Guiding Medium Radio Waves in the Magnetosphere: Features and Geophysical Conditions

Abstract

We present experimental results related to the features and geophysical conditions for the occurrence of the long-delay echo (LDE) signals in the medium-wave (MW) frequency range observed on 20 January 2025, at the Gor’kovskaya observatory near St. Petersburg (60.27° N, 29.38° E). A total of 19 series of experiments on guiding MF in the magnetosphere were carried out, while LDE signals were only registered on January 20, 2025, in evening hours, when the most disturbed conditions were observed (Kp = 4+, ΣKp = 27−). It was found that the LDE signals, with delay times of 310–322 ms, were observed in the evening hours under disturbed magnetic conditions. In such a case, the MW propagates into the magnetosphere to the magnetically conjugate point, is reflected from the topside ionosphere, and returns. The frequency of sounding signal f_SS exceeded the critical frequency of the F2 layer at Gor’kovskaya observatory foF2_GRK but was less than the critical frequency at the magnetic conjugated point foF2_MCP, foF2_GRK < f_SS < foF2_MCP. The LDE signals were observed in the narrow frequency range from 2100 to 2400 kHz. The background geophysical conditions during the occurrence of LDE signals were analyzed using the CADI ionosonde data and Swarm satellite observations. The plausible generation mechanisms for MW guiding in the magnetosphere are discussed.

1. Introduction

During the long-distance propagation of the medium and short radio waves (MWs and SWs), radio echoes with large delay times from hundreds of milliseconds to units of seconds (the so-called long-delayed echoes, LDEs) are sometimes registered [1,2]. Radio amateurs have also observed such phenomena [3,4]. Such LDE signals may be induced by the round-the-world propagation of radio waves, reflection from space objects, and magnetospheric propagation along magnetic field lines.

During the round-the-world propagation of signals, the radio wave is captured in the ionosphere–earth waveguide, can propagate around the Earth one or even several times, and be registered at the transmission point. The time of the round-the-globe propagation of signals is within the range of τ = 136–139 ms, with differences between different experiments being within 5% [2,4,5]. With multiple propagations around the globe, the delays of echo signals are determined as τ_n = nτ, where n is the number of the round-the-globe propagation of signals.

It is also possible to observe echo signals when an ordinary (O-mode) polarized HF radio wave is converted into a slow extraordinary (Z-mode) wave and propagated above the maximum height of the F2 layer (hmF2). A similar conversion mechanism and its modeling are described in [6]. The waves propagating by such a mechanism are received strongly attenuated.

During the magnetospheric propagation, a radio wave radiates from the ground surface, having passed through the ionosphere, penetrates into the magnetosphere and is guided by the duct along the Earth’s magnetic field line. The duct with enhanced or reduced electron density may be created by the medium- and large-scale plasma irregularities along magnetic field lines. In such a waveguide, the signal propagates to the magnetically conjugate point. It passes through the topside ionosphere, is reflected in the F2 region and propagates back to the observation point. Such returned signals, or long-delayed echo signals, LDEs, were observed in some experiments concerning investigations of the anomalous propagation mechanisms of medium and short radio waves [7,8]. Most often, LDEs were observed in the frequency range from 1.8 to 3 MHz. In some cases, injection of radio waves into the magnetosphere and their further propagation were observed even in the frequency range of 9–12 MHz [9].

Satellite-based instruments also registered the MF waves passing through the magnetosphere between magnetically conjugate points. Based on the results of ionosphere probing from above on the ALUETT satellite, it was shown that probing signals at frequencies below 4 MHz could penetrate into narrow ducts oriented along magnetic field lines and propagate to the magnetically conjugate point of the opposite hemisphere. Moreover, most often, LDEs were recorded at frequencies up to 2 MHz. LDEs were observed at L-shell less than 4 with the delay times of 500 ms [10].

Signal propagation by the described mechanism is rare and constitutes a few percent of the total observations. Experimental studies of guiding short radio waves at frequencies of 4.9, 5.4 and 7.9 MHz at the EISCAT/Heating facility, used as a magnetospheric radar, did not demonstrate a positive result [11].

