The paper presents the results of a joint analysis of ground-based and satellite observations of ... more The paper presents the results of a joint analysis of ground-based and satellite observations of very low-frequency and low-frequency (VLF/LF) signals during periods of strong seismic activity in the region of Kuril Islands and Japan in 2004–2010. Ground and satellite data was processed using a method based on the difference between the real signal in nighttime and that of a model. The results of the analysis show a good correlation between ground-based and satellite data for several cases of strong (M P 6.8) earthquakes.
There are several more or less promising preseismic events but none of them could be separately u... more There are several more or less promising preseismic events but none of them could be separately used for the short-term EQ forecast in terms of simultaneous prediction of time, position and magnitude. For example the use of seismic or seismo-acoustic clustering alone suffers from alternative between real foreshock and swarm activity, preseismic ULF emission from troposphere is not sufficient for
The over‐horizon (out‐of‐sight) VHF transmitter signals are found to be received before an earthq... more The over‐horizon (out‐of‐sight) VHF transmitter signals are found to be received before an earthquake, and also natural VHF noises are known to be detected prior to an earthquake. In order to have a better understanding of these over‐horizon VHF signals and noises, an interferometric direction‐finding system has been, for the first time, introduced, and some preliminary observational results have been presented here. With FM Sendai as a target VHF transmitter (at 77.1 MHz), we have carried out the observation at Chofu (Tokyo). We can receive occasionally the over‐horizon VHF transmission signals in possible association with earthquakes. We have analyzed a few earthquakes since the commencement of our observation, including a large one named the Niigata Chuetsu‐oki earthquake. The azimuth distribution by means of the interferometer has enabled us to correlate any burst in the temporal evolution of electric field intensity to a certain earthquake. The over‐horizon VHF transmitter sign...
Journal of Geophysical Research: Space Physics, 1990
The propagation mechanism of low‐latitude and equatorial latitude whistlers is investigated on th... more The propagation mechanism of low‐latitude and equatorial latitude whistlers is investigated on the basis of spaced direction finding measurements in South China. Observations were made continuously in the local time (LT) interval from 0000h to 0400h during the period of January 5–11 1988 at the three stations (Zhanjiang (geomagnetic latitude 10.1°), Guilin (14.1°), and Wuchang (19.4°)) and two horizontal magnetic components and one vertical electric field component were simultaneously recorded over a wide frequency range to enable comprehensive direction finding. Two major occurrence peaks on January 5 and January 6 and two minor ones on January 9 and January 11 have been analyzed and the following experimental results have emerged. (1) The whistler occurrence at very low latitudes is generally very small compared with that at low latitude (geomagnetic latitude ≳20°), but once it occurs, the occurrence rate becomes comparable to that at low latitudes. (2) The whistler dispersion is ...
This paper reports on some propagation effects occurring while the ionosphere consists of several... more This paper reports on some propagation effects occurring while the ionosphere consists of several layers. A few simple model problems are considered to show in which way this multi‐layer structure can yield a new set of ELF resonance phenomena. The resonances appear in the space between the Earth and layer interfaces or inside the layers and are liable to give rise to peaks or minima in the frequency dependencies of ELF electromagnetic fields both for the cosmic and terrestrial sources. Calculations have been made for the real ionospheric models mid distant Earth's sources have shown that such a mechanism can explain some experimental data including the effect of “Evening Ionosphere Radiation” by Schecotov and Molchanov (1985).
It is known that electromagnetic emissions take place in association with earthquakes in a wide f... more It is known that electromagnetic emissions take place in association with earthquakes in a wide frequency range from DC/ULF to VHF. But the most promising frequency range from the standpoint of short-term earthquake prediction is ULF (ultra-low-frequency, f<10Hz), because those ULF emissions are definite to have come from the focal zone. While, the generation mechanisms of higher frequency seismogenic emissions are extremely poorly understood. We show our latest results on seismogenis ULF emissions. The first one is the principal component analysis (PCA) for the ULF emissions associated with the Izu islands earthquake swarm in July 2000. Then, the second will be the fractal (mono-and multi-) analysis for the Guam earthquake. These sophisticated signal processing would be of great importance in elucidating precursory weak seismogenic ULF emissions.
