We propose a method for local estimation of ionospheric parameters (vertical total electron content and critical frequencies f 0F2 ) from single-station observations of low-orbiting beacon satellites in the vicinity of the reception station. We demonstrate the capabilities of this technique using the data from two receiving stations located in Moscow (Russia) and Gakona (Alaska, USA). The critical frequencies f 0F2 calculated using the data from satellite signal receivers are compared with those based on the ionosondes in Moscow and Gakona. The correlation coefficients based on the ionosonde and satellite data processed by the proposed method are 0.85–0.86. A possibility for constructing local indicators of ionospheric disturbances, which are sensitive to geomagnetic conditions, is discussed.

Ionospheric satellite radio tomography (RT) is an effective method for studying the structure of the ionosphere during the periods of strong geomagnetic storms when other common methods of ionospheric sounding are inapplicable or unreliable. In this work, we used two modifications of the radio tomographic methods. One is low orbiting radio tomography (LORT). LORT is based on the coherent transmissions of radio signals of the previous generation navigational satellites such as Parus/TRANSIT received at the chains of the ground receivers installed along the satellite trajectories. Another modification is the high orbiting radio tomography (HORT) based on the processing of GNSS signals recorded by the global and regional receiving networks. We present the results of RT (LORT and HORT) imaging of the ionosphere in the different regions of the world for the periods of strong geomagnetic disturbances during 23rd and 24th solar cycles. RT methods revealed multi-extrema structures, wall-like gradients of electron density, variations in the position and depth of the ionization troughs, wavelike disturbances, and a variety of local features. RT methods allow to reconstruct not only the ionospheric irregularities of natural origin but also artificial disturbances induced by various anthropogenic sources (industrial explosions, rocket launches, high power HF heating). We also discuss the application of RT reconstructions for studying HF propagation in artificially disturbed ionosphere.

The satellite radio tomography 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 low orbital radio tomography approach to study ionospheric disturbances in Russian sector along Svalbard-Murmansk-Moscow line during famous Halloween Storm 2003. Obtained tomographic cross-sections of electron density served as a media to model the peculiarities of the formation of ray trajectories of HF radio waves during this storm.

In the present paper we study GNSS - reflectometry methods for estimation of sea level variations using a single GNSS-receiver, which are based on the multipath propagation effects caused by the reflection of navigational signals from the sea surface. Such multipath propagation results in the appearance of the interference pattern in the Signal-to-Noise Ratio (SNR) of GNSS signals at small satellite elevation angles, which parameters are determined by the wavelength of the navigational signal and height of the antenna phase center above the reflecting sea surface. In current work we used GPS and GLONASS signals and measurements at two working frequencies of both systems to study sea level variations which almost doubles the amount of observations compared to GPS-only tide gauge. For UNAVCO sc02 station and collocated Friday Harbor NOAA tide gauge we show good agreement between GNSS-reflectometry and traditional mareograph sea level data.

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.

Two approaches to using the GNSS data for the construction of new perturbation indices of the ionospheric plasma which take into account the perturbation on the spatial scale less than 2° are suggested. The first approach is based on the data of high orbital radio tomography (HORT). Considering the deviations of the electron density distributions provided by HORT from their averages over an undisturbed period, we construct the HORT-indices of ionospheric perturbations (perturbation maps and integral indices obtained by spatial averaging of the maps). The second approach is based on the analysis of the initial slant TEC data. Here, by applying special processing procedures, we separate the variational varying (quasi wavelike) component and then locally average these variations over space by the least squares technique. Just as in the first approach, the perturbation maps serve as a basis for constructing integral indices. The comparison of the constructed indices with the space weather parameters and geomagnetic perturbations demonstrates the high sensitivity of the first to space weather factors. The effect of the delay of the ionospheric perturbation relative to the geomagnetic disturbance on the different scales of ionospheric variations is revealed.

Methods are suggested for constructing ionospheric perturbation indices (IPIs) based on empirical radio tomographic (RT) electron density distributions taking into account spatiotemporal resolution, and coverage of low- and high-orbiting (LO and HO) RT data. The LORT-based IPIs are calculated as spatial root mean square values of electron density derivatives indicating the presence of local ionospheric structures on a spatial scale of dozens of kilometers. The HORT-based IPIs are based on the statistical characteristics, i.e., means and deviations from the means, of electron density or vertical TEC distributions with different normalizations and subsequent spatial averaging, which take into account the geomagnetic activity and seasonal behavior of ionospheric plasma. Various schemes of IPIs construction are considered, correlations between IPIs and geomagnetic Kp index are analyzed, the indices most sensitive to geomagnetic activity are identified, and additional modifications of computational algorithms enhancing this sensitivity are suggested.

