Search

The sporadic E (Es) layer is an ionospheric layer with a high electron density and thin altitudinal thickness and appears at altitudes around 100 km. The density is sometimes so high that even VHF radio waves from the ground can be reflected and propagate beyond the radio horizon. It has been shown that aeronautical VHF navigation signals in the 108–118 MHz band can propagate beyond the radio horizon to cause potential interference. However, there has been little technique to monitor the Es layer in wide area. The fine structures of the Es layers which reflects the dynamics of the thermosphere has not been well understood yet. This study shows that the ROTI (rate of TEC index) map by utilizing dense GNSS observations is effective in detecting the Es layer at least in the daytime. The Es layer studies has a well-defined frontal structure, when ROTI values are mapped at the typical Es layer height (100 km). A methodology of detecting and characterizing the Es layer frontal structure without manual operation is developed by utilizing the Hough transformation, and the front direction and velocity are successfully derived. Utilizing the characteristics of the frontal structure, fine structures of the Es layer can be studied, and a couple of interesting features are found. However, this technique would not always be applicable. Application to a homogeneous Es layer and nighttime Es layer are challenging problems. Nevertheless, it is still very useful, and is suitable for automated real-time Es layer monitoring system in a wide area.

We introduce a network observation of anomalous long-distance propagation of aeronautical navigation (NAV) very high-frequency (VHF) radio waves due to sporadic E layer (Es). The system has been operative since May 2019 at 6 stations in Japan. The receiver consists of a log-periodic antenna, air-band filter, software-defined radio and small computer which is capable of recording the strength of radio signals in the frequency band from 98 to 118 MHz with a temporal resolution of 2 s. The receiver covers not only the NAV band including VHF omnidirectional radio range (VOR), instrument landing system localizer (ILS LOC) and ground-based augmentation system VHF data broadcast (GBAS VDB) from 108 to 118 MHz, but also broadcasting channels from 98 to 108 MHz. Soon after the start of the full operation of the network observation, a strong Es event was detected by an ionosonde in Tokyo during the daytime on July 4, 2019, in which foEs (critical frequency of Es) was sometimes higher than 15 MHz. The VHF radio wave monitoring system detected multiple signatures of Es anomalous propagation (EsAP) at all the stations extending from Okinawa to Hokkaido. At some stations, the EsAP signatures continued for a few hours with some brief intervals of disappearance. The observed correspondence between the enhancement of foEs and the occurrence of anomalous propagation confirmed that an extreme electron density enhancement within Es caused the anomalous long-distance propagation of VHF NAV signals. The data from this network observation can be browsed at http://gwave.cei.uec.ac.jp/cgi-bin/vor/vhf.cgi in near real-time basis. This near real-time monitoring capability allows people in the aeronautical operation community such as air navigation service providers, pilots, and airline engineers to check the propagation environment of VHF NAV signals online, which contributes to a mitigation of ionospheric space weather impacts on the aeronautical navigation systems. Not only that the current method for detecting Es in a wide area can be used to visualize the spatial distribution of Es in two-dimensional fashion through a combination of other observation techniques such as ionosondes and total electron content (TEC) measurements using Global Navigation Satellite System (GNSS) signals.

High-frequency Doppler (HFD) sounding is one of the major remote sensing techniques used for monitoring the ionosphere. Conventional systems for HFDs mainly utilize analog circuits. However, existing analog systems have become difficult to maintain as the number of people capable of working with analog circuits has declined. To solve this problem, we developed an alternate HFD receiver system based on digital signal processing. The software-defined radio (SDR) technique enables the receiver to be set up without the knowledge of analog circuit devices. This approach also downsizes the system and reduces costs. A highly stabilized radio system for both the transmitter and receiver is necessary for stable long-term observations of various phenomena in the ionosphere. The global positioning system disciplined oscillator with an accuracy of $${10}^{-11}$$ 10 - 11 compensates for the frequency stability required by the new receiving system. In the new system, four frequencies are received and signal-processed simultaneously. The dynamic range of the new system is wider (> 130 dB) than that of the conventional system used in HFD observations conducted by the University of Electro-Communications in Japan. The signal-to-noise ratio significantly improved by 20 dB. The new digital system enables radio waves to be received with much smaller amplitudes at four different frequencies. The new digital receivers have been installed at some of the stations in the HFD observation network in Japan and have already captured various ionospheric phenomena, including medium-scale traveling ionospheric disturbances and sudden commencement induced electric field fluctuations, which indicates the feasibility of SDR for actual ionospheric observations. The new digital receiver is simple, inexpensive, and small in size, which makes it easy to deploy new receiving stations in Japan and elsewhere. These advantages of the new system will help drive the construction of a wide HFD observation network. Graphical Abstract

