The spatial distribution features of three Alpha transmitter signals at the topside ionosphere
2017; Wiley; Volume: 52; Issue: 5 Linguagem: Inglês
10.1002/2016rs006219
ISSN1944-799X
AutoresXuemin Zhang, Shufan Zhao, Yu. Ya. Ruzhin, Jing Liu‐Zeng, Rui Song,
Tópico(s)Seismic Waves and Analysis
ResumoRadio ScienceVolume 52, Issue 5 p. 653-662 Research ArticleFree Access The spatial distribution features of three Alpha transmitter signals at the topside ionosphere X. Zhang, Corresponding Author X. Zhang zhangxm96@hotmail.com orcid.org/0000-0001-9165-3506 Institute of Earthquake Science, China Earthquake Administration, Beijing, China Correspondence to: X. Zhang, zhangxm96@hotmail.comSearch for more papers by this authorS. F. Zhao, S. F. Zhao Institute of Earthquake Science, China Earthquake Administration, Beijing, ChinaSearch for more papers by this authorY. Ruzhin, Y. Ruzhin Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Science, Moscow, RussiaSearch for more papers by this authorJing Liu, Jing Liu orcid.org/0000-0002-3252-8630 Institute of Earthquake Science, China Earthquake Administration, Beijing, ChinaSearch for more papers by this authorR. Song, R. Song Institute of Earthquake Science, China Earthquake Administration, Beijing, ChinaSearch for more papers by this author X. Zhang, Corresponding Author X. Zhang zhangxm96@hotmail.com orcid.org/0000-0001-9165-3506 Institute of Earthquake Science, China Earthquake Administration, Beijing, China Correspondence to: X. Zhang, zhangxm96@hotmail.comSearch for more papers by this authorS. F. Zhao, S. F. Zhao Institute of Earthquake Science, China Earthquake Administration, Beijing, ChinaSearch for more papers by this authorY. Ruzhin, Y. Ruzhin Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Science, Moscow, RussiaSearch for more papers by this authorJing Liu, Jing Liu orcid.org/0000-0002-3252-8630 Institute of Earthquake Science, China Earthquake Administration, Beijing, ChinaSearch for more papers by this authorR. Song, R. Song Institute of Earthquake Science, China Earthquake Administration, Beijing, ChinaSearch for more papers by this author First published: 02 May 2017 https://doi.org/10.1002/2016RS006219Citations: 7AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract The spatial distribution features of electric field over three Alpha transmitters in Russia were analyzed based on the Demeter satellite records at local nighttime during the solar minimum in December of 2008, where the three transmitters are with the same emitted power of 500 kW and the same radio waves at 11.9 kHz, 12.6 kHz, and 14.9 kHz. The results of observations showed that the maximal electric field reached −80 to −70 dB (hereafter referred as to V/m) at 660 km altitude, and the horizontal covered area even exceeded 80° in longitude with electric field above −100 dB at 14.9 kHz. The lowest electric field and the smallest longitude scale were detected over Krasnodar (KRA), which is demonstrated that the lower ionosphere plays an important role in attenuating the energy as suggested by the simulation results from the full-wave propagation model. Another feature over KRA was the significant decrease in electromagnetic field strength at 11.9 kHz and 12.6 kHz, being one order of magnitude lower than the other two transmitters, where the lower hybrid resonance waves affected severely the whistler mode wave mode propagation. Compared with the ground very low frequency observations at Tonghai and Ya'an in China, the most complex variations were observed from KRA, while the east transmitter Khabarovsk maintained high strength of electromagnetic power in a longer distance than the middle transmitter Novosibirsk in local nighttime, which is consistent with the large covering scale in the topside ionosphere due to the enhancement by wave-particle interaction from the other transmitter. Key Points The electric field can reach −80 to −70 dB (V/m) with the emitted power being 500 kW in solar minimum winter season in local nighttime The big decrease at 11.9 kHz and 12.6 kHz over KRA was related to the higher electron density and LHR frequency in the lower ionosphere The large-scale strong electromagnetic intensity over KHA was verified by satellite and ground measurements possibly from NWC 1 Introduction For the purpose of navigation and communication, many very low frequency (VLF, 3–30 kHz) transmitters have been set up on the ground. These radio waves can propagate to a long distance in the Earth-ionosphere waveguide, and also they can penetrate