In this paper, we report recent experimental results concerning guiding medium radio waves, radiated from the Earth’s surface, in the magnetosphere along the magnetic field line. The aim of the study is to reveal the characteristic features of LDEs and conditions under which these can occur. Results are based on experiments carried out in 2024–2025 at the Gor’kovskaya observatory (60.27° N, 29.38° E) near St. Petersburg. The outline of the paper is as follows. A description of the experiment and the instruments are given in Section 2. In Section 3, we report the experimental results obtained at Gor’kovskaya observatory. Section 4 discusses the obtained results. Conclusions are given in Section 5.

2. Experiment Setup and Observations

Experiments on guiding medium radio waves in the magnetosphere were carried out in January 2024 and 2025 in the evening and night-time hours at the Gor’kovskaya observatory (60.27° N, 29.38° E) near St. Petersburg, which belongs to the Arctic and Antarctic Research Institute (AARI). During the experiments, the frequency of sounding signal f_SS should exceed the critical frequency of the F2 layer, f_SS > foF2_GRK, to pass through the ionosphere without being reflected. At the same time, the f_SS should be less than the critical frequency at the magnetically conjugate point, f_SS < foF2_MCP which will allow the signal to be reflected from the ionosphere and return to the observation point. Therefore, the choice of the sounding frequency is determined by the following condition:

foF2_GRK < f_SS < foF2_MCP

(1)

This condition is fulfilled in the winter in the northern hemisphere (Gor’kovskaya observatory), which corresponds to summer in the southern hemisphere. Over the course of the experiments, the values of foF2 and the absence of the sporadic Es layer at Gor’kovskaya (GRK) observatory were determined every 15 min from the vertical sounding ionosphere (VSI) by the CADI ionosonde (Scientific Instrumentation Ltd., Saskatoon, Canada), which operates in the network mode [12].

To calculate the location of the magnetically conjugate point (MCP), the L-shell number, the length of the radio path along the magnetic field line, and, consequently, the delay of the radio echo of the sounding signal, the GEOPACK T-89 magnetospheric magnetic field model [13] was used. According to calculations for magnetic activity, Kp = 3–5, the location of MCP is in the region of 51° S and 53° E, and the L-shell value is 3.4–3.5. The estimated distance from GRK to the magnetically conjugate point and back along the magnetic field line is 94,546–96,712 km, which corresponds to the delay times of the echo signals in the range of 315–322 ms.

The values of foF2 at MCP were found from data presented at the site of Australian Center of Space Weather forecast, https://www.sws.bom.gov.au/HF_Systems/6/5 (accessed on 29 January 2025). These data were compared with the ionosonde observations at the Mawson station, Antarctic (67.4° S, 62.5° E), https://www.sws.bom.gov.au/HF_Systems/1/3 (accessed on 29 January 2025), closest to the magnetically conjugate point.

Figure 1a,b illustrate the geometry of experiments on guiding medium radio waves in the magnetosphere. Figure 1a shows the location of Gor’kovskaya (GRK) observatory, Mawson station (MAW), and magnetically conjugate point (MCT) according to the GEOPACK T-89 model. Figure 1b demonstrates the propagation trajectory of sounding signal (SS) along the magnetic field line from GRK to MCP and back along the magnetic field line. The Earth is in the center of the drawing, and the night side of the Earth is shaded. The sounding signal propagates from GRK to MCP along the L-shell with a value of 3.4, is reflected from the ionosphere in MCT, and returns to GRK as an echo signal (LDE).