Prompt penetration electric fields were detected during the intense geomagnetic storm on 22 June, 2015 by the HF Doppler sounders at middle and low latitudes; Prague (Czech Republic), Iitate, Oarai, Onna (Japan), and Zhongli (Republic of China). The storm was initiated by the storm sudden commencement (SC) at 1833 UT, immediately followed by the main phase with the minimum SYM-H of − 139 nT at 2017 UT. The convection electric field was found to be up to 5.1 mV/m at low latitudes, which is enough to cause ionospheric disturbances such as the enhancement of the total electron content, equatorial ionization anomaly and so on. The electric fields of the SC and main phase are westward in the night (0330 MLT), while eastward in the evening (1930 MLT). The main phase electric field intensified the eastward equatorial electrojets (EEJ) at Huancayo in South America on the dayside (1330 MLT) and the westward EEJ on the nightside at Guam in the western Pacific (0330 MLT) and Tirunelveli in India (00 MLT). The eastward direction of the evening-time electric field agrees with a feature of the evening anomaly of the penetration electric field, while the intensity is suppressed because of the sunlit condition in the evening in the summer solstice. The sudden northward turning of the interplanetary magnetic field (IMF) caused a transition from the main phase to the recovery phase, when the EEJ turned into the counterelectrojet (CEJ). It was found that the HF Doppler sounders detected an impulsive eastward overshielding electric field at 2005 UT on the nightside during the northward IMF-driven CEJ with the duration of 17 min and intensity of 8.0 mV/m at Iitate (0500 MLT) and 11.8 mV/m at Prague (2130 MLT). The impulsive overshielding electric field was accompanied by the anti-sunward plasma flows at middle latitude (50°–60° GML) equatorward of the dusk sunward convection as observed with SuperDARN, which signifies intensification of the Region-2 field-aligned currents at the flow reversal. Substorm signatures are identified with intensified westward auroral electrojet and magnetic dipolarization as detected by the geosynchronous orbit satellite, ETS-VIII. Consequently, the stormtime substorm may provide strong overshielding electric fields to the middle latitude ionosphere on the nightside.

To understand the dynamical properties of sporadic E layer (Es), we have developed a method to visualize the two-dimensional spatial structure of Es by monitoring the occurrence of anomalous propagation of VHF waves used for aeronautical navigation systems in combination with the electron density disturbance index ROTI, that is the Rate of total electron content (TEC) Index obtained from ground-based GPS receivers. We introduce a case of strong Es layer which occurred for ~4 h during daytime of July 4, 2019. In this interval, we succeeded in imaging the structure of Es elongating more in the east–west direction and moving northward gradually. This result indicates that the combination of network observations of aeronautical navigation signals and ROTI is capable of imaging the 2D structure of Es in a wide area including the sea surface, which enables us to discuss the dynamical characteristics and generation mechanism of Es.

Many studies have reported that ionospheric disturbances occur after big earthquakes. One of the causes of these coseismic ionospheric disturbances (CIDs) is the infrasound wave excited by Rayleigh wave propagated on the ground from the epicenter. The infrasound wave propagates upward and produces CIDs. In this study, using HF Doppler sounding system (HFD), CIDs at the different altitudes associated with the foreshock of Tohoku Earthquake were examined.

The convection electric field increases during the growth phase of substorms, driving the DP 2 ionospheric currents at high-to-equatorial latitudes, intensifying the eastward equatorial electrojet (EEJ) on the dayside. During the expansion phase, the electric field is often reversed; i.e., overshielding occurs at subauroral-to-equatorial latitudes where the EEJ turns to the westward counterelectrojet (CEJ). In this paper, we show that the HF Doppler sounders detected the eastward overshielding electric field at low latitudes on the nightside simultaneously with the CEJ on the dayside. We also show that the overshielding often occurs during the substorm recovery due to the convection reduction, resulting in a two-step form in both the dayside CEJ and nightside electric field. The opposite direction of the electric field on the dayside and nightside is consistent with the dusk-to-dawn potential electric field associated with the region 2 field-aligned currents intensified by the substorm. The overshielding electric field was found to drive an eastward electrojet with appreciable magnitude in the nighttime equatorial ionosphere, which in turn causes an equatorial enhancement of the midnight positive bay.

