Investigation on a dayside poloidal ULF wave event on 29 May 2007

We investigate a strong poloidal ultralow frequency wave event in the noon sector observed by THEMIS and LANL satellites on 29 May 2007. From 07:00 to 10:00 UT, five THEMIS satellites that were lined up in similar outbound orbits consecutively observed narrow-band poloidal pulsations from 10 to 4 mHz. The wave activity covered a broad region from 09:00 to 13:30 LT azimuthally and 5 to 9.5 RE radially with an azimuthal mode number around 8. The radial extent and power of the wave decreased with time from 07:00 to 08:30 UT, suggesting a decay process with a time scale of hours. In the region outside the plasmapause, the wave power was observed to decrease then increase from 08:00 to 09:00 UT with a rapid temporal variation. The decrease in wave power, which suggests fast decay (within one hour), might be related to the evolution of the plasmasphere. The increase could be related to the regeneration by the surface wave at the plasmapause. We suggest that a coupling between the surface wave and the resonance of the field line around the plasmapause takes place when the density inside the plasmapause is twice of the density outside the plasmapause.