Eigenmodes of the Transverse Alfvénic Resonator at the Plasmapause: A Van Allen Probes Case Study

• Published in: Geophysical research letters 2018-10, Vol.45 (20), p.10,796-10,804
• Main Authors: Mager, Pavel N., Mikhailova, Olga S., Mager, Olga V., Klimushkin, Dmitri Yu
• Format: Article
• Language: English
• Subjects: Alfven waves; Amplitude; Amplitude modulation; Amplitudes; Case studies; Distribution; Drift; Earth; Electric fields; Equator; Extremely low frequencies; Fluxes; Geomagnetism; Harmonics; Instability; Magnetic equator; Magnetic resonance; Magnetohydrodynamics; magnetosphere; Oscillations; Plasmapause; poloidal Alfvén waves; Probes; Protons; Radiation; Resonance; Resonators; Sensors; Spacecraft; Stability; Storms; Superposition (mathematics); transverse resonator; ULF waves; Velocity; Wave number; Wave propagation; Wavelength; wave‐particle interaction
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL079596

A Pc4 ultralow frequency wave was detected at spacecraft B of the Van Allen Probes at the plasmapause. A distinctive feature of this wave is the strong periodical modulation of the wave. It is assumed that this modulation is a beating of oscillations close in frequency: at least two harmonics with frequencies of 15.3 and 13.6 MHz are found. It is shown that these harmonics can be the eigenmodes of the transverse resonator at the local maximum of the Alfvén velocity. In addition, the observed wave was in a drift resonance with energetic 80‐keV protons and could be generated by an unstable bump on tail distribution of protons simultaneously observed with the wave. The estimate of the azimuthal wave number m made from the drift resonance condition gives a value of about −100, that is, it is a westward propagating azimuthally small‐scale wave. Plain Language Summary The study presented in this paper can be considered as the first direct experimental confirmation of the existence of the transverse Alfvénic resonators on the plasmapause exited by the kinetic instabilities in the hot plasma component. An Alfvén wave was detected by the Radiation Belt Storm Probes spacecraft B spacecraft on 23 October 2012 at the outer edge of the plasmapause where Alfvén velocity has a local maximum. The wave had strong amplitude modulation of the magnetic and electric fields. This amplitude modulation was the result of the superposition of (at least) two harmonics with close frequencies 15.3 and 13.6 mHz. We suppose that these harmonics are eigenmodes of the transverse Alfvénic resonator at the plasmapause. Moreover, the energetic proton fluxes were strongly modulated by the wave at energies near 80 keV. Assuming the drift wave‐particle resonance, we found that the wave was azimuthally small‐scale westward propagating wave with the azimuthal wavelength at the geomagnetic equator about 0.3 Earth's radius. The wave was probably generated by the instability caused by the inverted (bump on tail) proton distribution for energies below 84.4 keV. Key Points An azimuthally small‐scale ULF wave with strong periodical modulation was detected at the plasmapause The wave consists of at least two harmonics with close frequencies 15.4 and 13.6 mHz The wave is a superposition of eigenmodes of the transverse Alfvenic resonator at the plasmapause where Alfven velocity has the local maximum