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We investigate an event on the evolution of the plasmasphere and its effect on ULF wave generation observed by THEMIS satellites during three successive outbound passes.On September 13 2011 after a moderate magnetic storm,TH-D observed a sharp plasmapause at L=3.4 with a density drop from ~104cm-3 to ~50 cm-3.The plasmasphere starts to be refilled and expand on September 14th.On September 15th,a plasmapause boundary layer is created with two density drops at L=4.5 and 6.5,respectively.We show that strong Br and Ephi oscillations are observed within the two density drops,suggesting poloidal ULF wave.The even-mode signature and bump-on-tail plasma distribution at ~10keV favour the mechanism of drift-bounce resonance.All the above features suggest that the plasmasphere may have impact on the generation of ULF wave and potential impact on radiation belt acceleration.