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[PEM11-15] Role of nonlinear scattering by lower band whistler chorus waves in the formation of relativistic electron butterfly distribution
Keywords:radiation belt, relativistic electrons, whistler chorus waves, nonlinear scattering
Radiation belt electrons have various pitch angle distributions in response to global/local processes arising in the magnetosphere. The butterfly distribution is one of the characteristic features of the electron pitch angle distribution, which has the maximum electron flux intensity at the pitch angle between the loss cone and 90 degrees. Wave-particle interactions play an important role in the formation of butterfly distributions through some nonadiabatic scattering processes. Xiao et al. (2015) and Li et al. (2016) proposed a quasi-linear diffusion process by magnetosonic (MS) waves with wavenumber almost perpendicular to the mean magnetic field to explain the butterfly PAD of relativistic electrons. Horne and Thorne (2003) discussed that the cyclotron resonant scattering by whistler chorus waves at high magnetic latitudes may contribute the formation of the butterfly pitch angle distribution. On the other hand, Yang (2016) showed that quasi-linear diffusion process by whistler chorus waves prevent the formation.
To evaluate a role of whistler chorus waves in the formation of the butterfly pitch angle distribution of relativistic electrons, we investigate contribution of nonlinear scattering process by whistler chorus waves on the butterfly pitch angle distribution, which are not in the quasi-linear diffusion model. We have conducted a test-particle simulation for the electron scattering by lower band whistler chorus (LBC) waves propagating parallel to a magnetic field line. The simulation results show that coherent LBC waves create the butterfly pitch angle distribution of relativistic electrons. We have found that electrons satisfying the phase trapping condition dominantly contribute to the formation of the butterfly pitch angle distribution. Further, we have evaluated quasilinear diffusion coefficients for the electron scattering, and have found that the diffusion process is insufficient to create the butterfly PAD in the test--particle simulation. Therefore, we conclude that the nonlinear phase trapping process by the LBC waves has an important role in the formation of relativistic electron PAD.
References
[1] Xiao, F., Yang, C., Su, Z. et al. Wave-driven butterfly distribution of Van Allen belt relativistic electrons. Nat Commun 6, 8590 (2015). https://doi.org/10.1038/ncomms9590
[2] Li, J., et al. (2016), Ultrarelativistic electron butterfly distributions created by parallel acceleration due to magnetosonic waves, J. Geophys. Res. Space Physics, 121, 3212– 3222, doi:10.1002/2016JA022370.
[3] Horne, R. B., Glauert, S. A., and Thorne, R. M. (2003), Resonant diffusion of radiation belt electrons by whistler-mode chorus, Geophys. Res. Lett., 30, 1493, doi:10.1029/2003GL016963, 9.
[4] Yang, C., et al. (2016), Rapid flattening of butterfly pitch angle distributions of radiation belt electrons by whistler-mode chorus, Geophys. Res. Lett., 43, 8339– 8347, doi:10.1002/2016GL070194.
To evaluate a role of whistler chorus waves in the formation of the butterfly pitch angle distribution of relativistic electrons, we investigate contribution of nonlinear scattering process by whistler chorus waves on the butterfly pitch angle distribution, which are not in the quasi-linear diffusion model. We have conducted a test-particle simulation for the electron scattering by lower band whistler chorus (LBC) waves propagating parallel to a magnetic field line. The simulation results show that coherent LBC waves create the butterfly pitch angle distribution of relativistic electrons. We have found that electrons satisfying the phase trapping condition dominantly contribute to the formation of the butterfly pitch angle distribution. Further, we have evaluated quasilinear diffusion coefficients for the electron scattering, and have found that the diffusion process is insufficient to create the butterfly PAD in the test--particle simulation. Therefore, we conclude that the nonlinear phase trapping process by the LBC waves has an important role in the formation of relativistic electron PAD.
References
[1] Xiao, F., Yang, C., Su, Z. et al. Wave-driven butterfly distribution of Van Allen belt relativistic electrons. Nat Commun 6, 8590 (2015). https://doi.org/10.1038/ncomms9590
[2] Li, J., et al. (2016), Ultrarelativistic electron butterfly distributions created by parallel acceleration due to magnetosonic waves, J. Geophys. Res. Space Physics, 121, 3212– 3222, doi:10.1002/2016JA022370.
[3] Horne, R. B., Glauert, S. A., and Thorne, R. M. (2003), Resonant diffusion of radiation belt electrons by whistler-mode chorus, Geophys. Res. Lett., 30, 1493, doi:10.1029/2003GL016963, 9.
[4] Yang, C., et al. (2016), Rapid flattening of butterfly pitch angle distributions of radiation belt electrons by whistler-mode chorus, Geophys. Res. Lett., 43, 8339– 8347, doi:10.1002/2016GL070194.