Anomalous sporadic-E ionization before a great earthquake

Abstract

Anomalous increases of f oEs above 8 MHz were observed at Shigaraki and Kokubunji of epicentral distance 120 and 500 km, respectively, in the daytime on January 15, 1995 before the M7.2 Hyogo-ken Nanbu earthquake of January 17, 1995. Geomagnetic conditions were very quiet and there was no solar event on January 15. Normally, the f oEs is low in the winter and increases above 6 MHz for May–August in Japan. An upward tornado-type seismic cloud with rapid air currents appeared at heights between about 680 and 2000 m over the epicenter region and anomalous radon ion density increases of about 100 times the normal one were also observed at epicentral distance of about 200 km before this earthquake. The occurrence epicentral-area of the anomalous f oEs increases in the daytime on January 15 is about the same as that of the terrestrial gas emanations from active faults at epicentral distances within 500 km before great earthquakes [J. Geophys. Res., 91 (1986) B12, 12269]. The radon ions carried up to cold high altitudes become core ions of positively charged ice crystals in the topside cloud. An electrostatic field is set up in the lower ionosphere by positive cloud-to-ground lightning discharges. The temporal electrostatic field above the air breakdown electric field causes ambient electron heating and ionization of neutrals in the lower ionosphere. Computed times for producing the anomalous f oEs increases in this process are comparable with seismic-flash duration before great earthquakes. So, the anomalous f oEs increases before the Hyogo-ken Nanbu earthquake seem to be an ionospheric seismic precursor.