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

We examine ionospheric electron temperatures (T_e) 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 T_e 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, f₀F₂ 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(N_e), that is observed simultaneously onboard HINOTORI satellite to find possible cause mechanisms of the abnormal reduction of electron temperatures. Behavior of HINOTORI T_e/N_e and ionosonde f₀F ₂/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.