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The response of the ionosphere to the earthquake of a moderate magnitude (M ≈ 5.9) that occurred in Japan at 11:23:50 UT on 7 July 2018 has been studied using a newly developed coherent multi-frequency radio diagnostic system for remotely probing the ionosphere at oblique incidence. The seismic activity in Japan on 7 July 2018 was accompanied by aperiodic processes in the ionosphere at distances of no less than (1–2) × 103 km from the epicentre, with an enhancement in multiple-mode propagation, and a significant Doppler spectrum broadening. Three ways of transporting disturbances from the earthquake to the changes in the character of Doppler spectra variations have been identified by examination. First, the disturbances are generated by a surface Raleigh wave launched at the earthquake epicentre. They have been ascertained in the infrasonic range (a 3- to 4-min period) of oscillations. The relative amplitude of these quasi-periodic oscillations in the electron density is equal to 1.7–9%. The duration of the oscillation trains is found to be in the range of 24–55 min. The wave disturbance speed of propagation is approximately 3 km/s. Second, wave disturbances have also been ascertained in a 15- to 30-min period range. They could be generated in the vicinity of the epicentre and then propagated as atmospheric gravity waves modulating the electron density in the ionosphere. The relative amplitude of the quasi-periodic disturbances in the electron density is equal to 14 – 34%. The wave train attains a temporal duration of about 100 min and a speed of approximately 0.3 km/s. Third, the broadening of the Doppler spectra toward negative Doppler shifts with the time delay estimated to be 49–124 min, depending on the orientation of the propagation path, is the most pronounced Doppler signature of the disturbances caused by the earthquake. This time delay corresponds to a speed of about 0.3 km/s, and consequently, it suggests that this effect most likely is caused by the atmospheric gravity waves launched at the earthquake epicentre. Apparently, the rearrangement of the ionosphere acts to reverse the sign of the Doppler spectrum shift when the atmospheric gravity waves arrive at the reflection level.

A multi-path multi-frequency passive HF Doppler radar for oblique-incidence sounding the ionosphere, that has been produced in collaboration between researchers at the Harbin Engineering University and V. N. Karazin Kharkiv National University, is briefly described. An original software package has been designed, and the implemented autoregressive spectrum analysis gives a root-mean-square Doppler line error of 0.02 Hz and 7.5-s time resolution. The radar has been tested over the course of several months in an ionospheric dynamical process monitoring mode.

The large V/m electric fields inherent in the lower mesosphere play an essential role in lower ionospheric electrodynamics. They must be the cause of large variations in the electron temperature and the electron collision frequency and consequently of the transition of the ionospheric plasma in the lower part of the D region into a nonisothermal state. This study is based on the datasets on large mesospheric electric fields collected with the 2.2-MHz radar of the Institute of Space and Atmospheric Studies, University of Saskatchewan, Canada (52°N geographic latitude, 60.4°N geomagnetic latitude), and with the 2.3-MHz radar of the Kharkiv V. Karazin National University, Ukraine (49.6°N geographic latitude, 45.6°N geomagnetic latitude). The statistical analysis of these data is presented by [Meek, C.E., Manson, A.H., Martynenko, S.I., Rozumenko, V.T., Tyrnov, O.F. Remote sensing of mesospheric electric fields using MF radars. J. Atmos. Solar-Terr. Phys. 66, 881–890, 2004. 10.1016/j.jastp.2004.02.002]. The large mesospheric electric fields in the 60–67-km altitude range are experimentally established to follow a Rayleigh distribution in the 0 < E < 2.5 V/m interval. These data have permitted the resulting differential distributions of relative disturbances in the electron temperature, θ, and the effective electron collision frequency, η, to be determined. The most probable θ and η values are found to be in the 1.4–2.2 interval, and hence the nonstationary state of the lower part of the D region needs to be accounted for in studying processes coupling the electrically active mesosphere and the lower ionospheric plasma.

Large mesospheric electric fields can play an essential role in middle atmospheric electrodynamics (see, e.g., Goldberg, R. A., Middle Atmospheric Electrodynamics during MAP, Adv. Space Res. 10 (10) (1990) 209). The V/m electric fields of atmospheric origin can be the possible cause of large variations in the electron collision frequency at mesospheric altitudes, and this provides a unique opportunity to take measurements of electric fields in the lower ionosphere by using remote sensing instruments employing radiowave techniques. A technique has been proposed for making estimates of large mesospheric electric field intensities on the lower edge of the ionosphere by using MF radar data and the inherent effective electron collision frequency. To do this, data collected in Canada and Ukraine were utilized. The developed technique permits the changes in mesospheric electric field intensities to be derived from MF radar data in real time. The statistical analysis of data consistent with large mesospheric electric field intensities in the 60– 67 km region resulted in the following inferences. There are at least two mechanisms for the generation of large mesospheric electric fields in the mesosphere. The most likely mechanism, with a probability of 60–70%, is the summation of random fields from a large number of elementary small-scale mesospheric generators, which results in a one-parameter Rayleigh distribution of the total large mesospheric electric field intensity E with a mean value of approximately 0.7– 0.9 V/m in the 60– 67 km altitude region, or in the corresponding one-parameter exponential distribution of the intensity squared E2 of large mesospheric electric fields. The second mechanism of unknown nature, with 5–15% probability, gives rise to the sporadic appearance of large mesospheric electric field intensities E>2.5 V/m with a mean of 4 V/m . Statistically significant seasonal differences in the averaged large mesospheric electric field parameters have not been revealed. The probability of the absence of local large mesospheric electric fields amounts to approximately 25% for Ukraine and approximately 30% for Canada. A comparison of the Ukrainian and Canadian data indicates the possible existence of a latitudinal dependence in mean large mesospheric electric field features. Hence, the large electric fields are an additional source of electron heating that must be taken into account in studying a disturbed lower ionosphere and radio wave propagation within it.

