Plasmapause positions observed by CLUSTER/WHISPER show a strong MLT dependence with the geomagnetic activity and solar wind indices. This is in very good agreement with the mechanism of interchange instability that predicted such a MLT dependence. Typical examples of plasmapause positions obtained with CLUSTER/WHISPER and the three dimensional dynamic model of the plasmasphere developed at BIRA-IASB illustrate the time delay and the MLT dependence associated to the formation of the plasmapause.