Poloidal ULF wave observed in the plasmasphere boundary layer
Thursday, 4 September 2014: 9:10 AM
Regency Ballroom (Hyatt Regency)
Wenlong Liu1, Jinbin Cao1, Xinlin Li2, Theodore E Sarris2, Qiugang Zong3, Kazue Takahashi4 and Michael Hartinger5, (1)Beihang University, Beijing, China, (2)Univ Colorado at Boulder, Boulder, CO, United States, (3)Peking University, Beijing, China, (4)Johns Hopkins University, Applied Physics Laboratory, Laurel, MD, United States, (5)University of Michigan, Ann Arbor, MI, United States
Abstract:
We report on a rare ultra-low-frequency (ULF) wave generation event associated with the formation of a plasmasphere boundary layer (PBL), which was well observed by one of the THEMIS satellites, TH-D, during subsequent outbound passes. On 13 September 2011, TH-D observed a sharp plasmapause at Lâ=â3.4. 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 Lâ=â4.4 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 âbump-on-tailâ plasma distributions at ~10âkeV. 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.
