Vertical and oblique sounding data for northeastern Russia have been used to analyze the conditions for the propagation of radio waves during weak geomagnetic storms observed in fall seasons of 2018–2020 at minimal solar activity. Even during weak storms, the maximum observed frequencies have been found to decrease by 25–35% in daytime and by 40–50% at night. Variations in the parameters of the distribution of high frequency radio waves during disturbances depend on the spatio-temporal dynamics of large scale structures of the high-latitude ionosphere, which, in turn, depends on the processes of magnetosphere–ionosphere interaction. Here, the depth and duration of the negative disturbance are larger if the geomagnetic storm occurs on a disturbed background.

Peculiarities of 557.7 and 630.0 nm emissions observed in the second step of the magnetic storm main phase at the mid-latitude observatory Tory (52° N, 103° E) on March 17, 2015 are compared with the changes in ionospheric parameters above this station, detected from ionospheric sounding data and total electron content maps. We have found that the intensity of the 557.7 and 630.0 nm emissions noticeably increased after the observatory entered into the longitudinal sector of the developed main ionospheric trough (MIT). The most powerful synchronous increases in intensities of the two emissions are associated with amplification of the westward electrojet during strengthening of the magnetospheric convection. We study the dependence of the ratios between the intensities of 630.0 nm emission recorded in the north, zenith, and south directions on the position of emitting regions relative to the MIT bottom. The SAR arc is shown to appear initially near the bottom of the MIT polar wall and approach the zenith of the station during registration of F3s reflections by an ionosonde, which indicate the presence of a polarization jet near the observation point.

Spectral-polarization analysis of interval of pulsations of diminishing periods (IPDP), observed on November 9, 2017 at Istok observatory have been carried out. Pulsation characteristics calculated from five-minute sets of magnetic data are compared with changes in the intensities of 486.1, 557.7, and 630.0 nm emissions in the north, south, east, west, and zenith directions as well as with all-sky camera images and keograms made at the same wavelengths. It is shown that the frequency of the pulsations increased when regions of the intensified emissions expanded toward the equator. A possible relationship between the appearance of east-west elongated auroral arcs and the IPDP spectral composition is considered.

On the base of oblique sounding data from three paths located in Siberia and the Russian Far East, we analyzed probability of sporadic layer occurrence during stratospheric warmings in 2009–2013. The probability of sporadic layer occurrence has been shown to increase during stratospheric warmings, especially in years of low solar activity.

We carried out comparative analysis of the ionospheric, optic and geomagnetic disturbances observed during night hours of the main phase of the 17–18 March magnetic storm in the East Asian sector. It has been revealed that synchronous peaks of 557.7 and 630.0 nm emission intensity were associated with substorm-like enhancements of the westward electrojet. The strongest two peaks were observed during the shift of electrojet center to the corrected geomagnetic latitude φ’ ~ 55° N.

Based on oblique-sounding data from three HF radio paths, variability of the radio wave propagation conditions over East Siberia was investigated during one strong and two moderate magnetic storms in June 2015. Contribution of the solar cosmic rays, magnetospheric convection enhancements and thermospheric heating to changes in characteristics of HF radio wave propagation has been revealed.

Using vertical sounding data obtained by the Irkutsk digisonde DPS-4 from 2003 to 2016, we have studied the frequency of occurrence of the F1 layer in winter conditions. The frequency of occurrence of the F1 layer in December–January is shown to be more than twice lower than that in February at any level of magnetic activity. At moderate and low solar activity under quiet geomagnetic conditions, the appearance of F1 layer in midlatitudes of the Northern Hemisphere may be caused by active thermodynamic processes, which lead to transformation or destruction of the circumpolar vortex at heights of the middle atmosphere. Such global dynamic changes occurring in the winter strato-mesosphere are often associated with sudden stratospheric warming events, which are accompanied by increased generation of atmospheric waves of various scales. These wave disturbances can propagate upward to the heights of the lower thermosphere and ionosphere, carrying a significant vertical flow of energy and causing variations in the composition, thermodynamic parameters of the neutral atmosphere and ionosphere.

In the paper we study the ionosphere response to the March 2015 severe geomagnetic storm according to Eurasian high-middle latitude ionosonde chain. On the basis of the ionosonde chain data we have restored the time/longitude dynamics of the high-middle latitude ionosphere over the Eurasian continent. The data showed that during the main storm phase that developed in the nighttime Siberian sector, a disturbed thermosphere region was created. This region moved westward with 50-70 m/s velocity and which was registered two-three days after the storm in the Eastern and Western Europe. Characteristics of the geomagnetic field variations showed that three active zones occurred during the storm. One is shifted from the geographic pole toward magnetic pole at similar to 270 degrees longitude. Two other zones were formed symmetrically opposite to the geomagnetic pole at longitudes similar to 40 degrees and similar to 130 degrees; the major electron density depletions at these longitudes were observed in the high-middle latitude ionosphere. On the contrary, longitudinal sector similar to 80-110 degrees showed electron density maximum in quiet conditions and fast ionosphere restoration after the storm decaying.

