Disturbance of O~+ Density Before Major Earthquake Detected by DEMETER Satellite

Based on the data observed by French DEMETER satellite,this paper analyzed anomalies which may be caused by Ms7.9 Tonga Islands earthquake(3 May 2006).Perturbations of electric field and plasma parameters above epicenter from observations six days,two days and one day before the earthquake can be seen by visual interpretation of level-2 images.The amplitude and duration of perturbations increased when the earthquake was approaching.Perturbation was also recorded two days after the earthquake and its amplitude was still large.The variation feature of O~+ density was more obvious than that of H~+ and He~+ density,and O~+ accounted for about 90%among total H~+, He~+ and O~+ through analyzing the data detected by IAP(Plasma Analyzer Instrument) onboard DEMETER satellite which has a revisiting period of sixteen days.Compared with the revisiting orbits of a half orbit one day before the earthquake from 2006 to 2008,it was found that variations of O~+ density can be classified into three types:density in northern hemisphere is higher than that in southern hemisphere in summer;the situation is quite the contrary in winter;and O~+ density is almost equal in both southern and northern hemisphere in equinoxes season.This paper presented different effects on O~+ density under influence of earthquake and geomagnetic storm.The earthquake affects the distribution of O~+ density at an orbit close to the epicenter,while geomagnetic storm affects O~+ density at all orbits in a storm day.O~+ density increased 20%more at the orbit closest to the epicenter than the other orbits one day before the earthquake,and it increased nearly an order of magnitude referred to the mean of O~+ density in May from 2006 to 2008.In addition,we obtained electron density detected simultaneously by ISL onboard DEMETER at the above-mentioned revisiting orbits and discovered that electron density increased synchronously with O~+ density.The increment of plasma density may be related to the electric field anomaly which resulted from the earthquake and led to modification of ionospheric structure.