To search for an echo signal using the autocorrelation method and its unambiguous identification, a five-pulse Barker amplitude sequence was used as a sounding signal. The IC-718 transceiver (ICOM, Osaka, Japan) and the PW-1 amplifier (ICOM, Osaka, Japan), providing signal output power from 500 to 1300 W, were used in the course of experiments. The IC-718 transceiver was modified with sounding pulse generator circuit. This circuit is based on the so-called “blue pill” board with STM32 microcontroller (ST Microelectronics International N.V., Geneva, Switzerland, https://stm32-base.org/boards/STM32F103C8T6-Blue-Pill.html, accessed on 29 January 2025). On this board, the precise timer is launched, which counts specified time delays for a Barker sequence generation. The duration of the sequence pulses was selected based on the condition of failure-free operation of the IC-718 and PW-1. Experience of operation has shown that the equipment can operate stably with pulses lasting at least 30 ms. Thus, the duration of the sounding sequence was 150 ms. The scanning range of sounding signals was selected from the possible frequency range from 1.6 to 7 MHz, based on the fulfillment of the condition foF2_GRK < f_SS < foF2_MCP. The frequency tuning step ranged from 25 to 100 kHz. The sounding signal was transmitted every 4 s and was repeated seven times.

The transmitting antenna is a broadband “delta-web” with an operating frequency range of 1–16 MHz. The antenna fabric consists of two inclined and two horizontal vibrators, which are mounted on a 33 m high mast. The antenna provides the radiation pattern oriented vertically upward relative to the earth’s surface.

The reception of sounding signals was carried out using an HF receiving equipment (ICOM, Osaka, Japan), which is a decameter range spectrum analyzer based on the IC-R75 receiver [14]. For use in guiding experiments, a software package was developed that allows recording, visualization, and correlation and spectral analysis of radio pulse sequences. The bandwidth of the received signals analysis was 250 Hz, which was set by a band pass filter at the intermediate frequency. The signals were recorded in a binary file for further analysis. Signal reception was carried out on a vertical rhombic antenna (Serpukhov electronics factory, Serpukhov, USSR) with one side length of 50.4 m and a central mast height of 42 m. The antenna and transmitter provide radiation of sounding signals with a power of up to 5 kW in the frequency range of 1–5 MHz.

To synchronize the operation of the transmitter and receiver parts, the appropriate software was developed, allowing scanning of the selected frequency range with a specified number of repetitions of the sounding sequences. The software uses the Real-Time Clock of computer, which is synchronized with the NTP server. A GNSS receiver is not used. The software allows you to quickly form a radiation grid by time and frequency, and synchronize the frequency setting of the transmitter and receiver. Thus, in terms of its functional design, the equipment resembles an ionosonde, with the difference that the expected delays of reflected pulses were not microseconds, but hundreds of milliseconds.

3. Results

During observations in January 2024 and 2025 in the evening and night-time hours, a total of 19 series of experiments were carried out. The duration of each series lasted between 4 and 10 h. The experiments were conducted under quiet geomagnetic conditions, with the exception of 15, 16 and 20 January 2025, when magnetic disturbances were observed. Under quiet conditions the values of three-hour planetary index of magnetic activity Kp ranged from 1− to 3+ with a daily sum of ΣKp = 8–16. Under weakly disturbed conditions, Kp-index was about Kp ≈ 4, with ΣKp = 22–27−. The most disturbed conditions were observed on 20 January 2025, when the maximum Kp-index was 4+ with ΣKp = 27−.

According to observations, LDE signals with delay times of 310–322 ms were recorded only on 20 January 2025 from 17:45 to 18:00 UT (20:45–21:00 LT). During the experiment, the critical frequency of the F2 layer (foF2) was 2.1 MHz, the sporadic E layer was not observed on the CADI ionograms at Gor’kovskaya observatory. At the same time, the foF2 at the magnetically conjugate point was about 5 MHz. Sounding signals were radiated in the range from 1.8 to 3.5 MHz with a frequency step of 100 kHz. The radiated power was of 1 kW. Figure 2 demonstrates the behavior of foF2 at the Gor’kovskaya Observatory and magnetically conjugate point as well as the frequency changes of the sounding signal on 20 January 2025 from 17:30 to 18:30 UT.

