大矢 浩代

Abstract The submarine volcano Hunga Tonga–Hunga Ha’apai erupted explosively on January 15, 2022, offering a unique opportunity to investigate interactions between the atmosphere and ionosphere caused by Lamb and Pekeris waves. However, the resonance of Pekeris waves has not been previously detected. In this study, we applied a multi-point monitoring approach focusing on the lower ionosphere and atmospheric electric field. Here we show observed oscillations of 100–200 s in manmade transmitter signals and the magnetic and atmospheric electric fields, which were caused by Pekeris waves. However, no corresponding changes with the period of 100–200 s in atmospheric pressure due to Pekeris waves were observed on the ground. A simulation of neutral wind revealed Pekeris waves oscillating near the mesopause, suggesting resonance. Therefore, the oscillation in atmospheric electric field is interpreted that the resonance in the lower ionosphere was projected onto the Earth's surface via a global electric circuit.

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.

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.

We report the first observations of periodic oscillations of an atmospheric electric field simultaneously derived by field mills at four observation sites at a distance of 50-65 km in metropolitan Tokyo. Oscillations were detected during a snowfall event on 23-24 November, 2016. The main period of the oscillations of the atmospheric electric field at CHB was 78 min, which was similar to those at other sites. The periods of 39.0, 54.6, and 78.0 min observed at Chiba (CHB) were similar to those observed by W-band cloud radar (FALCON-I) reflectivity below a height of 5 km. High coherence of the 78-min period between the atmospheric electric field at CHB and the X-band phased array weather radar reflectivity suggest that the periodic oscillations of the atmospheric electric field during snowfall were caused by vertically convective cells in snow clouds with a radius of 60 km centered on CHB.

We report the first observations of similar to 100-s periodic oscillations of intensity in low-frequency (LF) standard radio waves over Japan at 05:51-05:56 UT, which was about 4min and 42s after mainshock onset of the 2011 off the Pacific coast of Tohoku earthquake on 11 March 2011 (Mw 9.0). The LF radio wave propagation paths used in this study were JJY (Japan, 60kHz)-Rikubetsu (RKB, Japan), and BPC (China, 68.5 kHz)-RKB, where the minimum distances between the epicenter and the LF propagation paths were 413.6 and 561.5 km, respectively. The observed modulations in intensity and phase were about 0.1 dB and 0.5 degrees, respectively. Based on a numerical simulation of the neutral atmosphere and the wave-hop method, the occurrence time of the similar to 100-s periodic oscillations was in good agreement with the total propagation time of the Rayleigh wave that spread concentrically from the epicenter to the LF propagation paths and acoustic waves propagating vertically from the Earth's surface to the bottom of the ionosphere. Variations in synthesized electric field intensity of ground waves and sky waves, calculated by the wave hop method up to 10 hops, reproduced similar similar to 100-s periodic oscillations that were caused by the difference in arrival time of the acoustic waves excited by the Rayleigh waves at each hop point. The amplitude of the D region electron density variations at 70km altitude during the similar to 100-s periodic oscillation was estimated to be about 1% compared to the background electron density, based on the numerical simulation.

We report the first observation of daytime tweek atmospherics based on measurements at Moshiri (44.37 degrees N, 142.27 degrees E) and Kagoshima (31.48 degrees N, 130.72 degrees E), Japan, during nonsolar eclipse days for 5months in 1980-1994. The daytime tweeks were observed on geomagnetically quiet and stormy days. The daytime tweeks had clear frequency dispersion with an average duration of 12ms, which was shorter than that in the nighttime (50ms). The average occurrences of the daytime tweeks at Moshiri and Kagoshima were 0.6 and 0.1 tweeks per minute during 10:00-15:00 LT, respectively. Daytime tweeks up to the second-order mode were visible. There was no difference in the occurrence of each visible mode between storm time and magnetically quiet time. The daytime reflection heights were similar to those at night (85-100km) but with greater variation. We evaluated the attenuation rate ((n)) of tweeks by strictly taking the ionospheric reflection coefficient into account. For each frequency, (n) was evaluated as a function of the electron density, electron density gradient, and ionospheric height. We found that (n) had an inverse relationship with the electron density (or conductivity), electron density gradient, and ionospheric height. We suggest that the best conditions for daytime tweek observations are when the bottomside of the ionosphere is sharply defined and the ionospheric height is high.

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 +/- 13.7 km at Moshiri and 87.2 +/- 12.9 km at Kagoshima. The reflection height at Moshiri was almost the same as that for normal nighttime conditions in July (96.7 +/- 12.6 km) 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 degrees for the paths that crossed the eclipse path and 27 degrees for the paths that did not cross the eclipse path. Assuming a normal daytime height for LF waves of 65 km, a ray tracing analysis indicates that the variations in phase correspond to a height increase of 5-6 km for the paths across the eclipse and 1-2 km 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.

