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Electron butterfly distribution modulation by magnetosonic waves

The butterfly pitch angle distribution is observed as a dip in an otherwise normal distribution of electrons centered about αeq=90\textdegree. During storm times, the formation of the butterfly distribution on the nightside magnetosphere has been attributed to L shell splitting combined with magnetopause shadowing and strong positive radial flux gradients. It has been shown that this distribution can be caused by combined chorus and magnetosonic wave scattering where the two waves work together but at different local times. Presented in our study is an event on 21 August 2013, using Van Allen Probe measurements, where a butterfly distribution formation is modulated by local magnetosonic coherent magnetosonic waves intensity. Transition from normal to butterfly distributions coincides with rising magnetosonic wave intensity while an opposite transition occurs when wave intensity diminishes. We propose that bounce resonance with waves is the underlying process responsible for such rapid modulation, which is confirmed by our test particle simulation.

Maldonado, Armando; Chen, Lunjin; Claudepierre, Seth; Bortnik, Jacob; Thorne, Richard; Spence, Harlan;

Published by: Geophysical Research Letters      Published on: 04/2016

YEAR: 2016     DOI: 10.1002/2016GL068161

butterfly; electron; magnetosonic; Magnetosphere; Van Allen Probes; wave particle interaction


Nonlinear Bounce Resonances between Magnetosonic Waves and Equatorially Mirroring Electrons

Equatorially mirroring energetic electrons pose an interesting scientific problem, since they generally cannot resonate with any known plasma waves and hence cannot be scattered down to lower pitch angles. Observationally it is well known that the fluxof these equatorial particles does not simply continue to build up indefinitely, and so a mechanism must necessarily exist that transports these particles from a equatorial pitch angle of 90 degrees down to lower values. However this mechanism has not been uniquely identified yet. Here, we investigate the mechanism of bounce resonance with equatorial noise (or fast magnetosonic waves). A test particle simulation is used to examine the effects of monochromatic magnetosonic waves on the equatorially mirroring energetic electrons, with a special interest in characterizing the effectiveness of bounce resonances. Our analysis shows that bounce resonances can occur at the first three harmonics of the bounce frequency (nωb, n = 1 , 2, and 3 ) and can effectively reduce the equatorial pitch angle to values where resonant scattering by whistler-mode waves becomes possible. We demonstrate that the nature of bounce resonance is nonlinear and we propose a nonlinear oscillation model for characterizing bounce resonances using two key parameters, effective wave amplitude \~A and normalized wave number inline image. The threshold for higher harmonic resonance is more strict, favoring higher \~A and inline image and the change in equatorial pitch angle is strongly controlled by inline image. We also investigate the dependence of bounce resonance effects on various physical parameters, including wave amplitude, frequency, wave normal angle and initial phase, plasmadensity, and electron energy. It is found that the effect of bounce resonance is sensitive to the wave normal angle. We suggest that the bounce resonant interaction might lead to an observed pitch angle distribution with a minimum at 90o.

Chen, Lunjin; Maldonado, Armando; Bortnik, Jacob; Thorne, Richard; Li, Jinxing; Dai, Lei; Zhan, Xiaoya;

Published by: Journal of Geophysical Research: Space Physics      Published on: 06/2015

YEAR: 2015     DOI: 10.1002/2015JA021174

bounce resonance; equatorioal noise; magnetosonic waves; nonlinear; Radiation belt; wave particle interaction