Figure 3 presents oscillograms of received signals at frequencies of 2300 and 2400 kHz recorded from 17:46 UT on 20 January 2025. Figure 3 shows the signals after amplitude detection, low-pass filtering and amplification. According to Figure 3, the sounding signal (SS) with a large amplitude of 1.5 V was observed at zero second. SS propagates from transmitter antenna to the receiver antenna as a direct ground wave. The distance between transmitter and receiver antennas is 100 m. According to Figure 3, at frequency of 2300 kHz at 310 ms after the start of sounding, an echo signal (long-delayed echo, LDE) was registered, which has the same temporal behavior as the radiated signal, but a smaller amplitude. At a frequency of 2400 kHz, the echo signal was not recorded. Further we consider the frequency range in which LDEs were registered.

Figure 4a shows the amplitude of the received signal depending on the delay time and time of observations on 20 January 2025 from 17:44 to 18:00 UT. The first sounding cycle began at 17:44:00 UT, and the frequency was 1800 kHz. At this frequency, seven soundings were performed with a period of 4 s. After 28 s, f_SS was 1900 kHz. At this frequency, seven soundings were also performed with an interval of 4 s. Thus, f_SS, was retuned every 28 s until 17:52:00 UT, when the frequency reached the upper sounding limit of 3500 kHz. The SS frequency changes during sounding cycles are shown in Figure 4b. From 17:52:25 UT, the sounding cycle was repeated from the frequency of 1800 kHz. In total, three sounding cycles were performed from 17:44 to 18:09 UT. Only the first two cycles, in which echo signals were observed, are shown in Figure 4. The amplitudes of the received signals correspond to the color bar in Figure 4a. LDE signals were registered 310–322 ms after the start of sounding.

In the first sounding cycle, LDE signals began to be observed from 17:45:25 UT at a frequency of 2100 kHz and were recorded in five sounding sequences out of seven. At a frequency of 2200 kHz, six LDE signals were rerecorded, and at 2300 kHz six LDE signals were also recorded.

In the second sounding cycle from 17:52:25 to 18:00:50 UT, LDE signals were recorded in the frequency range of 2100–2400 kHz. At frequencies of 2100, 2200, 2300, and 2400 kHz, six, seven, six, and two LDE signals were recorded, respectively. In the third sounding cycle, the LDE signals were not observed, so it is not shown in Figure 4.

Therefore, on 20 January 2025, the LDE signals were only observed in the narrow frequency range from 2100 to 2400 kHz. In the range below 2100 kHz, signals from various radiation sources were recorded due to the sounding signals that were able to be reflected from the ionosphere, which resulted in a high noise level.

4. Discussion

In January 2024 and 2025, a total of 19 series of experiments on guiding MF in the magnetosphere were carried out. Experiments were conducted on 21 December 2023; 9, 11, 16, 18, 22, 25 and 30 January 2024; 1 February 2024; 26 December 2024; 9, 10, 15, 16, 20, 22, 24, 29 and 31 January 2025. The condition foF2_GRK < f_SS < foF2_MCP was only fulfilled on 20 January 2025 from 17:45 to 18:00 UT, when the maximum Kp-index was 4+ with ΣKp = 27−. Analysis of the background geophysical conditions, in which LDE signals were observed, revealed some features of this day compared to other days.

4.1. Analysis of the Background Geophysical Conditions Using the Ionosonde Data

The analysis of the behavior of the critical frequency of the F2 layer (foF2) showed that LDE signals were observed after a sharp decrease in foF2, accompanied by a strong increase in the F2 layer maximum virtual height (h’F2). We compared the behavior of median values of foF2 and h’F2 for January 2025 and values of foF2 and h’F2 on 29 January 2025, with the observed values for 20 January 2025 (see Figure 5). The median value for January 2025 is based on data for the whole month from 1 to 30 January 2025. On 29 January 2025, an experiment was carried out but LDE signals were not registered. According to Figure 5, the median foF2 values in January 2025 from 16:45 to 18 UT decreased from 4.5 MHz to 2.8 MHz. In this case, the maximum gradient of the foF2 decrease was 0.7 MHz over 15 min. On 20 January 2025, the foF2 values dropped sharply from 4.9 to 2.1 MHz with the maximum gradient of 1.1 MHz over 15 min. Simultaneously, the h’F2 values rise steeply.