We investigated, for the first time, long-term (1976-2010) variations in reflection heights of tweek atmospherics based on very low frequency (VLF) observations at Kagoshima, Japan. The results revealed the effects of the solar cycle on the nighttime lower ionosphere at low to middle latitudes. The tweek reflection heights on geomagnetically quiet days were analyzed every month over three solar cycles by using an automated spectral fitting procedure to estimate the cutoff frequency. The average and standard deviation of the reflection height were 95.9 km and +/-3.1 km, respectively. Typical periods of time variation for the reflection height were 13.3, 3.2, 1.3, 1.0, 0.6, and 0.5 years. The variations in tweek reflection heights did not show simple anticorrelation with solar activity. The correlation coefficient between tweek reflection height and sunspot number was 0.03 throughout the three solar cycles. Hilbert-Huang transform analysis successfully indicated the presence of 0.5-1.5 year and similar to 10 year variations as intrinsic mode functions (IMF). The decomposed IMF with the similar to 10 year variation had a positive correlation with sunspot numbers and a negative correlation with galactic cosmic rays (GCRs). We hypothesize that these variations in tweek reflection heights could be caused by coupling of several ionization effects at the D and lower E regions, effects such as geocorona, GCRs, particle precipitation, and variations in neutral density in the lower thermosphere. Among these processes, the geocorona and particle precipitation could show negative correlation, while the GCRs and neutral density could show positive correlation with solar activities.

This paper presents an automated procedure to estimate apparent reflection height h (from the cutoff frequency for the first waveguide mode, f(c)), horizontal propagation distance d, and propagation time T-g of tweek atmospherics. Tweek data recorded at the Kagoshima Observatory (31.48 degrees N, 130.72 degrees E), Japan, were used to evaluate the procedure by comparing the results estimated by the automatic method to those read manually by an operator. The two types of results showed differences (automatic-manual) of +0.58 km, -9.9 Hz, and +3058.9 km for mean h, fc, and d, respectively. The difference in h(f(c)) was less than the resolution of the fast Fourier transform used to obtain the tweek spectra. These comparisons indicate that the automatic estimation procedure of tweek parameters developed in this paper performs well and is a useful toot for studying long-term height variations of the ionospheric D and lower E regions using very low frequency (VLF) and extremely low frequency (ELF) records observed in Japan over the past 30 years.

Tweek atmospherics are pulse signals from ELF/VLF electromagnetic waves caused by lightning discharges; these signals have characteristic cut-off frequencies due to long-distance propagation by the Earth-ionosphere waveguide mode. We developed a method of estimating tweek reflection heights (equivalent electron densities) in the low-middle latitude D-region ionosphere by accurate measurement of their first-order mode cut-off frequency (∼1.8 kHz). This research is the first to show reflection-height measurements of the D-region ionosphere's response to the great geomagnetic storm in October 2000 using tweek atmospherics observations. We found transient lowering/ rising in the small bay Dst-variation and rising near the maximum depression of the major bay Dst-variation. Comparing this response in tweek reflection heights with the intensities of 40 kHz radio-wave signals, electron density profiles from the IRI-2001 model and measured by MF radars, and the ionospheric F-region parameter (h′F) from ionosonde data, we determined two cases where the lowering/rising of the D-region was coupled with vertical motion of the F-region and one where it was not. Based on simultaneous total electron content (TEC) perturbation data, we suggest the following possible mechanisms for these two results: E × B vertical plasma drift due to ionospheric electric fields, and the propagation of large-scale traveling ionospheric disturbances (LSTIDs) from the auroral to low-latitude ionosphere, respectively. © 2005 Elsevier Ltd. All rights reserved.

Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide mode over long distances. In this paper, we estimate equivalent nighttime electron densities at reflection heights in D-region ionosphere at low-middle latitudes by accurately reading the first-order mode cut-off frequency of tweek atmospherics. The estimation method was applied to tweek atmospherics received simultaneously at Moshiri and Kagoshima in Japan. Equivalent electron densities ranged from 20-28 el./cm(3) at ionospheric reflection heights of 80-85 km. Comparing our estimates with electron density profiles obtained from the IRI-95 model, MF radar measurements, and rocket experiments revealed almost consistent results for the lower part of the D-region ionosphere. The tweek method has the unique advantage of enabling reflection-height (equivalent electron densities) monitoring over a wide area of several thousand kilometers.