For the purpose of studying the radiation effects on electronic devices on the ground, observations of total-time radiation counts of natural and secondary cosmic rays up to an energy of 12 MeV were carried out at two locations with different altitudes: Chofu (53 m) and Sugadaira (1313 m) in Japan, using a multichannel analyzer (MCA) with a NaI detector. Sugadaira recorded more counts by 15% for the natural rays and by 20% for the cosmic rays than Chofu. This small increasing rate in cosmic rays compared with a 3-times increase in the soft error rate (SER) that is predicted at the altitude at Sugadaira suggested that the SER might originate from the higher energy neutrons above 12 MeV, which account for only 3% (0.3 countsh-1cm-2) of the effect below 12 MeV in the neutrons reaching the ground. Moreover, the spatial angle dependences of radiation were investigated by changing the angle between the NaI detector tube and the ground plane, and it was found that in the center of the direction parallel to the ground plane, the counts indicated a maximum immediately under the ground plane for the natural rays, and a maximum at the zenith for the cosmic rays.

Since the middle of 1996, we have made a routine observation of TEC using GPS in Chofu (35.65°N, 139.54°E), Tokyo. We examine in this paper the relationship between the perturbation components of TEC and geomagnetic field variations during 11 storm events for 1997–1998 to clarify the mid-latitude characteristics of TEC variations at the time of magnetic storms. The perturbation components of TEC were derived every 20 seconds by subtracting quiet-time TEC values which are estimated as the average of both 3 days just before and after the storm period. The magnetic field data from Memambetsu Magnetic Observatory (45.92°N, 144.20°E) are used for the identification of the storm-time variations. Our results show that (1) the amplitude level of the TEC variation tends to increase during the first 24 hr of storm and then decrease below its usual-day level with recovery in one or two days later for the typical magnetic storm, and (2) during a negative-value phase of the TEC variation, which follows the initial positive hump structure, the perturbation amplitude of TEC shows a remarkable reduction in summer compared to in winter.

A new-type source of power line harmonic radiation (PLHR) is described in this paper to point out its significance to the magnetospheric emissions. Characteristic electromagnetic fields were observed over the Kola Peninsula, northern Russia, during the flight of the balloon B15-2N in 1982. The source is characterized by (a) the fundamental frequency of 24.4 Hz, (b) the harmonic spectrum continuously extending up to VLF, and (c) the isolation from commercial power lines. These characteristics are different from the previously reported PLHR sources that could trigger the magnetospheric emissions. Based on the low impedance field and the amplitude variation along the balloon trajectory, the source can be modeled by a horizontal electric dipole or a horizontal magnetic dipole, and then the source distance is estimated within the induction area of a few hundred kilometers. The most probable source is an electric railroad or a single-phase power line, driven by an isolated power generator, and located on the Kola Peninsula. Estimating the radiation power through the ionosphere, it is shown that enough power to trigger magnetospheric emissions may be radiated into the magnetosphere from the source. Therefore, it is suggested that the source can be recognized as a new-type PLHR source possibly related to magnetospheric emissions.

Since 1980, we have detected impulsive noise bursts of seismogenic emissions at 82 kHz, 1525 Hz and 36 Hz with a multipoint detection network around the Tokyo region and the Izu peninsula. In earlier times, this system has recorded EM signals prior to several earthquake events. And it has also recorded volcanic eruptions on 15 and 21 November 1986 at Mt. Mihara on Ohshima Island, on 12 July 1989 in Itoh Bay in the Izu peninsula region, and since June 1991 at Mt. Unzen on the Shimabara peninsula. To obtain a clear explanation of the source mechanism of these emissions, we are progressing with statistical analyses of these emission characteristics using the data of 29 events over the last 5 years.