Thus, 15 min before the observation of LDE signals, a sharp decrease in the foF2 occurred with an increase in the h’F2 on 20 January 2025, which are not typical for their median values in January 2025, and for changes in the foF2 and on 29 January 2025, when LDE signals were not registered.

Next, we consider in more detail ionograms taken at Gor’kovskaya observatory on 20 January 2025 from 16: 45 to 18:30 UT (see Figure 6). As seen from Figure 6, the occurrence of an additional track, associated with F3S layer, was observed in ionograms at 16:45, 17:15, 17:45 and 18:15 UT. Moreover, the lowest value of the minimum frequency f_min, of 1.0 MHz was observed at 17:45 UT, while in other ionograms f_min was about 1.5–1.7 MHz. This means that the LDEs were registered when the absorption level was lower than usual.

It was found that the presence of the F3S layer on vertical sounding ionograms is the main signature of the development of a Subauroral Ion Drift (SAID), also known as Polarization Jet (PJ) near the station’s zenith [15]. A SAID/JP consists of narrow flow of very fast ion drifts to the west near the projection of the plasmapause at altitudes of the F-region of the ionosphere [16]. The SAID/JP features were discussed in detail in [15,17,18,19,20]. The SAID/JP has an extension along the meridian of 100–200 km (1–2° in latitude) and is always registered equatorward from the boundary of the auroral oval. It was observed mainly in the evening and pre-midnight sector and shifted to lower latitudes with the growth of the geomagnetic activity.

According to ground-based and satellite observations, the subauroral flow leads to the rapid formation of narrow and deep ionization trough or deepening of the already existing main ionization trough near its polar wall; that is, the SAID/JP is always located inside the main trough, equatorial to its polar wall [17,21]. The difference in latitude between SAID and polar wall varies from 1° to 10° and depends on geomagnetic conditions. During moderate geomagnetic disturbances (Kp = 4–5), the SAID/PJ is located southward of the polar wall of the trough by a few degrees [17].

4.2. Analysis of the Background Geophysical Conditions Using Swarm Satellite Data

The location of the main ionospheric trough (MIT) during the experiment can be estimated using data from the Swarm satellites, which are equipped with the Langmuir probe, operating at a measurement frequency of 16 Hz [22]. The orbit of the Swarm A and C, launched by the European Space Agency in 2014, has an altitude of 451–481 km and an inclination of 87.3°. The distance between the spacecrafts in longitude is 1.4°.

Twenty minutes before observing the guiding effect on 20 January 2025, Swarm A and C passed along a trajectory 5° eastward from the Gor’kovskaya observatory, and the altitude of the flight was approximately 460 km. Figure 7a demonstrates plasma frequency values (f_pl) and the corresponding electron density values (Ne), based on measurements by Swarm A and C on 20 January 2025 from 17:29 to 17:34 UT. For comparison, the behavior of f_pl and Ne under quiet magnetic conditions on 29 January 2025 from 16:53 to 16:58 UT, when the guiding effect was not observed, is also shown. Swarm C is closest to the Gor’kovskaya station (see Figure 7b,c).

Comparing the data from Swarm A and C measurements spaced apart by longitude, the f_pl and Ne values are very close. As a result, it can be expected that the fpl and Ne values above the Gor’kovskaya observatory will also be close to the fpl and Ne values measured on Swarm C. According to Figure 7, on 20 January 2025, the equatorial boundary of the MIT T_E, corresponding to a 20% decrease in the electron density [23], was located at a latitude of 56.3°, and the bottom of the MIT was in the latitude range from 60.3° (T_BE)–62.7° (T_BP) corresponding to the bottom width of 2.4°, and the MIT poleward wall T_Pol was at 63.4°. Thus, the Gor’kovskaya observatory was located at the bottom of the MIT, near its equatorial boundary. The projection of the plasmapause P_pp was also located at the MIT bottom, between its polar boundary and Gor’kovskaya.

