Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation

K. Yamamoto; M. Nosé; K. Keika; D. P. Hartley; C. W. Smith; R. J. MacDowall; L. J. Lanzerotti; D. G. Mitchell; H. E. Spence; G. D. Reeves; J. R. Wygant; J. W. Bonnell; S. Oimatsu

doi:10.1029/2019JA027158

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Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation

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Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation

K. Yamamoto, M. Nosé, K. Keika, D. P. Hartley, C. W. Smith, R. J. MacDowall, L. J. Lanzerotti, D. G. Mitchell, H. E. Spence, G. D. Reeves, …

Journal of geophysical research. Space physics, Vol.124(12), pp.9904-9923

12/2019

DOI: 10.1029/2019JA027158

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Abstract

Two wave packets of second harmonic poloidal Pc 4 waves with a wave frequency of ~7 mHz were detected by Van Allen Probe A at a radial distance of ~5.8 RE and magnetic local time of 13 hr near the magnetic equator, where plasmaspheric refilling was in progress. Proton butterfly distributions with energy dispersions were also measured at the same time; the proton fluxes at 10–30 keV oscillated with the same frequency as the Pc 4 waves. Using the ion sounding technique, we find that the Pc 4 waves propagated eastward with an azimuthal wave number (m number) of ~220 and ~260 for each wave packet, respectively. Such eastward propagating high‐m (m > 100) waves were seldom reported in previous studies. The condition of drift‐bounce resonance is well satisfied for the estimated m numbers in both events. Proton phase space density was also examined to understand the wave excitation mechanism. We obtained temporal variations of the energy and radial gradient of the proton phase space density and find that temporal intensification of the radial gradient can generate the two wave packets. The cold electron density around the spacecraft apogee was >100 cm−3 in the present events, and hence the eigenfrequency of the Pc 4 waves became lower. This causes the increase of the m number which satisfies the resonance condition of drift‐bounce resonance for 10–30 keV protons and meets the condition for destabilization due to gyrokinetic effect. Key Points The first direct observation of drift‐bounce resonance that excites eastward propagating second harmonic poloidal waves with high m number These waves are excited by intensification of radial gradient of proton phase space density due to substorm injection Cold electrons also contribute to the wave excitation by increasing the m number to satisfy gyro‐kinetic destabilization condition

drift‐bounce resonance

geomagnetic storm

plasmasphere

ring current

substorm

ULF wave

Details

Title: Subtitle: Eastward Propagating Second Harmonic Poloidal Waves Triggered by Temporary Outward Gradient of Proton Phase Space Density: Van Allen Probe A Observation
Creators: K. Yamamoto - Kyoto University
M. Nosé - Nagoya University
K. Keika - The University of Tokyo
D. P. Hartley - University of Iowa
C. W. Smith - University of New Hampshire
R. J. MacDowall - Goddard Space Flight Center
L. J. Lanzerotti - New Jersey Institute of Technology
D. G. Mitchell - Johns Hopkins University Applied Physics Laboratory
H. E. Spence - University of New Hampshire
G. D. Reeves - Los Alamos National Laboratory
J. R. Wygant - University of Minnesota
J. W. Bonnell - University of California, Berkeley
S. Oimatsu - Kyoto University
Resource Type: Journal article
Publication Details: Journal of geophysical research. Space physics, Vol.124(12), pp.9904-9923
DOI: 10.1029/2019JA027158
ISSN: 2169-9380
eISSN: 2169-9402
Number of pages: 20
Grant note: Japan Society for the Promotion of Science (19J14222) MEXT | Japan Society for the Promotion of Science (JSPS) JHU/APL (131803) Grants‐in‐Aid for Scientific Research (16H04057) Challenging Research (17K18804) Energetic Particle, Composition, and Thermal Plasma (RBSP‐ECT) Grants‐in‐Aid for Specially Promoted Research
Language: English
Date published: 12/2019
Academic Unit: Physics and Astronomy
Record Identifier: 9984548859502771

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