Monitoring the Earth’s ionosphere is an important, fundamental and applied problem. Global Navigation Satellite Systems (GNSS) provide a way of measuring the ionospheric total electron content (TEC), but real-time single-station absolute TEC measurements are still a problem. This study describes a single-station system to measure the absolute TEC, based on the GNSS–MITIGATOR (MonITorInG the Absolute TOtal electRon content) system. The latter enables real-time measurements for the absolute TEC and its derivatives in time and in space to be obtained. The system is implemented by using JAVAD receivers. The convergence time and the run-mode retention time is ~8 h. We provide potential methods for using the system to estimate the critical frequency of the ionosphere, foF2, at oblique paths in the Siberian region. The developed tool could be useful for supporting real-time multi-instrumental ionosphere monitoring or for compensating for the ionospheric errors of radio equipment.

Using high frequency oblique sounding data, we study variations of the maximum observed frequencies (MOFs) as responses to the passage of large-scale traveling ionospheric disturbances (LS TIDs) during weak and moderate magnetic storms in 2006-2011. For this, the relative deviations of MOFs from the median values are calculated. Twenty-three magnetic storms were considered, during 11 of which LS TIDs were recorded, and their average amplitudes and periods were determined.

In this work, we describe the wave ionospheric disturbance connected with large scale traveling disturbance registered using oblique incidence data over Siberian and Far Eastern regions on April 20, 2018. We analyze diurnal variations of maximum observed frequency (MOF) of HF signals. MOF variations over the paths studied have reached 10 MHz during the main phase of the magnetic storm. The wave disturbance period is 2-3 hours.

For ten solar flares of M- and X- classes we study responses in characteristics of HF-signals propagation over three oblique incidence sounding paths. We consider time variations of the lowest observed frequency (LOF) and sums of amplitudes calculated for each ionogram. During studied solar X-rays flares we observe LOF increase and sums of amplitudes decrease. We obtain nonlinear dependence of the sums of amplitudes on LOF.

A variant of the direct optimization method for point-to-point ionospheric ray tracing is presented. The method is well suited for applications where the launch direction of the radio wave ray is unknown, but the position of the receiver is specified instead. Iterative transformation of a candidate path to the sought-for ray is guided by a generalized force, where the definition of the force depends on the ray type. For high rays, the negative gradient of the optical path functional is used. For low rays, the transformation of the gradient is applied, converting the neighborhood of a saddle point to that of a local minimum. Knowledge about the character of the rays is used to establish a scheme for systematic identification of all relevant rays between the given points, without the need to provide an accurate initial estimate for each solution. Various applications of the method to isotropic ionosphere demonstrate its ability to resolve complex ray configurations including 3-D propagation and multi-path propagation where rays are close in the launch direction. Results of the application of the method to ray tracing between Khabarovsk and Tory show good quantitative agreement with the measured oblique ionograms.

In this study, we analyze a specific type of propagation of decameter radio waves connected with lateral reflections of radio signals from ionospheric irregularities at the F-region altitudes. This feature is appeared in registration at the distance-frequency characteristics (ionograms) flat tracks with group paths exceeding ones for standard reflections from F2-region altitudes and maximum observed frequencies (MOF) exceeding MOF1F2. Non-great-circle HF radio signals propagation is considered for the Dikson - Norilsk, Amderma - Norilsk, and Salekhard - Norilsk paths using ionograms obtained in January and April 2016. 79 events of NGC-reflections during the studied period were registered. Among them, 45 events were observed in evening and night hours. Other NGC-events were registered in the day time.

In this work, the morphological investigation of maximum observed frequencies (MOF) and delays of interlayer signal over latitudinal path Khabarovsk - Tory in February 2014, has been carried out. Along with MOF and delays of regular 1F, 2F, 2E modes of high-frequency propagation we registered parameters of 1FN and 2Es modes. As a result of our study, we conclude that the interlayer signal is registered most often in the day time. This is connected with the E layer presence. At night time registration of 1FN mode is stipulated with the Es-layer presence over the decameter wave signal propagation path. Wave-like variations of MOF1FN and MOF2Es are registered on February 24 and February 28, 2014.

On 6 September 2017, the Sun emitted two significant solar flares (SFs). The first SF, classified X2.2, peaked at 09: 10 UT. The second one, X9.3, which is the most intensive SF in the current solar cycle, peaked at 12: 02 UT and was accompanied by solar radio emission. In this work, we study ionospheric response to the two X-class SFs and their impact on the Global Navigation Satellite Systems and high-frequency (HF) propagation. In the ionospheric absolute vertical total electron content (TEC), the X2.2 SF caused an overall increase of 2-4 TECU on the dayside. The X9.3 SF produced a sudden increase of similar to 8-10 TECU at midlatitudes and of similar to 15-16 TECU enhancement at low latitudes. These vertical TEC enhancements lasted longer than the duration of the EUV emission. In TEC variations within 2-20 min range, the two SFs provoked sudden increases of similar to 0.2 TECU and 1.3 TECU. Variations in TEC from geostationary and GPS/GLONASS satellites show similar results with TEC derivative of similar to 1.3-1.7 TECU/min for X9.3 and 0.18-0.24 TECU/min for X2.2 in the subsolar region. Further, analysis of the impact of the two SFs on the Global Navigation Satellite Systems-based navigation showed that the SF did not cause losses-of-lock in the GPS, GLONASS, or Galileo systems, while the positioning error increased by similar to 3 times in GPS precise point positioning solution. The two X-class SFs had an impact on HF radio wave propagation causing blackouts at < 30 MHz in the subsolar region and < 15 MHz in the postmidday sector.