On 29 January 2025 (see Figure 7), the MIT poleward boundary was located at a latitude of 65°, and the width of the MIT bottom was 1.2°. Plasma frequencies at the equatorial boundary and bottom of the MIT were higher by 0.9 and 0.3 MHz accordingly, compared to 20 January 2025 during the observation of the guiding effect.

Comparison of the location of the MIT main structures depending on geomagnetic conditions shows that, on 20 January 2025 under disturbed magnetic conditions, the MIT was located further south than on 29 January 2025 under quiet conditions. With increasing magnetic activity, the width of the MIT bottom also increased, accompanied by the decrease in the plasma frequencies on the equatorial boundary and the bottom of the MIT.

4.3. Plasmapause Location

The L-shell number of the plasmapause location Lp can be approximately estimated from the expression in [24] as

Lp = 5.6 − 0.46 Kp_max,

(2)

where Kp_max is the maximum value of the Kp-index for the previous 24 h. The expression is valid for the dipole magnetic field.

The geocentric distance of the plasmapause varies depending on the geomagnetic activity and decreases with an increase in the Kp-index that corresponds to the approach of the plasmapause to the Earth. When LDEs were registered on 20 January 2025, the maximum value of Kp-index was Kp_max = 4+, which corresponds to the location of the inner boundary of the plasmapause at Lp = 3.6. The L-shell for Gor’kovskaya observatory according to the GEOPACK T-89 model [13] is L = 3.4–3.5. Thus, the projection of the plasmapause onto the ionosphere was located to the north by one degree of latitude from Gor’kovskaya observatory. On other days of experiments under quiet geomagnetic conditions corresponding to a Kp-index value of 1–3, the Lp was 4.2–5.1.

Stepanov et al. [25,26] studied the dynamics and relative location of the poleward boundary of the main ionospheric trough, T_pol, the projection of the plasmapause onto ionosphere, P_pp, and the fast subauroral drift (SAID/JP), using a large number of examples. It was found that under disturbed magnetic conditions, SAID/PJ was located in the bottom of the main ionospheric trough, and P_pp was between SAID/PJ and T_pol.

The poleward boundary of the trough is formed by soft diffuse electron precipitation and can coincide with the projection of the plasmapause, as indicated by a number of studies based on both ground-based [27] and satellite observations [28,29,30]. Under disturbed magnetic conditions, the projection of the plasmapause shifts to the equator from the MIT poleward boundary and tends toward its bottom due to the plasmapause location changes much faster than the restructuring of the MIT.

What are the plausible mechanisms for the propagation of medium radio waves (MW) in the magnetosphere?

Blagoveshchensky [31] proposed a mechanism of guiding MW along magnetic field lines in the plasmapause. In this case, the favorable conditions for MW guiding are realized when the plasmapause projection is located inside the MIT, and the transmitter is located in the trough, equatorward of the plasmapause. The MW propagation occurs along the ionization step formed by the MIT structures and the plasmapause. Over the course of the experiment on 20 January 2025, the geophysical conditions given in [32] could indeed have developed and the propagation of radio waves was carried out along the ionization step between the minimum of the MIT, enhanced by the influence of the subauroral flow (SAID/PJ) and plasmapause.

Based on ALOUETTE satellite observations [10], the mechanism of radio wave channeling between the electron density irregularities oriented along the Earth’s magnetic field lines was proposed. The propagation channel was formed between field-aligned electron density irregularities, in which electron density could exceed only 1% of the background value. According to the recent satellite observations, the scale of the irregularities transverse to the magnetic field ranged from tens of meters to several kilometers [19,20].