Statistically improved results of power line radiation (PLR) over Eastern Asia observed at 50 and 60 Hz are described in this paper. A total number of 150 orbits, which had been observed from June 1984 to January 1986, by the Japanese scientific satellite OHZORA, are used to detect PLR over Eastern Asia around the Japanese Islands. Depending on the increase in the number of data points, the statistical characteristics of the background noise can be precisely determined by using the improved technique compared with the initial analysis. Statistically reasonable data points are detected as PLR based on the +3σ criterion, where σ is the standard deviation of the background noise. Therefore, the statistical reliability for rejecting the background noise is 99.85%. Then, these detected data are applied to the cause-and-effect test. When the statistically detected data points are placed on the map of Eastern Asia, the points cover Eastern Japan and the east coast of China for 50 Hz, and they cover Western Japan for 60 Hz. The maps of the detection ratios and those of the average field strengths indicate the positive correlation with the ground maps of the electric power generation at 50 and 60 Hz. The positive correlation is more clearly seen at 50 Hz since the background noise is somewhat weaker than that at 60 Hz. This close relationship between the satellite observation and the electric power generation suggests that the detection of PLR is not caused by chance, and that PLR penetrates into the ionosphere and propagates approximately just upward. The decrease of field strength with altitude can be interpreted as the gradual decrease of the refractive index from to 700 km. Therefore, the detection ratio and the average field strength with respect to the satellite altitude suggest PLR propagating from the bottom of the ionosphere. According to these observational results, it is concluded that PLR in Eastern Asia is high above the high electric power generating regions over Japan and China, and that the satellite observation is capable of estimating PLR field below the ionosphere. These results are the first direct indication that the PLR field is enhanced over the high electric power generation region, and is penetrating into the ionosphere.

The direction finding of precursory radio emissions associated with earthquakes is proposed, based on the simultaneous measurement of the waveforms of multiple electric and magnetic field components at ELF (1.525 kHz ± 5 Hz) and ULF (0–10 Hz) at a few stations. This measurement enables us to determine the wave mode of the observed signal (for example, its direction of arrival) by measuring such parameters as the wave impedance, wave polarization and wave coherence. From these observations we hope to be able to obtain the first convincing evidence on the precursory emissions of earthquakes.

At the time of the major volcanic eruptions on November 15 and 21, 1986 at Mt. Mihara, Izu-Ohshima Island, Tokyo territory, Japan, a detector from a multipoint direction-finding network to detect electromagnetic emissions was operating. Since October 20, 1986 impulsive noises at 82kHz were recorded, and these anomalous burst-like noise observations increased after November 3. At 10-16 JST (Japanese Standard Time, UT+9 hours) on November 14, several strong noise bursts were observed, and the first major eruption occurred at the main summit crater of this volcano at 16 JST on November 15. At 10-12 JST on November 21, noise bursts were observed and four hours later, 19 new craters erupted suddenly on the side slope of the mountain, both in and outside the caldera, simultaneously with the intrusion of dykes in the mountain body. These records are the first observations of electromagnetic emissions as precursors to volcanic eruptions ever recorded. The authors present this paper in the form of a short note for data presentation, because analysis of these data and comparison with other observations is still continuing.

[1] We present evidence for ionospheric effects caused by the gamma ray burst that originated at a cosmological distance. At the time of the strong cosmic gamma-ray burst of 29 March 2003 (GRB030329) that took place in the nighttime in Japan we observed a transient decrease in the strength of the radio noise coming from extraterrestrial sources (cosmic noise) at 38 MHz. We also observed a sudden field-amplitude decrease of an 8.006 MHz transmission signal recorded at a distance of 690 km from the transmitter. These phenomena are interpreted as a result of an ionospheric absorption enhancement due to transient ionization caused by GRB030329. We also report no appreciable effect on the ionospheric electron column content derived using GPS (Global Positioning System) microwave signals.

