Electron scattering by magnetosonic waves in the inner magnetosphere

PUBLICATIONS Journal of Geophysical Research: Space Physics RESEARCH ARTICLE 10.1002/2015JA021992 Key Points: • A new statistics of magnetosonic waves is performed • Magnetosonic waves can lead to the butterﬂy distributions of energetic electrons • Typical acceleration of electrons by magnetosonic waves is slower than energization by chorus Correspondence to: Q. Ma, qianlima@atmos.ucla.edu Citation: Ma, Q., W. Li, R. M. Thorne, J. Bortnik, C. A. Kletzing, W. S. Kurth, and G. B. Hospodarsky (2016), Electron scattering by magnetosonic waves in the inner magnetosphere, J. Geophys. Res. Space Physics, 121, 274–285, doi:10.1002/2015JA021992. Received 4 OCT 2015 Accepted 21 DEC 2015 Accepted article online 28 DEC 2015 Published online 20 JAN 2016 Electron scattering by magnetosonic waves in the inner magnetosphere Qianli Ma 1 , Wen Li 1 , Richard M. Thorne 1 , Jacob Bortnik 1 , C. A. Kletzing 2 , W. S. Kurth 2 , and G. B. Hospodarsky 2 Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, California, USA, 2 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, USA Abstract We investigate the importance of electron scattering by magnetosonic waves in the Earth’s inner magnetosphere. A statistical survey of the magnetosonic wave amplitude and wave frequency spectrum, as a function of geomagnetic activity, is performed using the Van Allen Probes wave measurements and is found to be generally consistent with the wave distribution obtained from previous spacecraft missions. Outside the plasmapause the statistical frequency distribution of magnetosonic waves follows the variation of the lower hybrid resonance frequency, but this trend is not observed inside the plasmasphere. Drift and bounce averaged electron diffusion rates due to magnetosonic waves are calculated using a recently developed analytical formula. The resulting timescale of electron energization during disturbed conditions (when AE* > 300 nT) is more than 10 days. We perform a 2-D simulation of the electron phase space density evolution due to magnetosonic wave scattering during disturbed conditions. Outside the plasmapause, the waves accelerate electrons with pitch angles between 50° and 70° and form butterﬂy pitch angle distributions at energies from ~100 keV to a few MeV over a timescale of several days; whereas inside the plasmapause, the electron acceleration is very weak. Our study suggests that intense magnetosonic waves may cause the butterﬂy distribution of radiation belt electrons especially outside the plasmapause, but electron acceleration due to magnetosonic waves is generally not as effective as chorus wave acceleration. 1. Introduction Fast magnetosonic waves are highly oblique whistler-mode electromagnetic emissions generated in the frequency range between the local proton gyrofrequency (f cp ) and the lower hybrid resonance frequency (f LHR ) [e.g., Perraut et al., 1982; Laakso et al., 1990; Santolik et al., 2002] and are primarily observed by spacecraft within 5° of the Earth’s magnetic equator [Russell and Holzer, 1970; Němec et al., 2005, 2006; Pokhotelov et al., 2008; Santolik et al., 2004]. The waves occur over a broad spatial region between 2 R E and 8 R E both inside and outside the plasmapause, with highest wave intensities observed near the dayside during geomagnetically disturbed conditions [Meredith et al., 2008; Ma et al., 2013; Němec et al., 2015]. Ion ring distributions provide free energy for the local excitation of magnetosonic waves when the ion ring energy is close to the local Alfven energy and typically occur over a broad region outside the plasmapause and near the outer edge of the plasmasphere [Chen et al., 2010; Jordanova et al., 2012; Ma et al., 2014a]. The injected ion populations may account for the excitation of magnetosonic waves especially on the dayside outside the plasmapause [Boardsen et al., 1992; Chen et al., 2011; Thomsen et al., 2011; Xiao et al., 2013], and the excited waves then propagate in both radial and azimuthal directions [e.g., Xiao et al., 2012]. Magnetosonic waves propagate nearly perpendicularly to the background magnetic ