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- Title
Observational Evidence of the Excitation of Magnetosonic Waves by an He<sup>++</sup> Ion Ring Distribution.
- Authors
Claudepierre, S. G.; Liu, X.; Chen, L.; Takahashi, K.
- Abstract
We report plasma wave observations of equatorial magnetosonic waves at integer harmonics of the local gyrofrequency of doubly ionized helium (He++). The waves were observed by Van Allen Probe A on 08 Feb 2014 when the spacecraft was in the afternoon magnetic local time sector near L≈4 inside of the plasmasphere. Analysis of the complementary in‐situ energetic ion measurements (1–300 keV) reveals the presence of a helium ion ring distribution centered near 30 keV. Theoretical linear growth rate calculations suggest that the local plasma and field conditions can support the excitation of the magnetosonic waves from the unstable ring distribution. This represents the first report of the generation of magnetosonic equatorial noise via a ring distribution in energetic He++ ions in the near‐Earth space plasma environment. Plain Language Summary: In nature, an object displaced from its stable equilibrium typically responds by oscillating at its natural frequency (e.g., a plucked guitar string). The ionized gas (plasma) in outer space where Earth‐orbiting satellites fly is no different. In music, we have octaves or the overtone series of pure tones, where each frequency is an integer multiple of a fundamental. In this study, we report on such "musical notes" in space, but instead of sound waves propagating through air (music), these musical notes are propagating in the plasma in space. And while music is heard because of the sound waves compressing air, these space plasma waves are the result of subatomic particles oscillating about (and cannot be heard). The oscillating particles are helium atoms that have two of their electrons stripped away by the violent processes that occur near the Sun's surface. These particles have made their way through interplanetary space all the way to Earth where they are detected by sophisticated sensors on NASA's Van Allen Probe satellites. Key Points: Equatorial magnetosonic waves are observed at harmonics of the He++ gyrofrequency near L = 4 on the daysideParticle observations suggest the presence of a helium ion ring distribution at energies ∼30–300 keVLinear growth rate calculations indicate that the MS waves are generated from the unstable ring distribution
- Subjects
PLASMA waves; HELIUM; VAN Allen radiation belts; PLASMASPHERE; SPACE plasmas
- Publication
Journal of Geophysical Research. Space Physics, 2021, Vol 126, Issue 8, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2021JA029532