Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 8 1 2008 10.5194/nhess-8-129-2008 http://www.nat-hazards-earth-syst-sci.net/8/129/2008/ http://www.nat-hazards-earth-syst-sci.net/8/129/2008/nhess-8-129-2008.html http://www.nat-hazards-earth-syst-sci.net/8/129/2008/nhess-8-129-2008.pdf 129 134 2008-02-25 On the generation mechanism of terminator times in subionospheric VLF/LF propagation and its possible application to seismogenic effects M. Yoshida T. Yamauchi T. Horie M. Hayakawa hayakawa@whistler.ee.uec.ac.jp Department of Electronic Engineering and Research Station on Seismo Electromagnetics, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu Tokyo 182-8585, Japan The signals from VLF/LF transmitters are known to propagate in the Earth-ionosphere waveguide, so that the subionospheric propagation characteristics are very sensitive to the condition of the lower ionosphere. We know that there appear the terminator times just around the sunrise and sunset in the diurnal variation of subionospheric VLF/LF signal (amplitude and phase). These terminator times are found to shift significantly just around an earthquake, which enables us to infer the change in the ionosphere during the earthquake. In this paper we try to understand the physical mechanism on the generation of those terminator times for relatively short propagation path (less than 2000 km) by means of wave-hop method. It is found that the lowering of the ionosphere boundary during an earthquake decreases the path length of the sky wave and this alters the interference condition of this wave with the ground wave, which lead to an appearance of terminator times as the destructive interference between the ground and sky waves. Finally, we suggest a possible use of terminator time shifts to investigate the lower ionospheric plasma changes during the earthquakes. Belrose, J. S.: Low and very low frequency radio wave propagation, in Radio Wave Propagation, AGARD Lecture Ser., 29, 97–115, Advisory Group for Aerospace Research and Development, NATO, France, 1968. Biagi, P. F., Castella, L., Maggipinto, T., Ermini, A., Perna, G., and Capozzi, V.: Electric field strength analysis of 216 and 270 kHz broadcast signals recorded during 9 years, Radio Sci., 41, RS4013, doi:10.1029/2005RS003296, 2006. Budden, K. G.: The Wave-Guide Mode Theory of Wave Propagation, Logos Press, London, 325 pp., 1961. Hayakawa, M. and Molchanov, O. A.: Seismo Electromagnetics: Lithosphere – Atmosphere – Ionosphere Coupling, TERRAPUB, Tokyo, 477 pp., 2002. Hayakawa, M. and Fujinawa, Y.: Electromagnetic Phenomena Related to Earthquake Prediction, Terra Sci. Pub. Co., Tokyo, 667 pp., 1994. Hayakawa, M., Molchanov, O. A., Ondoh, T., and Kawai, E.: The precursory signature effect of the Kobe earthquake on VLF subionospheric signals, J. Comm. Res. Lab., Tokyo, 43, 169–180, 1996. Hayakawa, M.: Atmospheric and Ionospheric Electromagnetic Phenomena Associated with Earthquakes, Terra Sci. Pub. Co., Tokyo, 996 pp., 1999. Hayakawa, M. and Molchanov, O. A.: NASDA/UEC team, Summary report of NASDA's earthquake remote sensing frontier project, Phys. Chem. Earth, 29, 617–625, 2004. International Telecommunication Union, Recommendation ITU-R, Ground wave propagation curves for frequencies between 10 kHz and 3 DMHz,Geneva, Switzerland, P. Series, p. 368–7,1992. International Telecommunication Union, Recommendation ITU-R, Prediction of field strength at frequencies below 150 kHz, Geneva, Switzerland, P. Series, p. 684-3, 2002. Maekawa, S., Horie, T., Yamauchi, T., Sawaya, T., Ishikawa, M., Hayakawa, M., and Sasaki, H.: A statistical study on the effect of earthquakes on the ionosphere, based on the subionospheric LF propagation data in Japan, Ann. Geophys., 24, 2219–2225, 2006. Molchanov, O. A. and Hayakawa, M.: Subionospheric VLF signal perturbations possibly related to earthquakes, J. Geophys. Res., 103, 17 489–17 504, 1998. Molchanov, O. A., Hayakawa, M., Ondoh, T., and Kawai, E.: Precursory effects in the subionospheric VLF signals for the Kobe earthquake, Phys. Earth Planet. In., 105, 239–248, 1998. Rozhnoi, A., Solovieva, M. S., Molchanov, O. A., and Hayakawa, M.: Middle latitude LF (40 kHz) phase variations associated with earthquakes for quiet and disturbed geomagnetic conditions, Phys. Chem. Earth, 29, 589–598, 2004. Shvets, A. V., Hayakawa, M., Molchanov, O. A., and Ando, Y.: A study of ionospheric response to regional seismic activity by VLF radio sounding, Phys. Chem. Earth, 29, 627–637, 2004. Wakai, N., Kurihara, N., Otsuka, A., and Imamura, K.: Prediction method of LF/MF field strangths, Inst. Electr. Information Comm. Engrs. Japan, Technical Report, AP-2003-271, 35–40, 2004.