The Kharkiv V. N. Karazin National University MF radar has been widely employed to study mesospheric electrodynamics in the altitude range from 60 to 70 km where the electron density values remain below 100 cm–3. On the other hand, the technique developed at the University for investigating mesospheric electrodynamics demands signal-to-noise ratios greater than five. In addition, a major contribution to signal degradations makes radio interference, and consequently the radar has to operate in a frequency range at the upper end of the medium frequency band, and the antenna array cannot be designed to be resonant. The purpose of this article is to report the upgrades to the MF radar antenna array to achieve maximum signal-to-noise ratio values. First, the replacement of a point resistive load by a resistor-capacitor load has provided the improvement in the impedance match of the existing antenna to the feeder. The voltage standing wave ratio in the 1.9–3 MHz frequency band could reach 3 before the replacement and does not exceed a value of 1.75 thereafter. Further, the upgrades put forward to the individual antenna arrangement resulted in converting three-lobe antenna pattern only into one-lobe symmetric antenna pattern and low levels of sidelobes over the entire 1.5–3.5 MHz frequency band.

The MF radar developed for remotely probing the near-Earth space environment and the technique for diagnosing plasma parameters in the electrically active mesosphere have enabled the determination of the basic electrodynamic characteristics of this region, which are required for achieving the fundamental physical understanding and can be used in practical applications (Sun-Earth connections, modeling the ionosphere as well as natural and anthropogenic global disturbances, near-Earth environment monitoring, earthquake predictions, and radio propagation forecasting). In particular, the average daily electric fields intrinsic to the mesosphere have been determined for the first time.

The Department of Space Radio Physics that was established in 1964 at the Kharkiv V. N. Karazin National University conducts studies of the near-Earth space environment at its constituent Radiophysical Observatory that is continuously upgraded. The Radiophysical Observatory (49deg38'N, 36deg20'E) is a facility that includes the MF radar, the HF Doppler radar (at vertical and oblique incidence), the satellite radio beacon receivers, the magnetometers, and the ionosonde. The instrumentation allows to investigate the various interacting processes operating in the ionosphere within an altitude range of ~60-1000 km and the characteristics of radio-wave propagation within a frequency band of up to approximately 2 GHz, simultaneously with magnetometer measurements. The Radiophysical Observatory is included in the register of the most valuable scientific facilities in Ukraine

Inverse problems in radiowave probing of the ionospheric plasma amount to solving integral equations, the right-hand part of which originates from measurements. Therefore, the solutions to such equations are unstable and require the application of regularization algorithms. As an example, we consider the partial reflection technique.

The distribution of variations in the effective electron collision frequency was obtained in the 60-66 km altitude range in the lower ionosphere (experimental errors within this altitude range were less than 50%). A technique for estimating the variations in atmospheric electric fields at the lower boundary of the ionosphere was developed using the experimental values of the effective electron collision frequency. From our measurements follows that the electric field E>0.25 V/spl middot/m/sup -1/ in approximately 70% cases under quiet ionospheric and atmospheric conditions. These facts must be taken into account in the investigations of ionospheric processes, meteorological and propagation effects.

Measurements were made in 1995 to 2000 at the two frequencies of 2.8 MHz and 3.0 MHz. The moving solar terminator generated detectable effects only in the absence of other disturbances in the ionosphere, and, most of all, in the absence of magnetic storms. The response to the dawn terminator commenced immediately after sunrise in the lower part of the ionospheric F-region where the probing radio waves are reflected. HF Doppler radar investigations of a variety of processes associated with rocket launches were performed over the same period of time. The observations have been analysed of thirty two launches from different cosmodromes (Baikonur, Plesetsk, Cape Canaveral, Kourou, and others).

The results are discussed of MF radar remote sensing of disturbances caused by strong earthquakes at great distances in the plasma at mesospheric heights, and a mechanism is advanced for the development of such large-scale disturbances in ionospheric plasma parameters, which is a large-scale mesospheric electric potential redistribution due to an increase in the atmospheric conductivity over the seismic region.