ThispaperisrelatedtostudyoftheiondensityrecordedbythelowaltitudesatelliteDEMETER.Itwillpresentionosphericperturbationsobservedduringlargeseismicevents.Astheionosphereishighlyvariable,thepaperwillshowastatisticalanalysisperformedontheplasmaparametersduringnighttime.Analgorithmhasbeenimplementedtodetectcrestsandtroughsinthedatabeforeworld-wideearthquakes.Theearthquakeshavebeenclassifieddependingontheirmagnitude,depth,andlocation(land,belowthesea,closetoacoast).Duetotheorbit,DEMETERreturnsabovethesameareaeveryday(onceduringdaytime,onceduringnighttime)butnotatthesamedistanceofagivenepicenter.Then,foreachearthquake,datahavebeencheckeduntil15daysbeforetheshockwhenthedistancebetweenthetraceoftheorbitandtheepicenterislessthan1500km.Theresultsofthestatisticalanalysisarepresentedasfunctionsofvariousparameters.Acomparisonisdonewithtwootherdatabaseswhere,ononehand,thelocationoftheepicentershasbeenrandomlymodified,andontheotherhand,thelongitudeoftheepicentershasbeenshifted.Resultsshowthatthenumberandtheintensityoftheionosphericperturbationsarelargerpriortoearthquakesthanpriortorandomevents,andthattheperturbationsincreasewiththemagnitude.