When LDEs were registered, Gor’kovskaya observatory was located equatorward of the plasmapause projection and, due to the propagation channel, could have formed between irregularities on the inner boundary of the plasmapause. Within such irregularities, the electron density can exceed the background by up to 10% [33]. Possible reasons of protonospheric-ionospheric causes for the generation of such irregularities were examined [34]. According to work [35], only one mechanism is possible related to the formation of a local electric field at the base of the magnetic flux tube leading to the generation of the field-aligned irregularities.

By using data from the Yakut meridional network of ionospheric stations, it was shown that plasma is transferred along the magnetic flux tube from the ionosphere to the plasmasphere in the SAID/PJ band, which is significant for the F-layer ionization losses [18]. In addition, it was also shown with satellite observations that the SAID/PJ events recorded by ionospheric stations are accompanied by the generation of irregularities in the outer plasmasphere from satellite observations [19,20,36].

The SAID/PJ, observed at Gor’kovskaya observatory during the recording of the LDE signals, points to electron outflow from the ionosphere and the presence of a strong electric field. The SAID/PJ, consisting of the narrow intense streams of western ion drift, was repeatedly observed in the night sector near the boundary of convection and the poleward wall of the MIT [16,37]. Such a narrow stream of fast westward ion drift is associated with a strong poleward electric field. Thus, the existence of subauroral flow during the experiment on 20 January 2025 supports such a propagation mechanism.

5. Conclusions

We have analyzed the features and geophysical conditions for the occurrence of the echo signals in the medium-wave frequency range with large time delays, called the long-delay echo (LDE) signals. Observations were carried out in January 2024 and January 2025 at the Gor’kovskaya observatory (60.27° N, 29.38° E) near St. Petersburg. A total of 19 series of experiments were carried out. The LDE signals were only recorded on 20 January 2025 in the evening hours, when the most disturbed conditions were observed (Kp = 4+, ΣKp = 27−). They have time delays of 310–322 ms. The observed delays correspond to the propagation time of the MW, radiated from the ground surface in the northern hemisphere, to the magnetically conjugate point in the southern hemisphere along the magnetic field line. It passes through the topside ionosphere, is reflected in the F2 region and propagates back to the observation point traveling the distance of 94,546–96,712 km.

It was shown that LDE signals were registered when the frequency of sounding signal f_SS exceeded the critical frequency of the F2 layer at Gor’kovskaya observatory foF2_GRK but was less than the critical frequency at the magnetically conjugate point in the southern hemisphere: foF2_MCP, foF2_GRK < f_SS < foF2_MCP. Sounding signals were radiated in the range from 1.8 to 3.5 MHz with a frequency step of 100 kHz. LDE signals were registered with the use of transmitting and receiving measuring equipment developed at the AARI having a radiation power of 1 kW. The LDE signals were registered not at the only fixed frequency, but in a frequency band of 400 kHz, from 2100 to 2400 kHz.

The background geophysical conditions during the occurrence of LDE signals were analyzed using the CADI ionosonde data and Swarm satellite observations. It was found that LDE signals were observed after a sharp drop of foF2, accompanied by a strong increase in the F2 layer maximum virtual height (h’F2). The distinctive feature seen from the CADI ionograms was the presence of the F3S layer, which is the main signature of the development of a subauroral flow near the station’s zenith. According to Swarm satellite observations, Gor’kovskaya observatory was located at the bottom of the main ionospheric trough (MIT), near its equatorial boundary. The projection of the plasmapause was also located at the MIT bottom, between its poleward boundary and Gor’kovskaya.

The plausible generation mechanisms for MW guiding in the magnetosphere are considered. The first one is guiding MW along magnetic field lines when the plasmapause projection is located inside the MIT [32]. The second mechanism proposed by [10] is the radio wave channeling between the electron density irregularities oriented along the Earth’s magnetic field lines. The existence of subauroral flow accompanied by the generation of field-aligned irregularities in the outer plasmasphere supports such a propagation mechanism. This fact is consistent with the extensive previous VLF whistler results [38,39], which indicate an increase occurrence of ground-based whistlers following geomagnetic activity.