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The paper aims at an experimental study of the mechanisms of long-distance high-frequency (HF) propagation and spatial and temporal variations of the ionospheric parameters during the first hours of a severe geomagnetic storm of April 23, 2023 by spatially separated measuring equipment located at the research vessel (RV) Noosfera, the Ukrainian Antarctic Akademik Vernadsky station (hereinafter Vernadsky), and the LOFAR observatory PL610 in Borowiec (Poland). High-frequency vertical and oblique sounding techniques of the ionosphere were used. Geospace measurements were carried out synchronously. During the first hours of the geomagnetic storm of April 23-24, 2023, unexpectedly well-correlated variations in the Doppler frequency shifts of HF signals emitted from Vernadsky were observed at the RV Noosfera and the PL610 station. Furthermore, variations in Doppler frequency shifts of HF signals strongly correlate with magnetic field records in Antarctica and Poland. Variations in the frequency of HF signal spectral components, distinguishable during storm conditions, are utilized to clarify the mechanism of long-distance HF propagation and estimate the vertical velocity of ionospheric layers. Signals of HF CHU time radio station (Canada) at 7850 and 14670 kHz were unexpectedly observed in all receiving sites. Most probably, the CHU station radio signals registered during the initial stage of the geomagnetic storm were scattered on the polar ovals' ionospheric inhomogeneities and propagated further along the return (long arc of the great circle) paths. Redistribution of the ionospheric plasma during the geomagnetic storm leads to the formation of HF radio propagation channels absent under quiet conditions. [ABSTRACT FROM AUTHOR]

A statistical picture of the occurrence and characteristics of Traveling Ionospheric Disturbances (TIDs) over the Antarctic Peninsula is established using Global Navigation Satellite System Total Electron Content and High Frequency sounding observations. The measured parameters of the majority of the disturbances allow classifying them as medium scale TIDs (MSTIDs). Overall, the observed climatology of ionospheric disturbances in the Antarctic Peninsula region varies significantly with the season and makes it possible to differentiate two major types of the disturbances: winter daytime and summer nighttime, based on their occurrence periods and characteristics. During the Antarctic summer period, the disturbances are present mainly during the nighttime and morning hours, when the background plasma density is at maximum (due to Weddell Sea Anomaly). These disturbances predominantly propagate northwestward and their occurrence probability is well correlated with the sporadic E layer observations, suggesting that these are electrified MSTIDs. During the winter, the TID events are almost exclusively observed during the daytime. The propagation direction of the disturbances during the daytime shows a strong correlation with the background neutral wind direction in the thermosphere. A possible mechanism for this effect is wind filtering of the Atmospheric Gravity Waves originating in the troposphere, which indicates that their source is in the lower atmosphere. The periods of the TIDs also significantly differ between the seasons. Wintertime TIDs have noticeably shorter periods (10–50 min) than those observed during other parts of the year (30–140 min), which also likely reflects the fact that the two types of TIDs are generated by different physical mechanisms. Key Points: Majority of Traveling Ionospheric Disturbances (TIDs) observed in Antarctica are medium scale TIDsTwo major peaks in TID occurrence is observed: winter daytime and summer nighttimePropagation direction of daytime TIDs is primarily controlled by neutral wind [ABSTRACT FROM AUTHOR]

This paper describes a system for multi‐position Doppler ionosphere sounding implemented at the beginning of 2018 in the Kharkiv region of Ukraine, the data processing technique and initial results of data analysis. The system comprises a high frequency (HF) transmitter and three receiving sites making almost a right triangle with catheti of 32.8 and 58 km. The transmitter continuously emitting a monochromatic signal of a ∼20 W power at a frequency in most cases slightly lower than foF2 is located in the city of Kharkiv. The foF2 value was permanently controlled by the ionosonde installed in about 45 km from the transmitter. We report and discuss the initial results of traveling ionospheric disturbances (TIDs) observations for three years since January 2018 until December 2020. Statistical distributions of the frequency of appearance, periods, horizontal speeds, and propagation directions of TIDs are presented. The range of their most probable wavelengths is from 120 to 260 km, while their propagation velocities are from 80 to 220 m/s. The preferred direction of the TID propagation changes during the daytime and shows a tendency to rotate clockwise in azimuth. Notably, such regular long‐term measurements in the Eastern Europe have been carried out for the first time. Key Points: We describe a novel multi‐position high frequency Doppler facility in Ukraine and data processing technique capable of exploring the traveling ionospheric disturbances (TID) parametersVariations of the Doppler frequency shift from each of three sites revealed the salient TID characteristics in several case studiesStatistical distributions of the appearance frequency, periods, horizontal speeds, and directions of TIDs during a few years are presented [ABSTRACT FROM AUTHOR]

Results of investigating the scattering and propagation effects for the HF radiation from the HAARP heating facility (Alaska, USA) are presented. The latest data were collected during the HAARP measurement campaign that was conducted from February 21 through March 3, 2008. The transmissions from the HAARP HF heater were monitored at geographically dispersed sites in the USA, Europe, and Arctic, using digital Doppler receivers of the Institute of Radio Astronomy (Kharkov, Ukraine) and Digisondes of the University of Massachusetts Lowell (Lowell, USA). A number of characteristics features of the detected scattered signals suggest that the received radiowaves were scattered by ionospheric irregularities produced by the same HF emission. This effect is known as "self-scattering." Average relaxation times for the self-scattered signals were shown to be several tens of seconds (40-60 s), while the observed risetime was much longer, up to a few minutes. These numbers are characteristics of the types of the irregularities responsible for the scattering of the heater signal. A time constant of 50-60 s is usually associated with the relaxation time of small-scale irregularities with spatial scale sizes from a few meters to tens of meters. Several possibilities for explaining the observed longer risetime constant of two to three minutes are discussed. The power and spectral width of the scattered signals depend on the HAARP beam orientation and local ionospheric conditions. The possibility of extracting parameters of the stimulated plasma irregularities is discussed. [ABSTRACT FROM AUTHOR]