Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 9 4 2009 10.5194/nhess-9-1259-2009 http://www.nat-hazards-earth-syst-sci.net/9/1259/2009/ http://www.nat-hazards-earth-syst-sci.net/9/1259/2009/nhess-9-1259-2009.html http://www.nat-hazards-earth-syst-sci.net/9/1259/2009/nhess-9-1259-2009.pdf 1259 1266 2009-07-28 Analysis of ionospheric plasma perturbations before Wenchuan earthquake X. Zhang zhangxm69@yahoo.cn X. Shen J. Liu X. Ouyang J. Qian S. Zhao Institute of Earthquake Science, China Earthquake Administration, China Based on data recorded by the French DEMETER satellite, plasma perturbations have been analyzed before the Wenchuan 8.0 earthquake which occurred on 12 May 2008. Using the revisited orbits, the seasonal variations of the O⁺ density at daytime and nighttime were obtained respectively. There mainly exist two kinds of shapes of O⁺ density, with peak values in the Northern Hemisphere during May to September and reversely in the Southern Hemisphere during other months. Analysis on local daytime O⁺ density showed that it reached its lowest values 3 days prior to the earthquake, i.e. on 9 May. With studies in more than 3 month time series and comparison with those in 2006 and 2007, it was found that the significant decrease of the O⁺ density on 9 May 2008 was different from the trend variations of 2007 and 2006 both with relatively higher O⁺ density in May. In addition, the ion density and temperature was also analyzed before the earthquake at local nighttime. They showed smallest value in O⁺ density and fast and short-term variations in ion temperature on 9 May above the epicenter area. All these plasma anomalies may be related to the Wenchuan earthquake. 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