directly into the ionosphere and magnetosphere and then be detected by satellites there [Helliwell and Katsufrakis, 1974; Clilverd et al., 2008]. The French satellite Demeter operated during 2004–2010 at the altitude of 660–710 km and had observed the worldwide VLF transmitters in electric and magnetic field [Berthelier et al., 2006; Parrot et al., 2006]. It is known that VLF propagation depends on the plasma state of the ionosphere, so these VLF signals recorded at satellite were used in many research areas, for example, the wave-particle interaction [Sauvaud et al., 2008; Li et al., 2012], the VLF-induced ionospheric heating phenomena [Parrot et al., 2007; Bell et al., 2011], the lightning propagation in VLF band [Nemec et al., 2010; Colman and Starks, 2013], and also the earthquake research by studying the decrease of the signal-noise-ratio over the epicenters to verify the seismic precursors correlated with plasma perturbations [Molchanov et al., 2006; Rozhnoi et al., 2007]. Studying the propagation features in VLF transmitters at the ionosphere can help us to further understand the coupling process between electromagnetic waves and plasma parameters. Inan et al. [1984] used a test particle computer model of the gyroresonant wave-particle interaction to interpret the satellite-based observations of electron precipitation zones induced by VLF transmitters. Based on the full-wave method, Cohen et al. [2012] presented the theoretical simulation results and compared them with the actual observation on Demeter, and they discussed the errors that resulted from the conversion of wave energy into electrostatic modes. In this paper, the three Alpha transmitters in Russia have been collected by the record of Demeter, which have the same power and with the same three emitting frequency waves, but located at different latitudes and longitudes. It provides a good opportunity for us to compare the similarity and difference of VLF radio wave propagation features in the topside ionosphere at different frequencies from one place and at one same frequency from different positions and to investigate more effect factors on VLF wave propagation in the ionosphere. 2 Data Collection and Analysis In order to reveal the strong penetration features of Alpha stations at the topside ionosphere, the electric field data detected by the Instrument Champ Electrique [ICE; see Berthelier et al., 2006] onboard the Demeter satellite was collected in local nighttime of December, 2008, at the winter season of the Northern Hemisphere and in the extremely solar minimum year. At the VLF frequency band, the continuous data along each orbit were only stored with power spectrum density (PSD) in one component of electric field by ICE. Table 1 lists the information of the three Alpha transmitters (Novosibirsk (NOV), Krasnodar (KRA), and Khabarovsk (KHA)), with power of 500 kW and same three frequency points at 11.9 kHz, 12.6 kHz, and 14.9 kHz. The electric field PSD was selected individually at these three frequencies. Because Demeter satellite only provided one component of three directions in the electric field and magnetic field, for example, E12 along the flying direction of the satellite in electric field generally, here we did not transfer them into electromagnetic power flux to avoid more errors in the data processing. The three frequencies transmitted from Russia are within a quite narrow band; the VLF electric field spectrum data onboard Demeter were processed by computing their RMS with two closest frequency PSD around each transmitted frequency. Table 1. The Information of Alpha Transmitters in Russia Name Abbr. Longitude/°E Latitude/°N Power/kW Emitted Frequency/kHz Novosibirsk NOV 84.43 55.75 500 11.9, 12.6, 14.9 Krasnodar KRA 38.1 45.4 500 11.9, 12.6, 14.9 Khabarovsk KHA 136.6 50.07 500 11.9, 12.6, 14.9 As shown in Figure 1, the three transmitters at 11.9 kHz can be clearly seen from their intensity centers and the spatial distribution scale around them. Since the three transmitters are located at middle latitudes of Northern Hemisphere, it can be seen that the intensity centers have a little shift to south direction relative to the transmitter latitudes due to the effects of geomagnetic field inclination on whistler wave propagation. Also in Figures 2 and 3, the three Alpha transmitters illustrate the similar spatial distribution features at 12.6 and 14.9 kHz, but with much higher intensity points larger than −95 dB in electric field and larger covered areas with the increase of emitted