7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Frequency Chirping of Electromagnetic Ion Cyclotron Waves in Earth's
Magnetosphere
Xin Tao1, Zeyu An1, Fulvio Zonca2,3, and Liu Chen3,21
Department of Geophysics and Planetary Sciences, University of Science and Technology ofChina, Hefei, China
2 Center for Nonlinear Plasma Science and C.R. ENEA Frascati, C.P. 65, 00044 Frascati, Italy
3 Institute of Fusion Theory and Simulation, School of Physics, Zhejiang University, Hangzhou,Chinae-mail: xtao@ustc.edu.cn
Frequency chirping, a rapid change in frequency overtime, has been observed in several wave modes in Earth's
magnetosphere and laboratory plasmas. In themagnetosphere, examples of frequency chirping waves
include whistler mode chorus waves, electromagnetic ion
cyclotron (EMIC) waves, magnetosonic waves, andelectron cyclotron harmonic waves. Similarly, in fusion
plasmas, Alfven modes have been observed to exhibit
frequency chirping on different time scales. These
chirping waves generally consist of discrete packets thatare narrowband and quasi-coherent. Interaction withthese chirping modes often results in the rapid transport
of energetic particles in phase space, affecting space
weather or deteriorating particle confinement in fusion
devices. Therefore, there is significant research interestin understanding the physical mechanism of frequency
chirping in both communities of space and laboratory
plasmas.
In this work, we focus on the frequency chirping of
EMIC waves in the magnetosphere, which has beenstudied only preliminarily. The only theoretical model(Omura et al., 2010) of EMIC wave chirping we know ofis based on nonlinear wave-particle interactions, which
produces a chirping rate proportional to EMIC wave
amplitude. Here, we propose a new model of EMIC
wave chirping based on nonlinear wave-particle
interaction theories of EMIC waves and the principle of
the recently proposed “Trap-Release-Amplify” (TaRA)
model (Tao, Zonca and Chen, 2021) of whistler modechorus waves. Our new model relates the chirping rate ofEMIC waves not only to wave amplitude via nonlinear
wave-particle interactions but also to the backgroundmagnetic field inhomogeneity for the first time,
providing a more comprehensive understanding of theunderlying mechanisms of EMIC wave chirping.
Additionally, we provide a direct quantitative
comparison between theoretical chirping rates andobservations. The comparison shows good agreement,suggesting the validity of our new model and that thesame underlying principle of chirping mechanisms exists
between EMIC waves and chorus waves. Our results,
therefore, not only improve the understanding of the
chirping of EMIC waves but also open new perspectives
for further investigating the chirping mechanism of otherwave modes in both space and laboratory plasmas.

References:Omura, Y. et al. (2010) ‘Theory and observation ofelectromagnetic ion cyclotron triggered emissions in themagnetosphere, Journal of Geophysical Research: Space
Physics,115(A7).
Availableat:https://doi.org/10.1029/2010JA015300.
Tao, X., Zonca, F. and Chen, L. (2021) ‘A “Trap‐Release‐Amplify” Model of Chorus Waves’, Journal ofGeophysical Research: Space Physics, 126(9). Available
at: https://doi.org/10.1029/2021JA029585.