Equatorial Source of Oblique Electromagnetic Ion Cyclotron Waves: Peculiarities in the Ion Distribution Function

Electromagnetic ion cyclotron (EMIC) waves are important for Earth's inner magnetosphere as they can effectively drive relativistic electron losses to the atmosphere and energetic (ring current) ion scattering and isotropization. EMIC waves are generated by transversely anisotropic ion populati...

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Bibliographic Details
Published in:Journal of geophysical research. Space physics 2024-11, Vol.129 (11), p.n/a
Main Authors: Tonoian, David S., Zhang, Xiao‐Jia, Artemyev, Anton, An, Xin
Format: Article
Language:English
Subjects:
Cyclotrons
Dipoles
Distribution functions
Earth magnetosphere
EMIC waves
Equatorial regions
Ion cyclotron waves
Ion distribution
Ion scattering
Ionosphere
Ions
Landau damping
Magnetic fields
Obliquity
ring current ions
Ring currents
Spacecraft
Wave generation
Wave propagation
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Summary:Electromagnetic ion cyclotron (EMIC) waves are important for Earth's inner magnetosphere as they can effectively drive relativistic electron losses to the atmosphere and energetic (ring current) ion scattering and isotropization. EMIC waves are generated by transversely anisotropic ion populations around the equatorial source region, and for typical magnetospheric conditions this almost always produces field‐aligned waves. For many specific occasions, however, oblique EMIC waves are observed, and such obliquity has been commonly attributed to the wave off‐equatorial propagation in curved dipole magnetic fields. In this study, we report that very oblique EMIC waves can be directly generated at the equatorial source region. Using THEMIS spacecraft observations at the dawn flank, we show that such oblique wave generation is possible in the presence of a field‐aligned thermal ion population, likely of ionospheric origin, which can reduce Landau damping of oblique EMIC waves and cyclotron generation of field‐aligned waves. This generation mechanism underlines the importance of magnetosphere‐ionosphere coupling processes in controlling wave characteristics in the inner magnetosphere. Key Points We report observations of very oblique electromagnetic ion cyclotron (EMIC) waves around their equatorial source region Oblique EMIC wave generation is associated with field‐aligned thermal ion streams and hot, transversely anisotropic ions The presence of field‐aligned thermal ion streams attributes oblique EMIC wave generation to its possible ionospheric source
ISSN:2169-9380
2169-9402
DOI:10.1029/2024JA032848