frequency, especially at 14.9 kHz. For the conjugate regions, at 11.9 kHz, only the east transmitter KHA can be seen (Figure 1), but at 14.9 kHz (Figure 3), the middle station NOV also presented high intensity. Comparing the three frequencies' features, it reveals that the 14.9 kHz VLF radio waves can bring more energy into the ionosphere than those at 11.9 kHz, including their conjugate areas. Figure 1Open in figure viewerPowerPoint Spatial distribution of electric field at 11.9 kHz of the three Alpha transmitters (yellow stars) at 660 km altitude in December 2008. Figure 2Open in figure viewerPowerPoint Spatial distribution of electric field at 12.6 kHz of the three Alpha transmitters at 660 km altitude in December 2008. Figure 3Open in figure viewerPowerPoint Spatial distribution of electric field at 14.9 kHz of the three Alpha transmitters at 660 km altitude in December 2008. To compare the difference in spatial distribution of the three transmitters, −100 dB was taken as a lower boundary of their covered area, and the detailed information was summarized in Table 2. From Table 2, we can find the big differences among them in their covered spatial scale, in which the east station KHA exhibited the effected longitudinal area at 14.9 kHz at longitude of 100–180°E, followed by the middle station NOV at 70–100°E, and the west station KRA was the smallest at 20–43°E. As for the latitudinal scale, KRA was still the smallest one at 35–50°N at 11.9 and 12.6 kHz, while KHA showed the high electric field strength from 25°N to 60°N at 14.9 kHz. To obtain the detailed spatial features of E-field associated with each of the three transmitters, the electric field strength at 14.9 kHz is presented in Figure 4 along latitude and longitude separately. To avoid the high latitude effects at the north of each station, the latitude scales were limited in 50°N for the west station KRA, 63°N for NOV and 65°N for KHA, and the longitude scales were limited within 20–50°E of KRA, 70–100°E of NOV, and 110–170°E of KHA. From the profiles along the latitude (the first row of Figure 4), the maximal electric field points are located to the south of each station as expected due to the geomagnetic inclination there. It can be seen that the maximal electric field intensity approximately equals to −70 dB at NOV and KHA, but being the smallest to −80 dB at KRA. In addition, clear sharp peaks occurred at NOV and KHA along their latitudinal profiles, while the peak of electric field strength at KRA was not so obvious near its latitude line. In addition, to the north of each station, the electric field was attenuated faster with sharp decreasing slope than that at the south part. Along the longitude (the second row of Figure 4), the electric field was generally symmetrical at the west and east sides of KHA transmitter, but more points with high electric field strength were located at the west of KRA and NOV, exhibiting the asymmetry at east-west direction with quick attenuation at their east sides. Table 2. The Covered Scale of Three Transmitters at Three Frequencies Transmitter 11.9 kHz 12.6 kHz 14.9 kHz Longitude/°E Latitude/°N Longitude/°E Latitude/°N Longitude/°E Latitude/°N KRA 30–42 35–50 20–43 35–50 20–43 32–50 NOV 70–90 45–62 70–90 45–62 55–100 42–62 KHA 120–150 38–58 110–170 35–58 100–180 25–60 Figure 4Open in figure viewerPowerPoint Electric field strength of the three transmitters along the latitude and longitude at 14.9 kHz (red vertical lines point the latitude and longitude of each station). In order to compare the differences in electric field intensity, the sliding median values of 11 observing points along the latitude were calculated with the longitudinal scale of ±5° around each transmitter, and the results were exhibited in Figure 5. It showed different features at three stations, in which the west station KRA observed similar flat maximal electric field around −105 dB to −100 dB at 11.9 and 12.6 kHz, and to −95 dB at 14.9 kHz with fast increase and clear peak; at the middle station NOV, the maximal electric field intensity was about −92 dB at 11.9 and 12.6 kHz, up to −88 dB at 14.9 kHz, with 4 dB increase in electric field strength as KRA between 11.9 kHz and higher frequency of 14.9 kHz; while at east station KRA, the maximal electric field did not change so much, with a little increase of −90 dB to −87 dB from 11.9 to 14.9 kHz. In a sum, the increasing trend of electric field is clear at 14.9 kHz relative to 11.9 kHz at all three stations, while 12.6 kHz was approximate to 11.9 kHz at all