Whistler mode chorus waves are particularly important in outer radiation belt dynamics due to their key role in controlling the acceleration and scattering of electrons over a very wide energy range. The efficiency of wave-particle resonant interactions is defined by whistler wave properties which have been described by the approximation of plane linear waves propagating through the cold plasma of the inner magnetosphere. However, recent observations of extremely high-amplitude whistlers suggest the importance of nonlinear wave-particle interactions for the dynamics of the outer radiation belt. Oblique chorus waves observed in the inner magnetosphere often exhibit drastically nonsinusoidal (with significant power in the higher harmonics) waveforms of the parallel electric field, presumably due to the feedback from hot resonant electrons. We have considered the nature and properties of such nonlinear whistler waves observed by the Van Allen Probes and Time History of Events and Macroscale Interactions define during Substorms in the inner magnetosphere, and we show that the significant enhancement of the wave electrostatic component can result from whistler wave coupling with the beam-driven electrostatic mode through the resonant interaction with hot electron beams. Being modulated by a whistler wave, the electron beam generates a driven electrostatic mode significantly enhancing the parallel electric field of the initial whistler wave. We confirm this mechanism using a self-consistent particle-in-cell simulation. The nonlinear electrostatic component manifests properties of the beam-driven electron acoustic mode and can be responsible for effective electron acceleration in the inhomogeneous magnetic field.
Bibliographical noteFunding Information:
This work was supported by the JHU/APL contract 922613 (RBSP-EFW), NASA grant NNX16AF85G, NASA contract NAS5-02099, and NSF grants AGS1202330 and AGS1219369. V. K. acknowledges financial support of CNES through grant STEREO-WAVES invited scientist. Van Allen Probe EMFISIS data are at http://emfisis.physics.uiowa.edu/ data/index. EFW data are at http://www. space.umn.edu/rbspefw-data/. We acknowledge NASA contract NAS5-02099 and V. Angelopoulos for use of data from the THEMIS Mission: J. W. Bonnell and F. S. Mozer for use of EFI data; C. W. Carlson and J. P. McFadden for use of ESA data; O. LeContel, and the late A. Roux for use of SCM data. THEMIS data can be found at themis.igpp.ucla.edu.
- electron acceleration
- electron acoustic waves
- induced scattering
- nonlinear wave-particle interactions
- wave steepening
- whistler waves