中田 裕之

Abstract Several studies have examined ionospheric variation associated with meteorites, meteoroids, or meteors based on Global Satellite Navigation System total electron content observations. However, there have been few quantitative studies of the D‐region of the ionosphere (60–90 km), which is associated with meteoroids. We investigated variation in the D‐region during the passage of a meteoroid over northeastern Hokkaido, Japan, at 11:55:55 UT on 18 October 2018, using very low‐frequency (VLF, 3–30 kHz) and low‐frequency (LF, 30–300 kHz) signals observed by three transmitters [JJY (40 kHz), JJY (60 kHz), and JJI (22.2 kHz)], at Rikubetsu, Japan. Periodic variation of 100–200 s was observed in the VLF and LF amplitudes upon arrival of the acoustic wave. The vertical seismic velocity of Hi‐net and F‐net data also showed acoustic waves. Although the main period of the acoustic wave was 0.1–0.5 s in the seismic data, a longer period component (100–200 s) remained during propagation up to the D‐region ionosphere. The estimated velocity of the acoustic waves was ∼340 m/s on the ground according to the Hi‐net seismic data. The acoustic wave originated near the endpoint (25 km altitude) of the meteoroid trajectory. Based on the observed propagation time of the acoustic waves and ray tracing results, the acoustic waves propagated obliquely from near the endpoint of the meteoroid trajectory up to a D‐region height (about ∼90 km altitude), south of the Rikubetsu receiver.

Abstract The explosive eruption of the Hunga Tonga-Hunga Ha’apai volcano on 15 January 2022 generated atmospheric waves traveling around the Earth, which caused ionospheric disturbances on various spatio-temporal scales. A HF Doppler sounding system in Japan detected characteristic ionospheric disturbances showing periodic oscillations in the Doppler frequency with a period of ~ 4 min. In this study, such periodic oscillations were examined by comparing Doppler frequency data with Total Electron Content data obtained by Global Navigation Satellite System. The observed periodic oscillations in the Doppler frequency were characterized by a sawtooth or S-letter shaped variation, implying the passage of the traveling ionospheric disturbances through the reflection points of the HF Doppler sounding system. It was also found that the periodic oscillations occurred prior to the arrival of the tropospheric Lamb wave excited by the Tonga eruption. From the total electron content data, the traveling ionospheric disturbances causing the periodic oscillations were excited by the tropospheric Lamb waves at the conjugate point in the southern hemisphere, namely, the electric field perturbations due to the Lamb waves in the southern hemisphere mapped onto the sensing area of the HF Doppler sounding system in the northern hemisphere along the magnetic field lines. The periodic oscillations were observed only in the path between Chofu transmitter and Sarobetsu receiver, whose the radio propagation path is almost aligned in the north–south direction. This suggests that the traveling ionospheric disturbance has a structure elongating in the meridional direction. The variation in the Doppler frequency was reproduced by using a simple model of the propagation of the traveling ionospheric disturbances and the resultant motion of the reflection point. As a result, the vertical motion of the reflection point associated with the periodic oscillations was estimated to be about 1 km. It is known that 4-min period variations are sometimes observed in association with earthquakes, which is due to resonances of acoustic mode waves propagating between the ground and the lower ionosphere. Therefore, a similar resonance structure in the southern hemisphere is a plausible source of the traveling ionospheric disturbances detected in the northern hemisphere. Graphical Abstract

Abstract High-Frequency Doppler (HFD) sounders at low-latitudes often detect characteristic oblique spreading Doppler traces in the spectrogram, known as Oblique Spread Structure (OSS). OSS has been expected to be generated by the dispersion of radio wave reflection due to equatorial plasma bubbles (EPBs). However, it has not yet been confirmed whether OSS is surely a manifestation of EPB by conducting simultaneous observations of EPB and OSS with different observational techniques. Additionally, it remains unclear what kinds of properties of EPB are reflected in the fine structure of OSS. In this study, we investigated three cases of OSSs and EPBs simultaneously observed by a HFD sounding system and an all-sky airglow imager in Taiwan. For the three cases presented here, the timing of OSS occurrence in the HFD data well coincided with that of the EPB appearance in the airglow data. The frequency shift of OSS is quantitatively explained assuming a radio wave reflection at 250–300 km altitudes. These results strongly indicate that OSS is formed by electron density variations at F-region altitudes accompanying EPB; thus, OSS is a manifestation of EPB in the HFD observations. Furthermore, it was suggested that the fine structure of OSS reflected the branching structure of EPB when the multiple branches of EPB reached the intermediate reflection point of the HFD observation. The detection of EPB occurrence and its fine structure using HFD observation enables monitoring of EPB regardless of weather conditions, which will contribute to monitoring the space weather impact of EPBs, for example, on GNSS navigation, in a wide area. Graphical Abstract