three frequency points. Comparing the three stations, the east station KHA exhibited the strongest electric field intensity, second was NOV, and the west station KRA was the lowest at all three frequencies, lower by 10–15 dB relative to KHA. The highest electric field at KHA may demonstrate the slowest attenuation feature at east-west direction at this station, and NOV reached the same level at 14.9 kHz. Figure 5Open in figure viewerPowerPoint Electric field strength of the three transmitters along the latitude at three frequency points: (a) 11.9 kHz, (b) 12.6 kHz, and (c) 14.9 kHz. 3 Simulation by Full Wave Propagation Model The full-wave model has been widely used to simulate the propagation process of VLF waves [Lehtinen and Inan, 2009; Cohen and Inan, 2012; Zhao et al., 2015]. By using the full wave model developed by Lehtinen and Inan [2008], we calculated the electromagnetic fields in a horizontally stratified ionosphere treated as magnetized plasma. This method is inherently stable against the "swamping" problem (the evanescent wave solutions with a large imaginary component of the vertical wave number "swamp" the waves of interest). The transfer matrix method was used to simulate the propagation of VLF waves, by considering the collision effects among electrons, ions, and neutral particles and the leading role of geomagnetic field above the D layer in the ionosphere [Lehtinen and Inan, 2008]. To simulate the spatial distribution feature of the three Alpha transmitters, we took the layer with altitude lower than 80 km as free space and 80–700 km altitude as homogeneous anisotropy stratified half space. The electron density profiles were calculated by the International Reference of Ionosphere model (http://omniweb.gsfc.nasa.gov/vitmo/iri2012_vitmo.html). The electron collision frequency employed the model from Cummer [2000] at 80–300 km height, and the Coulomb collision at 300–700 km was suggested by Helliwell [1965]. The geomagnetic field was computed by the International Geomagnetic Reference (IGRF-12; http://www.ngdc.noaa.gov/geomag-web/?model=igrf) model. The date was selected as 1 December 2008, and local time was 22:00 to be the same as the Demeter satellite in nighttime. The electron profiles at three Alpha stations were presented in Figure 6. It shows that the collisional frequency is about 107 Hz at the lower ionosphere of 60 km altitude in the same collision frequency curve over three transmitters and declines exponentially to 103 Hz at 120 km, while the electron density was the highest at the west station KRA at the lower ionosphere beyond 90 km, KHA in the middle, and NOV was the lowest. Figure 6Open in figure viewerPowerPoint The electron density and collision frequency profiles from the IRI2012 model on 1 December 2008. The final simulation results were as shown in Figure 7 at 660 km altitude, in which three stations with three frequencies were all computed along the north to south direction, by taking their positions as "0" point in the X axis. In the three pictures (Figures 7a–7c), NOV all exhibits the highest energy, and KRA is the lowest one, which illustrates its strong negative correlation with electron density in the lower ionosphere, while at the south direction from the origin points, the electric field component reached their maximal peaks at about −65 dB, being higher by a half order of magnitude than those peaks at their north direction, and these maximal electric fields were a little higher than those observed by the Demeter (Figures 1-4) of 5 dB. This higher predicted electric field at the satellite altitude coincided with the analysis results of Lehtinen and Inan [2009]. Compared the three frequencies, the electric field strength increased from 11.9 kHz to 14.9 kHz over all transmitters to 2–3 dB around the central area of each transmitter, with small differences in electric field due to the quite close transmitted frequency band of 11.9 to 14.9 kHz, and we can see that with the increasing frequency, the attenuation slope became slow, with the third peak at south direction moving horizontally from 500 km at 11.9 kHz to 700 km at 14.9 kHz. It illustrates that high frequency signals will cover larger area than the lower one when they are with the same emitted power, which is consistent with the recording results in this paper at topside ionosphere. Figure 7Open in figure viewerPowerPoint Simulation results of the three transmitters at 660 km altitude from north to south: (a–c) The three frequencies of 11.9, 12.6, and 14.9 kHz, respectively. From