The pattern of the ionospheric storm that was observed during the severe two-step geomagnetic storm on 19-22 December 2015 in East Asia is investigated. The study is performed using a combination of vertical and oblique incidence sounding, total electron content, riometer and magnetometer data obtained near 120 degrees E meridian in 19-66 degrees N latitude zone. The revealed ionospheric disturbances are compared with the features of ionospheric storm, developed over the same region during the severe one step magnetic storm on 14-16 December 2006. Compared magnetic storms are almost identical in the season and the onset time. They have approximately equal peak intensities (Dst = -155 nT and -162 nT), but differ noticeably in the duration of the main phases (19 h and 2.5 h) and the rate of the ring current field amplification. Through the comparison the ionospheric disturbances which are similar and dissimilar for both storms are revealed. Our study suggests that the main differences between ionospheric storms were observed during the initial and early recovery phases. They could be due to the differences between pre-storm states of the magnetosphere-ionosphere system as well as between interplanetary drivers.

Using data from ionosondes, located in East Asia, and total electron content maps, we have made a comparative analysis of ionospheric disturbances associated with the intense geomagnetic storms of December 14–16, 2006 and December 19–22, 2015. These storms had almost equal peak intensities (Dstmin=–162 and –155 nT), but different durations of the main phases (2.5 and 19 hr). At the beginning of both the storms, the region under study was located in the vicinity of the midnight meridian. Ionospheric responses to magnetic storms differed in: i) an increase in the F2-layer critical frequency at subauroral latitudes, caused by an increase in auroral precipitation, during the initial phase of the former storm and the absence of this effect in the latter; (ii) a sharp drop in the critical frequency in the evening hours of the main phase of the latter storm, caused by a shift of the main ionospheric trough to lower latitudes, and the absence of this effect during the former storm; (iii) generation of a short-term positive disturbance observed at subauroral latitudes only in the early recovery phase of the former storm after the negative ionospheric disturbance. During both the storms at middle latitudes there were positive disturbances and wave-like fluctuations of the critical frequency which increased in the vicinity of the dawn meridian. The main causes of the differences between the ionospheric storms are shown to be the differences between the initial conditions of the magnetosphere–ionosphere system and durations of the main phases of magnetic storms.

The low latitude ionospheric data observed by digisonde at Hainan station (19.5°N, 109.1°E) in a whole solar activity cycle period from 2002 to 2012 within Ap<20 have been analyzed to explore the diurnal, seasonal, annual variations and solar activity dependences of the ionospheric peak parameters (foF2, hmF2, and Chapman scale height Hm), as well as some quantitative comparison with IRI-2012 modeling predictions. The results show that the winter anomaly in the daytime foF2 appears at different levels of solar activity. The semiannual anomaly in the daytime and nighttime foF2 with two maxima in equinox seasons is present. The foF2 have a close correlation with a solar activity factor F107P=(F107+F107A)/2 and the correlation coefficients (r) in their diurnal variation are around 0.7. The slope of foF2 varying with F107P in daytime is usually smaller than in nighttime. The afternoon and evening hmF2 show good correlation with F107P (their r values exceed 0.6), but hmF2 at other time are low or poor related to F107P. The prominent character of hmF2 in equinox and summer seasons is its strong increase at sunset in high solar activity period, which may be due to pre-reversal enhancement (PRE) of local electric field. We also note that hmF2 values around midnight slightly decrease with increasing F107P index in equinox seasons. The diurnal variation of Hm usually has two peaks around noontime and pre-sunrise. The daytime Hm has an annual variation with maximum in summer and minimum in winter. Moreover, the dependence of the daytime Hm on solar activity is not strong due to meridional wind and other factors. The above results over Hainan are considerably different from those reported over Millstone Hill, which is attributed to their different geomagnetic locations. The quantitative results compared between IRI-2012 model predictions and observations show that the predicted foF2 values are basically underestimated and the magnitude of their deviations obviously increases with increasing solar activity. The predicted hmF2 obtained with measured M(3000)F2 inputs in low and moderate solar activity agree well with the observed ones. However, their deviations in high solar activity are significantly magnified.

Based on data from the Yekaterinburg radar (YeKB radar), ionospheric stations, located within the radar field of view and 20° eastward of it, as well as from 7 radio paths passing through the region under consideration a complex analysis of ionospheric disturbances during the 17-19 March 2015 magnetic storm have been done. By analyzing data from YeKB radar, vertical and oblique-incidence sounding ionograms, we found that on 17 March during the main and early recovery storm phases, the major contribution to the development of the ionospheric disturbances, observed at corrected geomagnetic latitudes 53-70°, was made by impact ionization of the ionosphere by precipitating magnetospheric particles. It had lead to an increase of foF2 at the stations of vertical sounding, and the appearance of diffuse reflections from the E and F regions and later, intense sporadic layers, alternating with intervals of total absorption as can be seen from riometric data. The main features of this magnetic storm were the large latitude width of auroral precipitation zone and its shifting to equator to geographic latitude ~ 50°. It could be connected with the high variability of interplanetary magnetic field and solar wind impacted on the magnetosphere. During the recovery phase of the storm in the morning at the stations of vertical sounding was recorded only sporadic layers. In daytime values of critical frequency and maximal observed frequency were less then the background values by a factor of 2. Such decreasing was observed in total electron content (TEC) data. Therefore, a simultaneous contraction of the electron density maximum in F-region and reduce of TEC may be due to common cause and the related change in the composition of the neutral atmosphere.