Abstract It has long been known that field-aligned irregularities within equatorial plasma bubbles (EPBs) can cause long-range propagation of radio waves in the VHF frequencies such as those used for TV broadcasting through the so-called forward scattering process. However, no attempt has been made to use such anomalous propagations of VHF radio waves for wide-area monitoring of EPBs. In this study, we investigated the feasibility of monitoring of EPBs using VHF radio waves used for aeronautical navigation systems such as VHF Omnidirectional radio Range (VOR). There are 370 VOR stations in the Eastern and Southeastern Asian region that can be potentially used as Tx stations for the observations of anomalous propagation. We have examined the forward scattering conditions of VHF waves using the magnetic field model and confirmed that it is possible to observe the EPB-related anomalous propagation if we set up Rx stations in Okinawa (Japan), Taiwan, and Thailand. During test observations conducted in Okinawa since 2021, no signal has been received that was clearly caused by anomalous propagation due to EPBs. This is simply because EPBs have not developed to high latitudes during the observation period due to the low solar activity. In March 2023, however, possible indications of EPB-related scattering were detected in Okinawa which implies the feasibility of observing EPBs with the current observation system. We plan to conduct pilot observations in Taiwan and Thailand in future to further evaluate the feasibility of this monitoring technique. Graphical Abstract

Summary The massive explosive eruption of the Hunga volcano on 15 January 2022 generated atmospheric waves that were recorded around the globe and affected the ionosphere. The paper focuses on observations of atmospheric waves in the troposphere and ionosphere in Europe, however, a comparison with observations in East Asia, South Africa and South America is also provided. Unlike most recent studies of waves in the ionosphere based on the detection of changes in the total electron content, this study builds on detection of ionospheric motions at specific altitudes using continuous Doppler sounding. In addition, much attention is paid to long-period infrasound (periods longer than ∼50 s), which in Europe is observed simultaneously in the troposphere and ionosphere about an hour after the arrival of the first horizontally propagating pressure pulse (Lamb wave). It is shown that the long-period infrasound propagated approximately along the shorter great circle path, similar to the previously detected pressure pulse in the troposphere. It is suggested that the infrasound propagated in the ionosphere probably due to imperfect refraction in the lower thermosphere. The observation of infrasound in the ionosphere at such large distances from the source (over 16 000 km) is rare and differs from ionospheric infrasound detected at large distances from the epicenters of strong earthquakes, because in the latter case the infrasound is generated locally by seismic waves. An unusually large traveling ionospheric disturbance (TID) observed in Europe and associated with the pressure pulse from the Hunga eruption is also discussed. Doppler sounders in East Asia, South Africa and South America did not record such a significant TID. However, TIDs were observed in East Asia around times when Lamb waves passed the magnetically conjugate points. A probable observation of wave in the mesopause region in Europe approximately 25 min after the arrival of pressure pulse in the troposphere using a 23.4 kHz signal from a transmitter 557 km away and a coincident pulse in electric field data are also discussed.