the simulation results, the three Alpha stations penetrate into the topside ionosphere in turn of NOV, KHA, and KRA with the energy from the largest to the smallest, which is consistent with the actual observation with KRA being the lowest (Figures 1-5). At three frequencies (Figure 5), KHA and NOV were larger than KRA by a half order of magnitude, but in the model, the amplitude does not differ so significantly. By the covered area, KHA was much more obvious than NOV in the observations, which cannot be explained by the present model. Due to the effects from geomagnetic inclination, the maximal intensity from the three transmitters always occurred at the south direction, whatever in the model or in the actual observations at the satellite. Figure 7 presents that the peak center of KRA moves to the southmost with the smallest geomagnetic inclination among the three transmitters. But the inhomogeneity at the west-east direction cannot be taken into account due to the complexity in computation, so more evidences need to be collected to understand their big differences detected by the satellite. 4 Comparison With Ground Observations Since 2009, three VLF receivers began to set up in China to collect the signals of the three Alpha transmitters in Russia, which includes Tonghai (TH) in Yunnan Province, Ya'an (YA) in Sichuan Province, and Beijing (BJ) inside our institute. Among them, each station will construct nine links with three transmitters at three frequency points independently. All the data have been published at the data sharing platform (http://123.127.64.18:8080/igmisp-admin/portal/vlf) supported by the Asia-Pacific Space Cooperation Organization since 2010. To compare with the satellite observation, the ground receiver data were selected in 2009 with similar low solar activity as 2008. Here the strength in 6 days of nine links at TH (24.1°N, 102.75°E) and Ya'an (30.0°N, 103.0°E) stations were presented in Figure 8, while Beijing station had not been constructed during that time period. It can be found that three transmitters showed significant differences at the same receiver station, especially on the west KRA to TH and YA links. The main features can be summarized from Figure 8 that (1) six links from KHA showed the best consistent curves at three frequency points whatever to TH or YA (Figures 8c and 8f), and the strength at 14.9 kHz was just a little smaller sometimes in local daytime and nighttime than those at 12.6 and 11.9 kHz; (2) the electric field strengths from NOV and KHA were almost the same in the local nighttime at about −70 dB (Figures 8b, 8c, 8e, and 8f), but it was about −75 dB or even much lower from KRA (Figures 8a and 8d), exhibiting significant difference; (3) the diurnal variation amplitudes from NOV were the smallest at about 10 dB both at NOV-TH and NOV-YA (Figures 8b and 8e), being 15 dB from KHA links with the sharp decrease at sunrise (Figures 8d and 8f), while they were 30 dB at KRA-TH (Figure 8a) and 15 dB at KRA-YA (Figure 8d). So from the ground-based observations, the electric field mainly by waveguide propagation exhibited similar strength, or with slow attenuations at lower frequency, which was different with satellite observation with high electric field and covered scale at higher frequency. Between the KHA and NOV, the KHA maintained equal power in local nighttime and higher power in local daytime than NOV with a longer distance relative to TH and YA, which verified the large horizontal covering scale over KHA at satellite altitude (Figures 1-3, Table 2). Among the three transmitters, the west station KRA exhibited the smallest strength, which may be mainly related to the longest distance among the three transmitters to the receivers. Figure 8Open in figure viewerPowerPoint Ground observations at Tonghai (TH) and Ya'an receivers during 5–10 December in 2009: (a–c) West KRA-TH, mid NOV-TH, and east KHA-TH, respectively; (d–f) KRA-YA, NOV-YA, and KHA-YA, respectively. 