Physics and Chemistry of the Earth, Parts A/B/C, 2004
ABSTRACT We analyze the ULF magnetic field emissions observed at three different field sites. Lar... more ABSTRACT We analyze the ULF magnetic field emissions observed at three different field sites. Large seismic events occurred near each station. Temporal dependence of the polarization ratio (ratio of horizontal to vertical magnetic field components) has been calculated in the frequency range between 1.17 mHz and 0.5 Hz with longer than 2-year data for each event. Two of three seismic events were associated with anomalous change in the polarization ratio. Remarkable similarities were found in the ratio between these two different seismic events: (1) The ratio has a significant increase about 1 month before the earthquake (about two times larger than usual level), (2) the increase is seen in the nighttime period (22–02 LT). The frequency range of increase in the ratio depends on the earthquake (0.01–0.022 Hz for the Izu Island earthquake swarms and 0.05–0.1 Hz for the Guam earthquake). Calculated ratio based on the simple theoretical model has a good agreement with observed values for these two earthquakes.
Journal of Geophysical Research: Space Physics, 2004
We have succeeded in observing sprites for winter lightning in the Hokuriku area (Japan Sea side)... more We have succeeded in observing sprites for winter lightning in the Hokuriku area (Japan Sea side) of Japan in the winter of 2001/2002. The optical results on 3 days are compared with the corresponding characteristics of parent (causative) lightning with particular attention to the significant differences between Hokuriku winter lightning and the more widely studied continental lightning. Despite significant differences with Hokuriku winter lightning, we have found nearly the same sprite properties as already observed in the U.S. continent with a significant difference (simpler shape for Hokuriku winter sprite). Then, we have also discussed the criteria for sprite occurrence. Specifically, two similar criteria are found: (1) cloud‐to‐ground discharges of positive polarity and (2) the presence of a certain threshold in vertical charge moment (200–300 C km) (roughly consistent with that for the U.S. continent). Mesoscale convective systems are not necessary to store the charge necessar...
ABSTRACT The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiv... more ABSTRACT The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiving station at Moshiri, Hokkaido, are used to detect the response of the lower ionosphere to the tsunami triggered by the 2011 Tohoku earthquake. Disturbances in the phase and amplitude of VLF signals propagating from the transmitter in Hawaiian Islands are observed during the tsunami wave passage, and these effects in the ionosphere are compared to the in situ sea-level global positioning system (GPS) measurements near Japan. The frequency of the maximum spectral amplitude both for the VLF and GPS data is found to be in the range of periods of 8-50 min, which is likely to correspond to the period of the internal gravity waves generated by the tsunami.
The paper presents the results of a joint analysis of ground-based and satellite observations of ... more The paper presents the results of a joint analysis of ground-based and satellite observations of very low-frequency and low-frequency (VLF/LF) signals during periods of strong seismic activity in the region of Kuril Islands and Japan in 2004–2010. Ground and satellite data was processed using a method based on the difference between the real signal in nighttime and that of a model. The results of the analysis show a good correlation between ground-based and satellite data for several cases of strong (M P 6.8) earthquakes.
There are several more or less promising preseismic events but none of them could be separately u... more There are several more or less promising preseismic events but none of them could be separately used for the short-term EQ forecast in terms of simultaneous prediction of time, position and magnitude. For example the use of seismic or seismo-acoustic clustering alone suffers from alternative between real foreshock and swarm activity, preseismic ULF emission from troposphere is not sufficient for
The over‐horizon (out‐of‐sight) VHF transmitter signals are found to be received before an earthq... more The over‐horizon (out‐of‐sight) VHF transmitter signals are found to be received before an earthquake, and also natural VHF noises are known to be detected prior to an earthquake. In order to have a better understanding of these over‐horizon VHF signals and noises, an interferometric direction‐finding system has been, for the first time, introduced, and some preliminary observational results have been presented here. With FM Sendai as a target VHF transmitter (at 77.1 MHz), we have carried out the observation at Chofu (Tokyo). We can receive occasionally the over‐horizon VHF transmission signals in possible association with earthquakes. We have analyzed a few earthquakes since the commencement of our observation, including a large one named the Niigata Chuetsu‐oki earthquake. The azimuth distribution by means of the interferometer has enabled us to correlate any burst in the temporal evolution of electric field intensity to a certain earthquake. The over‐horizon VHF transmitter sign...
Journal of Geophysical Research: Space Physics, 1990
The propagation mechanism of low‐latitude and equatorial latitude whistlers is investigated on th... more The propagation mechanism of low‐latitude and equatorial latitude whistlers is investigated on the basis of spaced direction finding measurements in South China. Observations were made continuously in the local time (LT) interval from 0000h to 0400h during the period of January 5–11 1988 at the three stations (Zhanjiang (geomagnetic latitude 10.1°), Guilin (14.1°), and Wuchang (19.4°)) and two horizontal magnetic components and one vertical electric field component were simultaneously recorded over a wide frequency range to enable comprehensive direction finding. Two major occurrence peaks on January 5 and January 6 and two minor ones on January 9 and January 11 have been analyzed and the following experimental results have emerged. (1) The whistler occurrence at very low latitudes is generally very small compared with that at low latitude (geomagnetic latitude ≳20°), but once it occurs, the occurrence rate becomes comparable to that at low latitudes. (2) The whistler dispersion is ...