Abstract 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 $${10}^{-11}$$ 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

<title>Abstract</title>Although solar activity may significantly impact the global environment and socioeconomic systems, the mechanisms for solar eruptions and the subsequent processes have not yet been fully understood. Thus, modern society supported by advanced information systems is at risk from severe space weather disturbances. Project for solar–terrestrial environment prediction (PSTEP) was launched to improve this situation through synergy between basic science research and operational forecast. The PSTEP is a nationwide research collaboration in Japan and was conducted from April 2015 to March 2020, supported by a Grant-in-Aid for Scientific Research on Innovative Areas from the Ministry of Education, Culture, Sports, Science and Technology of Japan. By this project, we sought to answer the fundamental questions concerning the solar–terrestrial environment and aimed to build a next-generation space weather forecast system to prepare for severe space weather disasters. The PSTEP consists of four research groups and proposal-based research units. It has made a significant progress in space weather research and operational forecasts, publishing over 500 refereed journal papers and organizing four international symposiums, various workshops and seminars, and summer school for graduate students at Rikubetsu in 2017. This paper is a summary report of the PSTEP and describes the major research achievements it produced.

© 2019, Springer Nature Singapore Pte Ltd. HF-START is HF Simulator Targeting for All-users’ Regional Telecommunications. This paper addresses the problems of worldwide existing radio propagation model and challenge on developing the HF-START. Its possible extension in the future is discussed. Meteorological information for aircraft in flight is broadcasted from worldwide radio stations in the frequency range of 3–15 MHz. Such frequency range is strongly affected from day-to-day bottom structure variation of the Earth’s ionosphere, where it is influenced by both space weather and upper atmosphere activities. Space weather is thus significant to aeronautical users, who deal with the critical radio application. Space weather data is, however, difficult to be understood. To translate research level data to user level data, radio propagation simulator named HF-START is being developed. Space weather disturbances-triggered failure of communications/navigation is of high priority to forecast.

Using 630 nm airglow data observed by an airglow imager on the International Space Station (ISS), the occurrence of equatorial plasma bubbles (EPBs) is studied. In order to examine the physical mechanisms in the boundary region between the Earth and the outer space, an ionosphere, mesosphere, upper atmosphere, and plasmasphere mapping (IMAP) mission had been conducted onboard the ISS since October 2012. The visible light and infrared spectrum imager (VISI) is utilized in the ISS-IMAP mission for nadir-looking observation of the earth's atmospheric airglow. In this study, we automatically select EPBs according to the criterion for extracting the tilted dark lines from VISI data. Using the selected events, the dependence of the occurrence rate of EPBs is examined. There is no other report of the occurrence rate of EPBs using downward-looking visible airglow data (630 nm). In this result, the occurrence rate is high at all longitudes in the equinoctial seasons. In the solstice seasons, in contrast, the occurrence rate is very small especially in the Pacific and American sectors. This result is basically consistent with previous studies, e.g., those determined by plasma density data on DMSP satellites.During the June solstice in 2013, EPBs were observed in association with geomagnetic storms that occurred due to a southward turning of the IMF Bz. Using these events, we examined the storm-time features of the occurrence of EPBs in the Pacific-American sectors during the June solstice. In these sectors, where the occurrence rate of EPBs is very small during solstice seasons, some EPBs were observed in the peak and recovery phases of the storms. This result shows that the prompt penetration of electric fields causes the development of EPBs, in the data we analyzed, the geomagnetic storms did not inhibit the generation of EPB in the Pacific-American sectors.

© 2016 The Institute of Electrical Engineers of Japan. In order to examine the direction of arrival of VHF radio waves dependent on the natural phenomena, the interferometers have been installed at Katsuyama, Chiba Prefecture and Numata. Gunma Prefecture. In this study, the radio wave it the frequency of 77.1 MHz transmitted from Tokyo Tower (assigned to FM broadcasting of the Open University of Japan) is observed. The interferometers consist of four Yagi antennas and estimate the directions of arrivals of the radio wave. The observational results show that the directions of the wave are affected by the disturbances in the ionosphere and the atmosphere. In this paper, the system of the interferometers is described and the examples of the observation results are presented.