Based on the data from the GPS receiving networks in Japan and America which have a high time resolution (2 min), two-dimensional (2D) distributions of the variations in the ionospheric total electron content (TEC) are constructed both close to and far from of the epicenter of the submarine earthquake of March 11, 2011 in Japan. Above the epicenter, a diverging multi-period disturbance appears after the main shock due to the acoustic gravity waves. Far from the epicenter, the wave trains associated with the tsunamigenic atmospheric internal gravity waves are revealed. These atmospheric waves significantly advance the arrival of the tsunami signal initially on the Hawaiian islands and then on the western coast of North America. The presence of the tsunami precursor in the form of atmospheric gravity waves is supported by the numerical calculations and by the analysis of the dispersion relation for the waves in the atmosphere. The detected ionospheric responses close and far from the epicenter can be used in the early tsunami warning systems.

The initial phase of a major geomagnetic storm on 14 December 2006 was selected in order to investigate the ionizing effect of energetic electrons in the ionosphere. The global network of GPS receivers was used to analyze the total electron content (TEC). A strong positive ionospheric storm of similar to 20 TEC units (TECU) with similar to 6 h duration was observed on the dayside during the interval of northward interplanetary magnetic field. At the same time, the NOAA/POES satellites observed long-lasting intense fluxes of >30 keV electrons in the topside ionosphere at middle and low latitudes, including a near-equatorial forbidden zone outside of the South Atlantic Anomaly (SAA). We found that the TEC increases overlapped well with the enhancements of energetic electrons. Modeling of the ionospheric response by using a Global Self-consistent Model of the Thermosphere, Ionosphere, and Protonosphere, based on the standard mechanisms of plasma transport, could only partially explain the ionospheric response and was unable to predict the long-duration increase of TEC. For the energetic electrons, we estimated the ionizing effect of similar to 45 TECU and similar to 23 TECU in the topside ionosphere, respectively, inside and outside of SAA. The ionizing effect contributed from 50% to 100% of TEC increases and provided the long duration and wide latitudinal extension of the positive ionospheric storm. This finding is a very important argument in supporting significant ionizing effect of energetic electrons in the storm time ionosphere both at middle and low latitudes.

The results of modeling the direct and inverse problems of low-orbital satellite ultraviolet (UV) tomography of the ionospheric 135.6 OI volume emission rate are presented. The direct problem was solved with the orbital geometry of DMSP block 5D3 satellites with SSUSI and SSULI UV spectrographs among the other payloads, the real operating parameters of these instruments (the scan rate and the interval of scan angles), and the set of the model distributions of the volume emission rate that contain irregularities on various scales. The solution of the direct problem yields the radiation intensities in the 135.6 nm line, which is used as the input data for reconstructing the initial (prototype) model distributions of the volume emission rates. The obtained system of linear equations (SLE) was solved using the Algebraic Reconstruction Technique (ART) and Simultaneous Iterative Reconstructive Technique (SIRT) algorithms, which are highly efficient in problems of the low-orbit radio tomography of the ionosphere. It is shown that the initial model distribution can be successively reconstructed if one takes the non-negativity condition of the solution into account, uses weighting functions to decrease the solution in the regions where it is known to be a priori small, and applies inter-iteration smoothing to eliminate the effects of the approximation errors. Here, the averaging parameters should decrease in the course of the iterations. With these constraints fulfilled, the computational costs of the ART- and SIRT-based solutions are similar, while the reconstruction error is approximately 6%. The influence of random errors and bias in the data on the results of the reconstruction is explored. It is shown that with a given error level of the initial data the parameters of the reconstruction algorithms can be adjusted in such a way as to efficiently suppress the influence of the noise with a relative amplitude of 2–3% on the solution.

We present the results of ionospheric imaging by the radio tomographic (RT) methods based on the Navigation Satellite Systems (GNSS). GNSS include the first-generation low orbiting (LO) systems (Tsikada, Transit, etc.) and second-generation high orbiting (HO) systems (GPS and GLONASS, which have been put in operation, and Galileo, BeiDou, and QZSS systems, which are currently under development in Europe, China, and Japan). The GNSS constellations and the networks of ground receivers are suitable for probing the ionosphere along different rays and processing the obtained data by tomographic inversion procedures. The results discussed in this work are obtained by the methods of low orbiting and high orbiting radio tomography (LORT and HORT, respectively). We present the examples of tomographic images of the subequatorial, midlatitude, subauroral, and auroral ionosphere in different regions of the world. The RT images of the Arctic ionosphere demonstrate different structures (characteristic circumpolar ring structures, ionization patches, tongues of ionization (TOIs), etc.) The GNSS RT methods are suitable for imaging the ionospheric disturbances caused by the tsunami wave propagation. We analyze the ionospheric disturbances after the strongest Tohoku earthquake in Japan (March 11, 2011). The RT reconstructions are compared to the measurements by the ionosondes and Global Ionospheric Maps (GIM).

