Ionospheric density and velocity anomalies before M ≥ 6.5 earthquakes observed by DEMETER satellite

The seismo-ionospheric density anomalies have been widely studied for last decades based on ground GPS and satellite in situ observations. In this paper, we analyze both ion density and ion velocity observed by the French satellite DEMETER before 49 earthquakes with M ≥ 6.5 in 2010. The results show that both ion density and ion velocity increase anomalously before the earthquakes. The differences before and after earthquakes are more significant with larger magnitude and shallower depth. The density variations have a positive correlation with the perpendicular velocity, which is related to the perpendicular electric field by E_⊥ = V_⊥B₀ (B₀ is the background geomagnetic field). The slopes of ΔN_i/N_i (in units of %) vs. V_⊥ (in units of m/s) ranges from 0.13 to 0.7. This suggests that the ion density variation is associated with the presence of ion velocity or electric field in the ionosphere. The observed peak perpendicular velocity can reach 300–1300 m/s before the earthquakes, corresponding to a perpendicular electric field E_⊥ ≈ 7.5–32.5 mV/m.