Chorus Wave Generation Modulated by Field Line Resonance and Mirror‐Mode ULF Waves. Issue 2 (31st January 2023)

• Record Type: Journal Article
• Authors: Li, Li; Omura, Yoshiharu; Zhou, Xu‐Zhi; Zong, Qiu‐Gang; Rankin, Robert; Yue, Chao; Fu, Sui‐Yan; Ren, Jie
• Is Part Of: Journal of geophysical research. Volume 128:Issue 2(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-01-31
• Subjects: ULF waves -- field line resonance -- drift‐mirror mode -- anticorrelated upper‐band and lower‐band chorus -- magnetic field configuration -- electron distributions; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2022JA031127
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: Earth's inner magnetosphere is a zoo of plasma waves where electromagnetic and electrostatic emissions with distinct frequencies coexist and interact. Spacecraft observations have shown that whistler‐mode chorus waves, one of the key components in the magnetospheric dynamics, are often modulated by ultralow frequency (ULF) waves. Here, we investigate the effects of two typical ULF wave modes (i.e., field line resonance and mirror mode) on the nonlinear generation process of chorus waves. We report for the first time periodic excitations of lower‐ and upper‐band chorus waves near ULF wave crests and troughs, respectively. Their anticorrelated occurrence is explained by the nonlinear theory, in which the threshold amplitude of nonlinear wave growth is modified by the ULF wave field configuration and the modulated electron distributions. In this framework, the newly observed feature of chorus wave occurrence near the ULF wave crests is attributed to the antisymmetric field profiles of mirror‐mode ULF waves, which periodically modulate the threshold amplitude by modifying the second‐order derivative of the background dipole field. In addition to the second‐order derivative, the first‐order derivative of the background magnetic field is also modulated by the ULF waves to regulate the size of the chorus wave source region. Plain Language Summary: Previous observations of wave modulation showed that chorus waves are often excited preferentially near the magnetic field troughs of ULF waves. In this study, we report for the first time the periodic occurrence of lower‐band chorus waves near the crests of ULF wave field, although upper‐band chorus waves are still excited near ULF wave troughs. This feature differs dramatically from previous observations, which is attributed to different modes of the observed ULF waves. In this event, ULF waves are identified to be drift‐mirror mode rather than field‐line resonances, which are characterized by the antisymmetric profile of wave magnetic field with respect to the equator. It is the antisymmetric profile that periodically adjusts the second‐order derivative of the background magnetic field, which in turn modifies the threshold amplitude for nonlinear growth of chorus waves and consequently fosters chorus wave excitation near ULF wave crests. We demonstrate that the electron distributions modulated by ULF waves also play a role in modifying the threshold amplitude, which contributes to the excitation of upper‐band chorus waves near ULF wave troughs. The good agreement between the theory and the observations highlights the combined effects of ULF wave field configuration and electron distributions in modulating chorus waves. Key Points: Chorus wave generation is affected differently by two typical ultralow frequency (ULF) waves, field line resonance (FLR) mode and mirror mode FLR mode results in lower band chorus in troughs, while the mirror mode causes lower‐ and upper‐band near the wave crest and troughs The modulation of ULF waves on chorus is due to variation of magnetic field configuration and electron distributions