## 1 Introduction  
The idea that electromagnetic pulses can be produced in the Earth's crust as a conversion result of the tectonic energy into electrical one, both before and during earthquake, has given rise in the 1970s to intensive searching for the characteristic signs of earthquake preparation among the variation of electromagnetic emission [1]. At that time, Prof. [PERSON] introduced the notion of the Earth's natural pulsed electromagnetic field (ENPEMF). Presently, the investigation of temporal variations in ENPEMF is one of the most promising directions to reveal the seismogenic effects of earthquake preparation in geophysical fields. Successive development of the methods for studying ENPEMF in the very low-frequency band revealed the presence of a substantial part of the lithospheric component in the structure of ENPEMF [2; 3; 4]. Application of the ENPEMF method augmented the capabilities of the research aimed the short-term earthquake forecasting. Based on a variety of experimental evidence, the structure of an earthquake precursor was specified as vanishing of the characteristic daily cycle of the electromagnetic field with a decrease in its amplitude, followed by a sharp jump. It was shown that the spatial distribution of the stations destined for monitoring geodynamic processes in the lithosphere allows for determining both the intensity of ENPEMF pulses and the bearing of incoming signals [3; 4; 5]. Consistent research is conducted for the development of special data processing and analyzing methods aimed at the detection of the signals with stable informational features of earthquake preparation in the ENPEMF variation [6; 7; 8; 9]. Detection of anomalies in ultra-low frequency (ULF) signals a few days and hours prior to an earthquake is one of the most frequent events in the series of confirmed electromagnetic precursors of major earthquakes. Among the candidates for short-term earthquake forecasting, much attention is paid to electromagnetic phenomena in the ULF (0.001-3 Hz) band [10; 11; 12]. Mostly, objective predictive expectations are connected with electromagnetic radiation propagating from an earthquake epicenter [13]. This applies to earthquakes of a magnitude \(M>6\), under the condition that the measuring equipment is located at a sufficiently close distance to the epicenter of future earthquakes [10]. It is especially true for ULF waves (\(f<1\) Hz), due to their deeper skin effect. According to [14; 15], at a distance of 10-50 km from the epicenter, magnetic and electric disturbances can reach the levels of 1-10 nT and 1-10 uV/m correspondingly, while less than a day before an earthquake intensity of electromagnetic signals can rise dramatically from several nT to tens of nT [10]. Such estimates are rather close to the values observed before the \(M\)7.1 Loma Prieta (1989) and \(M\)6.9 Spitak (1988) earthquakes [16]. Anomalous effects arising in the variation of ULF electromagnetic waves some days and hours before a seismic event were considered in detail in a number of reviews of historical and recent studies, where the connection of such a signal with an earthquake is actively discussed [10; 11; 17; 18; 19; 20]. With the development of modern technologies, such as satellite and ground-based methods for monitoring the ionosphere, it becomes noticeably more relevant to investigate the ionospheric anomalies on the eve of an earthquake, which could serve as a possible prognostic of a seismic event. Understanding the coupling between peculiarities of ionospheric phenomena and the mechanism of earthquake preparation in the lithosphere is widely discussed in the literature [21; 22; 23; 24; 25; 26], including perspectives of the Global Electric Circuit (GEC) concept [27; 28; 29]. The Doppler method is the most sensitive radiophysical technique for the detection of ionospheric instabilities.