Reduction of electron temperature in low-latitude ionosphere at 600 km before and after large earthquakes

摘要

We examine ionospheric electron temperatures (Te) observed by HINOTORI satellite during three earthquakes; M6.6 occurred in November 1981, M7.4 and M6.6 in January 1982 over Philippine, respectively. It is found that Te around the epicenters significantly decreases in the afternoon periods within 5 days before and after the three earthquakes. The region of ionosphere disturbance extends to 80–120 degrees in longitude. A tendency exists that duration of the disturbance becomes longer as the increase of earthquake magnitude. F2 peak frequency, foF2 and virtual height, h'F from a chain of 4 ionosonde stations located in the longitude zone of 120°E–130°E are used together with electron density(Ne), that is observed simultaneously onboard HINOTORI satellite to find possible cause mechanisms of the abnormal reduction of electron temperatures. Behavior of HINOTORI Te/Ne and ionosonde foF2/h'F implies the existence of westward electric field over epicentre. Our finding suggests that simple two plasma instruments might be able to play a fundamental role to study ionosphere disturbance associated with earthquake, if the constellation of small/mini satellites is organized and the orbits are properly chosen.