Effect of plasmapause on the generation of ultra low frequency (ULF) waves

We report two ULF wave events associate the evolution of plasmapause. In the first event on 29 May 2007, THEMIS and LANL satellites observed strong poloidal ULF waves around plasmapause. Multi-satellite observations provide information of the temporal evolution and spatial structure of the ULF waves. It is found that the movement of plasmapause leads to fast decay of ULF waves in the region outside of the plasmapause. In the second event, we observed ULF wave generation clearly associated with the formation of a plasmasphere boundary layer (PBL) based on THEMIS-D measurements of subsequent outbound passes. On 13 September 2011, THEMIS observed a sharp plasmapause at L=3.4. The plasmasphere started to expand and continued to be refilled on 14 September. On 15 September, a PBL was formed with two density gradients at L=4.4 and 6.5, respectively. Within the two density gradients, strong radial magnetic field and azimuthal electric field oscillations were observed, suggesting poloidal ULF waves. Based on the phase delay between magnetic and electric field signals, as well as the comparison between the observed wave frequency and predicted harmonic eigenfrequency, we find that the observed oscillations are second harmonic poloidal waves. Further investigation shows that the observed waves are likely generated by drift-bounce resonance with `bump-on-tail' plasma distributions at ~10keV. We demonstrate that the waves are excited within the PBL where the eigenfrequency is close to the bounce frequency of these hot protons, but not outside the PBL where the eigenfrequency deviates from the bounce frequency. Finally, we suggest that cold plasma density seems to be a controlling factor for ULF wave generation as well, in addition to the `bump-on-tail' energy source, by altering eigenfrequency of the local field lines.