We study a magnetosphere-ionosphere coupling at low latitudes during a moderate (corotating interaction regions/high-speed solar wind streams-driven) geomagnetic storm on 22 July 2009. Recently, it has been shown that during major (coronal mass ejection-driven) storms, quasi-trapped >30keV electrons largely enhance below the radiation belt in the forbidden zone and produce an additional ionization in the topside ionosphere. In this work, we examine a case of the recurrent storm when the magnetosphere-ionosphere coupling through the quasi-trapped electrons also may take place. Data from NOAA/Polar-orbiting Operational Environmental Satellite and Japanese Greenhouse gases Observing Satellite were used to identify the forbidden electron enhancement (FEE). We find a positive vertical gradient of the electron fluxes that indicates to the radiation belt as a source of FEE. Using global ionospheric maps, radiotomography reconstructions from beacon data and COSMIC/FORMOSAT-3 radio occultation measurements, we have observed an unusually large area in the nighttime ionosphere with increased total electron content (TEC) and prominent elevation of the F layer at low latitudes that coincides with FEEs spatially and temporarily. Ionizing particles are considered as an addition source of ionization along with generally accepted mechanisms for storm time TEC increase (a positive ionospheric storm). We discuss relative contributions of the FEE and disturbance dynamo electric field in the TEC increases during the storm recovery phase.

Observations of energetic electrons (10 – 300 keV) by NOAA/POES and DMSP satellites at heights <1000 km during the period from 1999 to 2010 allowed finding abnormal intense fluxes of ~10 6 – 10 7 cm −2 s −1 sr −1 for quasi‐trapped electrons appearing within the forbidden zone of low latitudes over the African, Indo‐China, and Pacific regions. Extreme fluxes appeared often in the early morning and persisted for several hours during the maximum and recovery phase of geomagnetic storms. We analyzed nine storm time events when extreme electron fluxes first appeared in the Eastern Hemisphere, then drifted further eastward toward the South‐Atlantic Anomaly. Using the electron spectra, we estimated the possible ionization effect produced by quasi‐trapped electrons in the topside ionosphere. The estimated ionization was found to be large enough to satisfy observed storm time increases in the ionospheric total electron content (TEC) determined for the same spatial and temporal ranges from global ionospheric maps. Additionally, extreme fluxes of quasi‐trapped electrons were accompanied by the significant elevation of the low‐latitude F‐layer obtained from COSMIC/FORMOSAT‐3 radio occultation measurements. We suggest that the storm time E x B drift of energetic electrons from the inner radiation belt is an important driver of positive ionospheric storms within low‐latitude and equatorial regions.

We present the results of application high-orbital (GPS/GLONASS) and low-orbital (Parus/TRANSIT) satellite navigation systems to the study of different-scale irregularities in the ionospheric total electron content (TEC), such as troughs, crests of equatorial anomaly, traveling ionospheric disturbances and so on. We also present the results of the comparison of the global ionospheric maps (GIM) of vertical TEC, which are now widely used in ionospheric research, with the results of low- and high-orbital radio tomographic ionospheric imaging and with the data of UV spectral imaging form GUVI instrument (Global Ultraviolet Imager). The data from low-orbital radiotomography systems in Russia (Moscow-Svalbard) and Alaska (Arctic Village-Cordova) were involved in the comparison as well as the data of the IGS (International GNSS Service) network. The comparisons cover the time interval from 2003 to 2008, which includes both geomagnetically quiet and disturbed periods. We also demonstrate the possibilities of GPS/GLONASS TEC studies in connection with solar flares and artificial ionospheric heating.

Two-dimensional distributions of the vertical total electron content in the ionosphere above the Japan under-sea mega-earthquake on March 11, 2011 are reconstructed using high-temporal-resolution (2-min) data from the Japan GPS network. A diverging ionospheric perturbation with multicomponent spectral composition is identified emerging after the main shock. The disturbances in the ionospheric electron concentration caused by acoustic gravity waves generated by the earthquake-related processes. The initial phase of this disturbance can be used as a marker in the tsunami early warning systems. The surface energy of the earthquake estimated from the amplitude of the ionospheric disturbance is close to the estimate based on the seismic data. Other disturbances (ionospheric responses to the Rayleigh and tsunami waves, a solitary ionospheric pulse) are also analyzed. Physical interpretation of the identified ionospheric disturbances is presented.

The specific features of the problem of ionospheric radio tomography based on the data from high-orbital navigation systems such as GPS and GLONASS are considered. An approach to solving this problem, which implies finding the solution with the minimum Sobolev’s norm (i.e., the smoothest solution) is proposed. The possibilities of the proposed approach are studied by numerical modeling with the use of real geometry of high-orbital satellite constellation and the layout of the receivers. The modeling shows that the proposed approach is very efficient in the regions with a sufficiently dense network of receivers. The resolution of possible radio tomographic systems over the territory of Russia is estimated. Examples of radiotomographic reconstructions of the ionosphere obtained from real high-orbital satellite data for the European region are presented.

Methods of radio tomography (RT) based on the low- and high-orbital navigational systems and radio occultation data are considered. Examples of RT imaging of the ionosphere in different regions of the world illustrate the use of low-orbital and high-orbital radio tomography (LORT and HORT, respectively) separately and in combination with each other. RT methods allow studying of various ionospheric structures: troughs, travelling ionospheric disturbances (TIDs), spots of enhanced ionizations, patches, blobs, wavelike structures, manifestations of particle precipitation. The possibilities for the application of RT systems together with other methods of UV and radio sounding are discussed.