© 2016 The Institute of Electrical Engineers of Japan. We performed the frequency analysis of time-series data of TEC. and examined the variations of TEC associated with earthquakes. Using Fast Fourier Transform processing, the spectral densities of coseismic TEC variations were calculated. The coseismic variations of TEC were detected in 16 events out of 26 events where the earthquakes larger than M6.4 occurred around Japan since 2000. In most events, the variations at the frequencies of 4.17, 5.21 mHz were observed. The spectral densities reached their maximums about 11 minutes after the earthquake. Since this delay corresponds to the propagation time of acoustic wave from the ground to the ionosphere, it is confirmed that the coseismic variations are due to the acoustic wave generated by the ground/sea surface perturbations. The enhancement of the signal density increases with the magnitude of earthquake and the height of tsunami. Using the enhancement of the TEC variation, the magnitude of wave source can be estimated.

In order to evaluate the relationship between the coseismic ionospheric perturbations and the ground perturbations, we compare the coseismic disturbances determined by a numerical simulation with those observed by HF Doppler. HF Doppler observation determines the ionospheric vertical drift speed of the reflection point. The numerical simulation calculates the temporal evolution of the neutral acoustic wave, In this numerical simulation, the ground perturbations observed by seismometers closest to the reflection points are used as sources of the perturbations. The speed estimated from HF Doppler is about an order larger than that calculated from numerical simulation. The difference is because the speed of the ionospheric vertical drift is not same as the neutral atmospheric wave. In order to fix this difference, the ionospheric neutral drift speed is modified considering the effects of the inclination of the magnetic field and the compression of ionospheric plasma. As a result, the both speed are comparable each other.

We report multipoint observations of daytime tweek atmospherics during the solar eclipse of 22 July 2009. Sixteen and sixty-three tweek atmospherics were observed at Moshiri and Kagoshima, Japan, where the magnitudes of the solar eclipse were 0.458 and 0.966, respectively. This was the first observation of tweek atmospherics during a low-magnitude eclipse (0.458). The average and standard deviation of the reflection height were 94.9 13.7km at Moshiri and 87.2 12.9km at Kagoshima. The reflection height at Moshiri was almost the same as that for normal nighttime conditions in July (96.7 12.6km) in spite of the low magnitude of the eclipse. The reflection height at Kagoshima seems be divided into two parts: propagation across the total solar eclipse path and propagation in the partial solar eclipse path. During the eclipse, we also observed the phase variation in the LF transmitter signals. The average change in the phase delay of the LF signals was 109 for the paths that crossed the eclipse path and 27 for the paths that did not cross the eclipse path. Assuming a normal daytime height for LF waves of 65km, a ray tracing analysis indicates that the variations in phase correspond to a height increase of 5-6km for the paths across the eclipse and 1-2km for partial eclipse paths. The wide range of estimated tweek reflection heights at Kagoshima also suggests a difference in electron density in the lower ionosphere between total and partial solar eclipses. © 2012. American Geophysical Union. All Rights Reserved.

We describe a new process of the magnetosphere-ionosphere (MI) coupling for global MHD simulations. In the MI coupling process, field-aligned currents and electric potentials interact with each other in the region between the ionosphere and the inner boundary of the magnetosphere. In order to examine these field-aligned currents and electric potentials selfconsistently, we consider the boundary condition as a problem of wave reflection, assuming that the changing of field-aligned currents as a result of MI-coupling process is associated with the reflection of shear Alfven waves. Separating the perturbed components from the correct solutions, the equation of current continuity of these components is considered. Then we determine the perturbed components generated by the MI coupling. Since the perturbed components of the electric potentials are associated with the reflected shear Alfven wave, we can describe a spatiotemporal development of ionospheric potential as summation of reflection processes of shear Alfven wave at the inner boundary, and then the new process is applicable to the general problems of the MI coupling.

The field-aligned current (FAC) is a key element in the magnetosphere-ionosphere coupling. A wave equation describing the generation of FAC in the magnetosphere has been derived by ITONAGA et al. (2000). This equation presents a deep insight into the coupling of the FAC with the inertial and diamagnetic currents associated with magnetosonic disturbances via the effects of inhomogeneous medium and curvilinear magnetic field line. It is assumed in the derivation of the wave equation that the now velocity disappears in the unperturbed state. Under the condition of μ≪V, where μ is the magnitude of unperturbed now velocity and V the propagation speed of MHD disturbance, this assumption is valid. The generation of quasi-oscillatory current wedge with a time scale of 1 to 2 min is discussed on the basis of the wave equation.