For the first time, by using a regression procedure, we analyzed the solar activity dependence of the winter anomaly intensity in the ionospheric F2-layer peak electron density (Nm F2) and in the Total Electron Content (TEC) on a global scale. We used the data from global ionospheric maps for 1998–2015, from GPS radio occultation observations with COSMIC, CHAMP, and GRACE satellites for 2001–2015, and ground-based ionosonde data. The fundamental features of the winter anomaly in Nm F2 and in TEC (spatial distribution and solar activity dependence) are similar for these parameters. We determined the regions, where the winter anomaly may be observed in principle, and the solar activity level, at which the winter anomaly may be recorded in different sectors. A growth in geomagnetic disturbance or in the solar activity level is shown to facilitate the winter anomaly intensity increase. Longitudinal variations in the winter anomaly intensity do not conform partly to the generally accepted Rishbeth theory. We consider the obtained results in the context of spatial and solar cycle variations in O/N2 ratio and thermospheric meridional wind. Additionally, we briefly discuss different definitions of the winter anomaly.

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We use ground‐based (GNSS, SuperDARN, and ionosondes) and space‐borne (Swarm, CSES, and DMSP) instruments to study ionospheric disturbances due to the 25–26 August 2018 geomagnetic storm. The strongest large‐scale storm‐time enhancements were detected over the Asian and Pacific regions during the main and early recovery phases of the storm. In the American sector, there occurred the most complex effects caused by the action of multiple drivers. At the beginning of the storm, a large positive disturbance occurred over North America at low and high latitudes, driven by the storm‐time reinforcement of the equatorial ionization anomaly (at low latitudes) and by particle precipitation (at high latitudes). During local nighttime hours, we observed numerous medium‐scale positive and negative ionospheric disturbances at middle and high latitudes that were attributed to a storm‐enhanced density (SED)‐plume, mid‐latitude ionospheric trough, and particle precipitation in the auroral zone. In South America, total electron content (TEC) maps clearly showed the presence of the equatorial plasma bubbles, that, however, were not seen in data of Rate‐of‐TEC‐change index (ROTI). Global ROTI maps revealed intensive small‐scale irregularities at high latitudes in both hemispheres within the auroral region. In general, the ROTI disturbance “imaged” quite well the auroral oval boundaries. The most intensive ionospheric fluctuations were observed at low and mid‐latitudes over the Pacific Ocean. The storm also affected the positioning accuracy by GPS receivers: during the main phase of the storm, the precise point positioning error exceeded 0.5 m, which is more than five times greater as compared to quiet days.

A Solar Radio Burst (SRB) is one of the most severe natural hazards affecting the performance of the global navigation satellite systems (GNSS). Considering the influence of different threat factors, the GNSS developers upgrade the systems to amend the accuracy and noise-proof features of the systems. In particular, GPS gradually replaces "old" satellites (GPS IIA, GPS IIR-A, GPS IIR-B) with new-generation equipment (GPS IIR-M, GPS IIF, GPS III) featured by an increase in the emitted signal power at L2 frequency and by new civilian codes. In this work, based on examples of the extreme SRB of September 24, 2011, and the severe SRB of September 6, 2017, we study how such modernization can improve the GPS system performance during solar flares accompanied by intense SRB. We recorded SRB-related drops in signal strength (S), which were 7.5/0 dB-Hz for the S1C, 10/7 dB-Hz for the S2X, 17/8 dB-Hz for the S2W and 9/7.5 for the S5/S5X in 2011/2017 correspondingly. The drop in the S2W signal strength for the modernized blocks was comparable in amplitude to those of the "old" blocks. However, the modernized IIR-M/IIF blocks were featured by about 5 dB-Hz higher signal strength. This resulted in a double and triple decrease in loss-of-lock density for the IIR-M/IIF satellites in 2011 and 2017, respectively, as compared to IIA/IIR-A during SRBs. Therefore, the increase in the emitted signal power and new civilian codes potentially enhance the stability of the GPS operation.

The focus of the paper is the ionospheric disturbances during sudden stratospheric warming (SSW) events in the Arctic region. This study examines the ionospheric behavior during 12 SSW events, which occurred in the Northern Hemisphere over 2006–2013, based on vertical sounding data from DPS‐4 ionosonde located in Norilsk (88.0°E, 69.2°N). Most of the addressed events show that despite generally quiet geomagnetic conditions, notable changes in the ionospheric behavior are observed during SSWs. During the SSW evolution and peak phases, there is a daytime decrease in NmF2 values at 10–20% relative to background level. After the SSW maxima, in contrast, midday NmF2 surpasses the average monthly values for 10–20 days. These changes in the electron density are observed for both strong and weak stratospheric warmings occurring at midwinter. The revealed SSW effects in the polar ionosphere are assumed to be associated with changes in the thermospheric neutral composition, affecting the F2‐layer electron density. Analysis of the Global Ultraviolet Imager data revealed the positive variations in the O/N2 ratio within the thermosphere during SSW peak and recovery periods. Probable mechanisms for SSW impact on the state of the high‐latitude neutral thermosphere and ionosphere are discussed.