A regional topside-ionospheric subsidence during the Wenchuan Earthquake of 2008 observed from aboard the Demeter spacecraft

Topside-ionospheric parameters recorded by the Instruments : Sonde de Langmuir (ISL), Analyseur de Plasma (IAP) and Capteur Electrique (ICE) onboard the DEMETER spacecraft during 85 days spanning the M8.0 Wenchuan Earthquake on 12 May, 2008 were investigated statistically. The influence of geomagnetic activity was removed from the records through confining the data investigated to periods characterized by (Dst ≥ -30 nT, Kp ≤ 3⁺, AE ≤ 200 nT). Spatiotemporal intensity distributions and two dimensional maps of topside-ionospheric parameters measured in the earthquake preparation area were analyzed. From 7 May 2008 (5 days before the M8.0 earthquake), electron and ion densities began to decrease at the 670 km altitude of the spacecraft while, simultaneously, the electron temperature and the vertical electric field started to increase. On 9 May and 10 May these parameters attained their respective minimum/maximum values and they decreased/increased again on the day of the earthquake itself. Also, on 13 and 15 May fluctuations in these seismic-sensitive parameters, which may have been related to aftershocks, were recorded and they reverted to their normal values on 19 May, 2008. Thereafter, during the Qingchuan M6.4 Earthquake on 25 May, 2008 a sequence of similar variations occurred at about the same location (1.6° north and 2.0° east of Wenchuan). The fact that the regional plasma density decreased and the electron temperature increased is interpreted to indicate a sinking of the ionosphere above the epicentral area. The observed increase in the local earthward vertical electric field would cause ionospheric ions to flow downward and simultaneously shift eastward due to ExB drift. This can explain the occurrence of top level ionosphere subsidence and also the fact that the center of this subsidence was not located directly above the epicenter but, rather, displaced to the south-east of it. Although in situ spacecraft m- - easurements are typically constrained with respect to providing measurements at high spatiotemporal resolution, in the light of the ionospheric plasma density variations that preceded the Wenchuan earthquake by five days, it is suggested that remote monitoring of the global ionospheric plasma might in future provide a useful means to detect ionospheric precursors of seismic activity.