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AbstractElectromagnetic ion cyclotron (EMIC) waves can cause the scattering loss of the relativistic electrons in the radiation belt. They can be classified into the guided mode and the unguided mode, according to waves propagation behavior. The guided mode waves have been widely investigated in the radiation belt, but the observation of the unguided mode waves have not been expected. Based on the observations of Van Allen Probes, we demonstrate for the first time the existence of the intense unguided L-mode EMIC waves in the radiation belt according to the polarization characteristics. Growth rate analyses indicate that the hot protons with energies of a few hundred keV may provide the free energy for wave growth. The reflection interface formed by the spatial locations of local helium cutoff frequencies can be nearly parallel to the equatorial plane when the proton abundance ratio decreases sharply with -shell. This structure combined with hot protons may lead to the trapping and significant amplification of the unguided mode waves. These results may help to understand the nature of EMIC waves and their dynamics in the radiation belt.
Published by: Journal of Geophysical Research: Space Physics Published on: 08/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2021JA029322
Abstract Whistler-mode waves are electromagnetic waves pervasively observed in the Earth s and other planetary magnetospheres. They are considered to be mainly responsible for producing the hazardous radiation and diffuse aurora, which heavily relies on their properties. Density irregularities, frequently observed in the Earth s magnetospheres, are found to change largely the properties of whistler-mode waves. Here we report, using Van Allen Probes measurements, whistler-mode waves strongly modulated by two different density enhancements. With particle-in-cell simulations, we propose wave trapping caused by field-aligned density irregularities (ducts) may account for this phenomenon. Simulation results show that whistler-mode waves can be trapped inside the enhanced density ducts. These trapped waves remain quasi-parallel and usually get much larger amplitudes than those unducted whistler waves during propagation away from the magnetic equator, and tend to focus at a spatially narrow channel, consistent with observations. Our results imply density irregularities may be significant to modulate radiation-belt electrons. This article is protected by copyright. All rights reserved.
Published by: Geophysical Research Letters Published on: 03/2021
YEAR: 2021   DOI: https://doi.org/10.1029/2020GL092305
Electromagnetic ion cyclotron waves and fast magnetosonic waves are found to be simultaneously modulated by background plasma density: both kinds of waves were observed in high plasma density regions but vanished in low density regions. Theoretical analysis based on Snell\textquoterights law and linear growth theory have been utilized to investigate the physical mechanisms driving such modulation. It is suggested that the modulation of fast magnetosonic waves might be due to trapping by plasma density structures, which results from a conservation of the parameter Q during their propagation. Here Q = nrsinψ, with n the refractive index, r the radial distance, and ψ the wave azimuthal angle. As for electromagnetic ion cyclotron waves, the modulation might be owed to the ion composition difference between different plasma density regions. Our results indicate the alternative mechanism for simultaneous appearance of electromagnetic ion cyclotron waves and fast magnetosonic waves (rather than wave excitations of both two wave emissions), which might take combined effects on the evolution of radiation belt electrons.
Published by: Journal of Geophysical Research: Space Physics Published on: 02/2019
YEAR: 2019   DOI: 10.1029/2018JA026149