical regions of the world under different space weather conditions. In this work, we used two modifications of the radio tomographic methods: the methods of the low orbiting radio tomography (LORT) and high orbiting radio tomography (HORT). The RT methods revealed a variety of the ionospheric structures: ionization troughs including the high-latitude trough, local plasma features, crests of the equatorial anomaly, large-scale electron density anomalies and areas marked with sharp gradients in electron concentration; complicated spotty plasma distributions with multiple extrema, etc. The analysis shows that the position, shape, and depth of the ionization trough widely vary depending on the space weather conditions. In some cases, complicated transformations and splitting of the trough were observed. The ionization gradients are highest and have most complicated morphology during the strong geomagnetic storms. Some of the RT reconstructions are compared to the independent observations by the ionosondes. We also present the examples of RT images comparison with the fluxes of ionizing particles measured onboard the DMSP satellites. We present numerous examples of RT reconstructions and discuss the structural features of the ionosphere revealed by HORT and LORT, demonstrating the broad variety of the ionospheric responses to space weather variations. These results can be of significant interest for physics of the ionosphere and for many practical applications.