This paper reports on some propagation effects occurring while the ionosphere consists of several... more This paper reports on some propagation effects occurring while the ionosphere consists of several layers. A few simple model problems are considered to show in which way this multi‐layer structure can yield a new set of ELF resonance phenomena. The resonances appear in the space between the Earth and layer interfaces or inside the layers and are liable to give rise to peaks or minima in the frequency dependencies of ELF electromagnetic fields both for the cosmic and terrestrial sources. Calculations have been made for the real ionospheric models mid distant Earth's sources have shown that such a mechanism can explain some experimental data including the effect of “Evening Ionosphere Radiation” by Schecotov and Molchanov (1985).
It is known that electromagnetic emissions take place in association with earthquakes in a wide f... more It is known that electromagnetic emissions take place in association with earthquakes in a wide frequency range from DC/ULF to VHF. But the most promising frequency range from the standpoint of short-term earthquake prediction is ULF (ultra-low-frequency, f<10Hz), because those ULF emissions are definite to have come from the focal zone. While, the generation mechanisms of higher frequency seismogenic emissions are extremely poorly understood. We show our latest results on seismogenis ULF emissions. The first one is the principal component analysis (PCA) for the ULF emissions associated with the Izu islands earthquake swarm in July 2000. Then, the second will be the fractal (mono-and multi-) analysis for the Guam earthquake. These sophisticated signal processing would be of great importance in elucidating precursory weak seismogenic ULF emissions.
Physics and Chemistry of the Earth, Parts A/B/C, 2004
ABSTRACT We analyze the ULF magnetic field emissions observed at three different field sites. Lar... more ABSTRACT We analyze the ULF magnetic field emissions observed at three different field sites. Large seismic events occurred near each station. Temporal dependence of the polarization ratio (ratio of horizontal to vertical magnetic field components) has been calculated in the frequency range between 1.17 mHz and 0.5 Hz with longer than 2-year data for each event. Two of three seismic events were associated with anomalous change in the polarization ratio. Remarkable similarities were found in the ratio between these two different seismic events: (1) The ratio has a significant increase about 1 month before the earthquake (about two times larger than usual level), (2) the increase is seen in the nighttime period (22–02 LT). The frequency range of increase in the ratio depends on the earthquake (0.01–0.022 Hz for the Izu Island earthquake swarms and 0.05–0.1 Hz for the Guam earthquake). Calculated ratio based on the simple theoretical model has a good agreement with observed values for these two earthquakes.
Journal of Geophysical Research: Space Physics, 2004
We have succeeded in observing sprites for winter lightning in the Hokuriku area (Japan Sea side)... more We have succeeded in observing sprites for winter lightning in the Hokuriku area (Japan Sea side) of Japan in the winter of 2001/2002. The optical results on 3 days are compared with the corresponding characteristics of parent (causative) lightning with particular attention to the significant differences between Hokuriku winter lightning and the more widely studied continental lightning. Despite significant differences with Hokuriku winter lightning, we have found nearly the same sprite properties as already observed in the U.S. continent with a significant difference (simpler shape for Hokuriku winter sprite). Then, we have also discussed the criteria for sprite occurrence. Specifically, two similar criteria are found: (1) cloud‐to‐ground discharges of positive polarity and (2) the presence of a certain threshold in vertical charge moment (200–300 C km) (roughly consistent with that for the U.S. continent). Mesoscale convective systems are not necessary to store the charge necessar...
ABSTRACT The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiv... more ABSTRACT The subionospheric data from a Japanese very low frequency/low frequency (VLF/LF) receiving station at Moshiri, Hokkaido, are used to detect the response of the lower ionosphere to the tsunami triggered by the 2011 Tohoku earthquake. Disturbances in the phase and amplitude of VLF signals propagating from the transmitter in Hawaiian Islands are observed during the tsunami wave passage, and these effects in the ionosphere are compared to the in situ sea-level global positioning system (GPS) measurements near Japan. The frequency of the maximum spectral amplitude both for the VLF and GPS data is found to be in the range of periods of 8-50 min, which is likely to correspond to the period of the internal gravity waves generated by the tsunami.
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Papers by M. Hayakawa