5 Discussion From the actual observations of Demeter and simulation results, it is illustrated that among the three frequencies, the electric field in the topside ionosphere at the lowest 11.9 kHz is the smallest, and the VLF waves at 14.9 kHz can penetrate into the ionosphere with much more energy. Comparing the three transmitters, the west station KRA showed quite smaller electric field in the ionosphere at all three frequencies, especially at 11.9 kHz and 12.6 kHz, almost an order of magnitude lower relative to the other two stations at the same frequencies. There are two reasons to explain it. The first one is the highest ionospheric density at lower ionosphere over KRA, leading to the lowest energy of KRA, which was demonstrated in our simulation. However, from the simulation results of the full wave model, the differences among the three transmitters are less than 4 dB in electric field due to the variations in electron density and collision frequency, which cannot explain the significant decrease by one order of magnitude at 11.9 and 12.6 kHz over KRA. Cohen et al. [2012] illustrated that the ionospheric VLF waves were robust against the variability of the electron density in local nighttime. Lehtinen and Inan [2009] considered in their model the effects from irregularities on the VLF waves due to the overestimation of VLF waves in the ionosphere and magnetosphere by the model, and Cohen et al. [2012] demonstrated that the irregularity scattering on whistler mode might be no larger than 6 dB. Combined with the observations in this paper, the variations in electron density or irregularity can verify the small differences of the three transmitters at 14.9 kHz, but they are not the main reason to reduce the electric field strength at 11.9 kHz and 12.6 kHz over KRA, because the waves over KRA are from the same transmitter, travelling the same path in three frequency points. The second reason is considered from the effects of the lower hybrid resonance (LHR) frequency in the ionosphere. Vavilov et al. [2013] had calculated the LHR frequency at satellite altitude by using the Demeter observing data from Langmuir Probe and Ion Analyzer for plasma densities. Their results revealed that over the west station KRA, the maximal LHR frequency reached about 13.2 kHz in winter night at 660 km altitude, which would enlarge the refractive index and strongly affect the penetration of VLF waves lower than this frequency into the ionosphere. To demonstrate the influence of fLHR to VLF radio wave propagation, the curves of fLHR have been calculated by using IRI2012 and IGRF12 models to obtain the plasma data and geomagnetic data at different altitudes from 400 km to 1000 km with an interval of 20 km. The results in Figure 9 show that, from west to east, the three stations present different features, in which the peak fLHR is 13.1 kHz at west station KRA around 600 km, while only being 9.2 kHz at NOV and 11.6 kHz at KHA. This illustrates that the 11.9 and 12.6 kHz signals emitted at these three stations will be affected strongly only at KRA, and a little influence at KHA due to its close fLHR to 11.9 kHz at the topside ionosphere. As for NOV, there is almost no effect of fLHR at all emitted three frequency points. It verifies again that the approximate intensity in electric field over the three stations is at 14.9 kHz, but the great difference among the three stations is at 11.9 kHz. Therefore, both the highest ionospheric density and the LHR frequency waves resulted in the lowest electric field at KRA among the three stations. Shao et al. [2012] studied the attenuation of whistler waves through conversion to lower hybrid waves by ray tracing model, and Cohen et al. [2012] used "smooth" full wave models to describe the transionospheric absorption due to plasma irregularities. Their researches demonstrated that short-scale density irregularities in lower ionosphere contributed to the significant attenuation of 2–10 dB in whistler mode waves at VLF frequency band above 5 kHz [Shao et al., 2012; Cohen et al., 2012]. It is suggested that over KRA the whistler waves scatter on plasma irregularities here and become transverse and then reflect at its conjugate region with high LHR frequency than the transmitted one [Kimura, 1966], so the energy will be lost before reaching the satellite. Figure 9Open in figure viewerPowerPoint The variation curves of fLHR with the altitude at 400–1000 km at the three stations in December of 2008: (left to right) KRA, NOV, and KHA; f1 = 11.9 kHz, f2 = 12.6 kHz. At the Southern Hemisphere, there is the most powerful VLF transmitter called NWC in Australia, located at 21°47′S, 114°09′E, operated at 19.8 kHz, with emitted power being 1 MW. NWC has induced a large amount of energetic particle precipitation in the whole world. According to the study of Li et al. [2012], two local electron belts are constructed at L of ~1.5–1.9 and longitude of 105–170°E at the Northern Hemisphere, while L is ~1.6–2.1 and longitude 110–180°E at the Southern Hemisphere. The L shell values of NOV and KHA are 1.88 and 