Using experimental data obtained over oblique-incidence sounding (OIS) paths we analyze parameters of large-scale traveling ionospheric disturbances (LS TIDs) registered during several days from October, 2015, till March, 2016. The transmitters are located near Magadan (60.1°N, 150.7E), Khabarovsk (47.6N, 134.7E) and Norilsk (69.4N, 88.4E). The receiver is located near Tory, Buryatia (51.8N, 103E). OIS experimental data processing was carried out by the program of automatic interpretation of OIS ionograms. This program permits to determine different modes of HF radio wave propagation. In this study LS TIDs are considered as significant wave-like variations of electron density of F2 ionospheric region near the middle point of the OIS path. Such electron density variations are appeared in corresponding wave-like changes of maximum observed frequencies reflected from F2 ionospheric layer by one hop (MOF1F2). In our study we determine mean amplitudes and periods of LS TIDs for quiet and magnetically disturbed conditions using relative deviations of MOF1F2 from median values. The electron density profiles near the middle point of OIS propagation paths have been calculated also. The analysis of the storm-time generated LS TID in November, 07, 2015, has been carried out.

In this work we study peculiarities of high frequency signals propagation over network of Linear Frequency Modulated (LFM) ionosondes located above 60N. The transmitters are located in Magadan (60.1N, 150.7E), Dikson (73.5N, 80.7E), Lovozero (68N, 35E), Amderma (69.6N, 60.2E), Salekhard (66.5N, 66.7E), Pevek (70N, 170.9E), the receiver is in Norilsk (69.4N, 88.4E). For Magadan-Norilsk and Pevek-Norilsk paths comparison between maximum observed frequencies reflected from F2-region of ionosphere and maximum usable frequencies calculated using program on the base of waveguide approach and International Reference Ionosphere has been carried out. By means of morphological analysis we consider intervals when HF signals are scattered from small-scale inhomogeneities. We consider also registration of abnormal signal modes reflected from the F region of the ionosphere. Such peculiarities of decameter radio wave propagation have not been taken into consideration by the standard long-term models of ionosphere at the present time.

In this work we study properties of RTW signals using data obtained over Magadan-Tory path during several days on June, October and December, 2000. The coordinates of the transmitter location are 60.1N, 150.7E, the coordinates of the receiver location are 51.8N, 103E. In our study we calculated F2-region critical frequencies (foF2) over the RTW path using International Reference Model (IRI). For the calculation of HF radio wave propagation parameters we used the program functioning on the basis of a waveguide approach. We compare experimentally obtained RTW frequency characteristics with calculated maximum usable frequencies (MUF). We analyze foF2 calculated over RTW propagation path with the aim to determine the most favorable conditions of RTW signals propagation.

The morphological features of wave-like ionospheric disturbances with periods of 1–2 h and the spatial extent exceeding 1000 km are studied. Oblique-incidence sounding data of the ionosphere, obtained in eastern Siberia during several continuous monthly experiments on three radio paths from 2006 to 2010, have been used. Large-scale traveling ionospheric disturbances generated during magnetic storms and large-scale wave-like ionospheric disturbances registered during geomagnetically quiet periods are considered. Small-scale ionospheric structures were also observed against a background of large-scale traveling iono-spheric disturbances considered in this study.

In this paper we study morphological peculiarities of wave-like ionospheric disturbances with period 1-2 hours and spatial dimentions more than 1000 kilometers. We use oblique-incidence ionospheric sounding data obtained over Siberian region of Russia during several monthly duration experiments in 2006-2010 years. Large-scale travelling ionospheric disturbances recorded in geomagnetically disturbed and geomagnetically quiet conditions over East Siberian region of Russia are investigated. The connection between large-scale travelling ionospheric disturbances and small-scale ionospheric structures is mentioned.

A morphological analysis of vertical sounding data obtained in Irkutsk from 2003 to 2008 has been performed. The AE index was used to determine the geomagnetic activity level, and the storm main phase onset was registered based on the D (st) index. The ionospheric response to a magnetic storm was estimated based on the relative deviation of the critical frequency and altitude of the ionospheric F2 region from the median values. Superstrong magnetic storms and storms without positive initial phases were not considered when the data were selected. We found that positive ionospheric disturbances, which were accompanied by an increase in the F2 region maximum altitude, predominated between the storm initial phase and main phases during all considered magnetic storms. Between these storm phases, negative disturbances were only registered at night. Predominance of positive ionospheric disturbances over negative ones can be related to the selection of storms for studies.

This work studies the variations of HF characteristics and ionospheric parameters recorded over mid-latitude paths in the Russian East-Siberian region during magnetic storms on May 15, 1997, and September 24, 2006. The sharp wave-like changes in maximum observed frequencies (MOF) were recorded during the main phase of the investigated storms. Assuming that observed MOF variations can be produced by ionospheric disturbances propagating from the northern to the southern latitudes, a simulation of HF propagation conditions was carried out.

An estimation of the F2 ionospheric region critical frequency (foF2) variations using analysis of round-the-world radio sounding data has been made. Experimental data obtained by the Russian chirp-sounders network have been used. For the first time, using experimental data and numerical simulation, the quantitative dependency between the minimum foF2 magnitudes over round-the-world propagation paths and round-the-world maximum usable frequencies has been obtained.