We have carried out a comprehensive analysis of data from the high-frequency coherent radar located near Yekaterinburg, ground-based ionospheric, riometric, and magnetic stations, situated within the radar field of view and in the vicinity of it, as well as from eight radio paths crossing the Asian region of Russia. Using these data, we studied dynamics of ionospheric disturbances over wide longitudinal sector during the first 3 days of the St. Patrick's two-step severe geomagnetic storm and determined the main mechanisms of their development. We showed that on 17 March during the main and early recovery storm phases, the major contribution to the generation of the ionospheric disturbances had been made by impact ionization by precipitating magnetospheric particles. This had lead to appearance of intense sporadic layers, alternating with intervals of total absorption. The main features of the storm were the large latitude width of the auroral precipitation zone and an expansion of this zone to corrected geomagnetic latitude similar to 45 degrees. We suppose that these peculiarities were due to high variability of interplanetary magnetic field and solar wind impacted on the magnetosphere. The most probable cause of the negative ionospheric disturbance on 18 March might have been a change in the neutral atmosphere composition. Significant differences between measured and simulated values of maximal electron concentration in F-2 layer point to the need to improve the existing empirical models of thermosphere, auroral precipitations, and magnetospheric convection in order to use them for modeling of ionospheric parameters during severe geomagnetic storms.

We analyzed ionospheric parameters including the critical frequency of the F2 layer (foF2), the peak height of the F2 layer (hmF2), and the scale height at hmF2 (HT) from 2006 to 2012 (ascending phase of solar activity) at Hainan (19.5° N, 109.1° E, MLat. 9.7° N), Irkutsk (52.4° N, 104.3° E, MLat. 42.5° N), and Norilsk (69.2° N, 88.0° E, MLat. 59.8° N) stations (low, middle and high latitudes). We have used manual scaled digisonde ionogram data. Studies of foF2 and hmF2 di-urnal-seasonal variations continue those made earlier for East Asia. Features peculiar for the ascending phase of solar activity are mostly consistent to those for de-scending phase, except for the features of sunset and nighttime hmF2 variations. Features of annual and semi-annual variations recorded by a digisonde agree with those obtained by a satellite occultation and TEC map. We also obtained seasonal, diurnal, annual, and semi-annual variations of the ionospheric parameter HT (scale height at hmF2) from digisonde data, which differ from foF2 variations and hmF2 features.

Современные проблемы дистанционного зондирования Земли из космоса

Based on vertical sounding data from nine ionosondes located at 19–66°N, 100–130°E we investigated the latitude-temporal dynamics of ionospheric disturbances during the 17–19 March 2015 severe two-step geomagnetic storm, and compared it with temporal dynamics of total electron content (TEC) profiles along 120°E. The phenomena that accompanied the main and early recovery storm phases were in particular focused on in this study. The distinct storm-related ionospheric disturbances began 2.5, 4 and 5h after onset of the storm main phase at subauroral, middle and low latitudes, respectively. To clarify the main mechanisms causing the disturbances at different latitudes we compared the changes in ionospheric parameters and TEC profiles with changes in the northern polar cap index and geomagnetic field in the vicinity of 120°E. The equatorward shift of the main ionospheric trough (MIT) and diffuse precipitations zone accompanied by an increase in precipitating particle flux was found to have a substantial influence on the subauroral ionosphere during the main and early recovery phases. The thermosphere Joule heating due to westward and polarized jets led to an increase in neutral wind velocity and generation of disturbed dynamo electric field. The strengthened wind was the main reason of the positive ionospheric disturbance observed at middle latitudes in the evening on 17 March. The further enhancement of magnetospheric convection caused the displacement of MIT and its associated negative ionospheric disturbance to middle latitudes. At low latitudes superposition of prompt penetration and disturbed dynamo electric fields play the decisive role in the ionosphere behavior till the end of the early recovery phase.

This paper examines the spatio-temporal dynamics of backscattering signals during St. Patrick’s Day two-step intense geomagnetic storm from the Yekaterinburg Coherent Radar (YeKB radar) data. It is found that a number of ground backscattering signals increased during the initial phase of the storm and decreased during the second step of its main phase and the first two days of its recovery phase. Changes in ionospheric backscattering signals started at the beginning of the main phase. During the first step, there was a six-hour sequence of ionospheric backscattering signals (BSi signals) the range of which decreased while the storm was in progress. During the last 5 hours of the main phase and the first 3 hours of the recovery phase, the YeKB radar observed only signals scattering in the E region of the ionosphere. We conduct a complex analysis of data from the YeKB radar, ground-based ionospheric, riometric, and magnetic stations located within the radar field of view. The analysis shows that the observed backscattering dynamics was caused by the magnetosphere compression, expansion of convection cells, impact ionization, and changes in atmospheric composition during the initial storm phase, first and second steps of the main phase, and the recovery phase respectively.