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This paper deals with the variations in the Doppler spectra and in the relative amplitudes of the signals observed at oblique incidence over the People's Republic of China (PRC) during the partial solar eclipse of 5–6 January 2019 and on reference days. The observations were made using the multifrequency multipath radio system for sounding the ionosphere at oblique incidence. The receiver system is located at the Harbin Engineering University, PRC, and 14 HF broadcasting station transmitters are used for taking measurements along the following radio-wave propagation paths: Lintong/Pucheng to Harbin, Hwaseong to Harbin, Chiba/Nagara to Harbin, Hailar/Nanmen to Harbin, Beijing to Harbin (three paths), Goyang to Harbin, Ulaanbaatar/Khonkhor to Harbin, Yakutsk to Harbin (two paths), Shijiazhuang to Harbin, Hohhot to Harbin, and Yamata to Harbin. The specific feature of this partial solar eclipse was that it occurred during the local morning with a geomagnetic disturbance (Kp ≈ 3−) in the background. The response of the ionosphere to the solar eclipse has been inferred from temporal variations in the Doppler spectra, the Doppler shift, and the signal relative amplitude. The partial solar eclipse was found to be associated with broadening of the Doppler spectrum, up to ± 1.5 Hz, alternating sign Doppler-shift variations, up to ± 0.5 Hz, in the main ray, and quasi-periodic Doppler-shift changes. The relative amplitude of electron density disturbances in the 15 min period of atmospheric gravity wave field and in the 4–5 min period of infrasound wave field is estimated to be 1.6 %–2.4 % and 0.2 %–0.3 %, respectively. The estimates of a maximum decrease in the electron density are in agreement with the observations.

The purpose of the present paper is to describe the observations of the variations in the parameters of HF radio waves propagating through the ionosphere when the action of the super typhoon Hagibis on 6–13 October 2019 occurred. The observations have been made with the Harbin Engineering University (the People's Republic of China) multi-frequency multiple path radio system involving the software-defined technology. The action of the super typhoon has been shown to be accompanied by enhanced atmospheric wave activity acting to generate wave processes with periods of 10 to 120 min. Coupling in the atmosphere–upper-atmosphere–ionosphere system has been confirmed to be carried out with atmospheric gravity waves. The ionosphere underwent the greatest impact on those days when the supertyphoon had maximum energy, on 8, 10, and especially 9 October 2019, and when it was found to be in an ∼2,500–3,000-km distance range from the propagation path midpoints. Under the action of wave processes, the height of the reflection region was observed to oscillate within the ±(30–90 km) limits. The amplitude of the quasi-periodic variations in the ionospheric F-region electron density was estimated to be 10–12% for periods of ∼20 min, and 30–60% for periods of ∼60–120 min. The joint action of the dusk terminator and the supertyphoon has been confirmed to enhance wave activity in the ionosphere. Similar effects for the dawn terminator have not been detected.

A brief description of the multipath coherent radio technique installation is presented. Its goal is continuous monitoring of the dynamical processes in the ionosphere caused by space weather variations and the impact of high-energy sources in the Earth–atmosphere–ionosphere–magnetosphere system. The variations of radio-wave characteristics (the Doppler spectra and signal amplitudes) in the HF range and the ionospheric parameters over China during the ionospheric storm of August 26, 2018, are described. An ∼50–100 km ascent in the radio-wave reflection region and its oscillations with an amplitude of ∼30–40 km were observed many times during the storm on all paths. The ascents were followed by descents of the radio-wave reflection region by many tens of kilometers. The ascents and descents of the reflection region were caused by a decrease in the electron concentration by a factor of 1.5–2 and its increase by a few times, respectively. The maximal increase in the electron concentration in the E and F regions reached a factor of 1.5 and 3, respectively. The relative amplitude of oscillations in the electron concentration reached many tens of percent. The amplitude of oscillations of frequency Doppler shift was several times lower on the reference days. The observed oscillations in the frequency Doppler shift were apparently caused by generation of the atmospheric gravity waves and their subsequent propagation to the latitudes of the means of observation. The velocity of wave disturbances was ∼275–480 m/s, whereas their period was ∼60 min. The ionospheric disturbances of August 27, 2018, were insignificant despite the repeated magnetic storm.

We acquired measurements with the oblique HF Doppler technique in the city of Harbin during the super typhoon Lekima event of 4 – 12 August 2019. The ionospheric response was observed by the Harbin...

The aim of this study is to observe the changes in the characteristics of HF radio waves traveling through the ionosphere perturbed by the super typhoon Hagibis during October 6–13, 2019 period. The measurements have been taken with the Harbin Engineering University (the People's Republic of China) multi-frequency multiple path software defined radio system. The super typhoon acted to enhance wave activity in the atmosphere and to generate wave processes with 10 to 120-min periods. The atmospheric gravity waves have been confirmed to conduct coupling within the atmosphere-upper-atmosphere-ionosphere system. The impact on the ionosphere was the largest when the super typhoon energy was a maximum, i.e., on October 8, 10, and particularly 9, 2019, and when the typhoon was at distances of ~2,500 km to ~3,000 km from the midpoints of the propagation paths. The wave processes acted to excite oscillations in the reflection level height from 30–50 km to 60–90 km. In the $F$ region, the ~20-min period harmonic in electron density changes were seen with amplitude of 10-12%, while the ~60-120-min period harmonics with amplitudes of 30-60%.

We have made measurements of HF radio waves at the Harbin Engineering University, the People's Republic of China, during the geospace storm of August 30-September 2, 2019. Further, we have performed a general analysis of space weather, and afterwards the data and disturbances in the ionosphere have been analysed in order to determine the effect that the ionospheric storm had on the parameters of radio waves. The main results of this analysis could be summarised as follows. The magnetic storm power and energy have been calculated to be 9 × 109W and 1.5 ×1015J, i.e., the storm was moderate. The geospace storm included a strong to moderate negative ionospheric storm. During August 31, 2019 and September 1, 2019, the $F$ region of the ionosphere showed a reduction in the electron density by a factor of 1.4 to 2.4 in comparison with the electron densities observed on the reference day. The geospace storm also caused notable disturbances in the midlatitude $E$ region of the ionosphere, and in the sporadic E s layer. During the ionospheric storm, the heights of reflection could undergo a drastic rise from ~150 km to ~300-310 km. The geospace storm launched atmospheric gravity waves that modulated the electron density in the ionosphere.

A brief description of multi-frequency multipath passive radar dedicated to sounding the ionosphere at oblique incidence is presented. Observations of variations caused by the ionospheric storms in the Doppler spectra and in amplitudes of the signal reflected from the ionosphere in 2018 – 2019 are described.

The observation of disturbances in the ionosphere during moderate (M ≈ 5.9 – 6.6) earthquakes of July 7, 2018, September 5 2018, and April 11, 2019 in Japan are described. Analysis of the signal amplitude, of the Doppler spectra variations, and of the Doppler shift of frequency in the signals traveling along the main ray path on the reference days and on the day when the earthquakes occurred have revealed that they are different. The disturbance propagation exhibits two apparent speeds, of 3 km/s characteristic of seismic waves and of ~300 – 1000 m/s characteristic of acoustic and atmospheric gravity waves in the Earth’s ionosphere. The amplitude of electron density disturbances caused by the infrasonic and atmospheric gravity waves has been estimated to be ~0.15 – 9% and 7 – 55%, respectively.