2.54, respectively. So the waves from KHA have the opportunity to interact with the energetic particles at Northern Hemisphere excited by NWC. Most particles induced by NWC move to the east to almost 170°E, which is consistent with the large scale of KHA signals in longitude from 100 to 180°E. Compared with the three transmitters in Figure 5, the electric field strength at the south of KHA in 20–30°N latitude was much larger than NOV, which demonstrated the enhancement of background electromagnetic field from 11.9 to 14.9 kHz frequency band over this region. As for the conjugate area, the geomagnetic field differences between conjugate points were calculated, with about −16340 nT at KRA, −2410 nT at NOV, and +3586 nT at KHA. The small geomagnetic field intensity at the conjugate Southern Hemisphere of KRA means small electron cyclotron frequency here, and the collision frequency profile will play the major role in VLF wave propagation, so it may cause the further attenuation of VLF waves. Taking account of the LHR at the conjugate area, the signals would be reflected magnetospherically from the region where f = fLHR [Vavilov et al., 2013], and they could not be observed at the lower ionosphere with high fLHR, so the almost disappeared signals at the Southern Hemisphere might also contribute to its fLHR over the KRA conjugate area. 6 Conclusions The spatial distribution features over three Alpha transmitters in Russia were analyzed based on the Demeter satellite records and compared with the simulation results and ground observations to understand their similarity and variability. Three frequency VLF waves with same radiated power can be compared at the same time and same place at the first time, and the main points can be concluded as follows: At VLF frequency band, more power can be brought into the ionosphere with the increase of the frequency of electromagnetic waves through whistler mode propagation, which is verified by both satellite records and simulation results between 11.9 kHz and 14.9 kHz. The electric field can reach −80 to −70 dB at 14.9 kHz with the emitted power being 500 kW at satellite, while being higher of −67 to −62 dB by simulation over three transmitters. However, the ground observations do not show obvious differences among different frequency points in the same transmitter-receive link, with −70 dB in local nighttime. The simulation results revealed the effects of background plasma parameters and geomagnetic field on the VLF propagation into the ionosphere. Negative relationship is illustrated between electron density and the intensity of electric field in the topside ionosphere, which well explains the lowest electromagnetic wave intensity over the west transmitter KRA at all three frequency points. The geomagnetic inclination leads the maximal intensity moving to the south of the three transmitters and fast attenuation at the north direction. When the low hybrid resonance frequency is close to the VLF transmitter, it will strongly affect the VLF radio wave propagation in the ionosphere. The maximal LHR frequency exceeds 13 kHz at 600 km altitude at the west station KRA, and it attenuates the radio waves at 11.9 kHz and 12.6 kHz here, with the electric field decreasing by about 10 dB, being one order of magnitude rather than the others. The close LHR wave frequency over KRA and plasma irregularities here may be the main factors to cause the whistler mode conversion and the significant attenuation in electric field at 11.9 and 12.6 kHz. The covered areas differ extensively from west to east, especially along the longitudinal direction, with 20° scale over KRA, but 80° region over the biggest one KHA at the same emitted frequency, which is also verified by the large strength at ground observations from KHA to TH and YA in local daytime. The west-east expanding feature over KHA may be related to the wave-particle interaction from the most powerful transmitter NWC, but it still needs further investigation and the inhomogeneous model should be considered in the future. Acknowledgments This paper is supported by the National Natural Science Foundation of China (41674156, 4141104064) and the International Cooperation Project (2014DFR21480). The authors thank the Demeter center for providing the ICE data (http://demeter.cnrs-orleans.fr/). The data used in this paper can be found by the website link shown in the paper and the acknowledgement. References Bell, T. F., K. Graf, U. S. Inan, D. Piddyachiy, and M. 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