To investigate whether blood pressure differs by taking preferred alcoholic beverage among habitual drinkers, systolic and diastolic blood pressure (SBP, DBP) were compared among groups with different beverage types in 563 middle-aged Japanese males using data from a cross-sectional health survey conducted from February, 1989 through March, 1991 in five areas of Japan. Mean values of SBP and DBP, adjusted for residence, age and body mass index (BMI), were significantly greater in 'exclusively sake' drinkers (adjusted SBP: 127.2 mmHg, adjusted DBP: 83.0 mmHg) and in 'exclusively shochu' drinkers (adjusted SBP: 127.5 mmHg, adjusted DBP: 84.2 mmHg) than in non-drinkers (adjusted SBP: 120.9 mmHg, adjusted DBP: 77.3 mmHg). Adjusted SBP and DBP of 'exclusively beer' drinkers (adjusted SBP: 121.9 mmHg, adjusted DBP: 79.1 mmHg) were significantly (for SBP: p = 0.016, for DBP: p = 0.008) lower than those of 'exclusively sake' drinkers. Similar patterns of blood pressure differences between five beverage types of habitual drinkers were found especially in the group with less than 150 g of weekly ethyl-alcohol consumption. Even after adding ethyl-alcohol consumption as a covariate among 479 habitual drinkers, the significant differences in adjusted SBP and DBP between 'exclusively beer' drinkers and 'exclusively sake' drinkers (for SBP: p = 0.032, for DBP: p = 0.044) were noted. These results may suggest that the effects of drinking on blood pressure differ by beverage type in middle-aged Japanese males.

Observations of the electromagnetic field strength of power line radiation over northern Europe were conducted under the geomagnetically active conditions using the balloons B_-1N on March 20,1982 and B_-2N on November 23 and 24,1982. Substorms occurred during the both flights. A substorm effect on the strength of the electromagnetic field of the power line radiation was investigated at the harmonic frequencies of commercial power line frequency, 50Hz in the frequency range from 40Hz to 1kHz. The strength of the power line radiation at the harmonics of 50Hz in the frequency range from 200 to 900 Hz enhanced during the substorms. A great increase of strength was observed at 300,450 and 600Hz on the balloon B_-1N, and at 300,450Hz on B_-2N. However, the strength at 50,100,and 150 Hz did not enhance more than 3dB during the substorms. It is concluded that the strength of the power line radiation enhanced at higher harmonics of 50Hz during a geomagnetic substorm.

利用は著作権の範囲内に限られる

At the time of the major volcanic eruptions on November 15 and 21, 1986, at Mt. Mihara, Izu-Ohshima Island, in the Tokyo territory of Japan, one detector of a multi-point (multi-static) direction finding network for detection of electromagnetic emissions was operating. Since October 20, impulsive noise bursts at 82 kHz were recorded, and the occurrence of these anomalous burst-like noise observations increased after November 3. At 10–16 JST (Japanese Standard Time, UT + 9 hr) on November 14, several strong noise bursts were observed, and the first major eruption occurred at the main summit crater of this volcano at 16 JST on November 15. At 10–12 JST on November 21, noise bursts were observed, and 4 hr later, 19 new craters erupted suddenly on the side of the mountain, both inside and outside the caldera. These data are the first observations of electromagnetic emissions as precursors to volcanic eruptions correlated in time with the intrusion of dykes in the mountain body.

Measurements of power line radiation were carried out in northern Europe by means of balloon B_-1N. The balloon was launched during the period of magnetic substorm on March 20,1982. Relationship between the power line radiation and a magnetic substorm was investigated by the use of magnetic field intensity spectra up to 1kHz. It revealed that the power line radiation at the fundamental frequency of 50Hz is not enhanced, but, the harmonic frequencies at 300Hz (6th), 450Hz (9th) and 600Hz (12th) are enhanced in association with the magnetic substorm.

The authors are building a multi-point network system for the prediction of earthquake epicenter location by means of the seismogenic emission phenomena around Tokyo area. The network system consists of eight observation points with 50 km from each other and at each point a new digital type direction finding detector is set up with two loop sensors tuned to 82kHz. The output signals of the receivers are added into the digital vector composition circuit to obtain the direction angle of the source point,and this signal is transmitted to the center computer through a telemeter line. In order to protect from local man-made noise interferences,the warning is only anounced when the computation results of the center computer pointed one small area with high cross-corelation values for area from between all points.The source mechanism of emission related to precursors of earthquakes can be explained as electtromagnetic emissions for the rocks around the focus when they are crushed completely by the distortion pressure.And these emission energies are propagated in the ground along the boun-dary surface of the fault as a surface wave mode and radiated from the slit antenna which consists of the boundary at the ground surface of the fault.The authors will present the computer flow and try to explain the source mechanism of these emission in this paper., 利用は著作権の範囲内に限られる