Multiple satellites observation evidence: High-<i>m</i> Poloidal ULF waves with time-varying polarization states

TitleMultiple satellites observation evidence: High-m Poloidal ULF waves with time-varying polarization states
Publication TypeJournal Article
Year of Publication2019
AuthorsWei, C, Dai, L, Duan, S, Wang, C, Wang, YX
Corporate Authors,State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China, ,College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
JournalEarth and Planetary Physics
Pagination190 - 203
Date Published05/2019
Keywordsbump-on-tail; inward gradient; polarization rotation; poloidal waves; Van Allen Probes
AbstractWe report multi‐spacecraft observations of ULF waves from Van Allen Probes (RBSP), Magnetospheric Multiscale (MMS), Time History of Events and Macroscale Interactions during Substorm (THEMIS), and Geostationary Operational Environmental Satellites (GOES). On August 31, 2015, global‐scale poloidal waves were observed in data from RBSP‐B, GOES and THEMIS from L=4 to L=8 over a wide range of magnetic local time (MLT). The polarization states varied towards purely poloidal polarity. In two consecutive orbits over 18 hours, RBSP‐A and RBSP‐B recorded gradual variation of the polarization states of the poloidal waves; the ratio (|Ba|/|Br|) decreased from 0.82 to 0.13. After the variation of polarization states, the poloidal ULF waves became very purely poloidal waves, localized in both L and MLT. We identify the poloidal wave as second harmonic mode with a large azimuthal wave number (m) of –232. From RBSP particle measurements we find evidence that the high‐ m poloidal waves during the polarization variations were powered by inward radial gradients and bump‐on‐tail ion distributions through the N=1 drift‐bounce resonance. Most of the time, the dominant free energy source was inward radial gradients, compared with the positive gradient in the energy distribution of the bump‐on‐tail ion distributions.

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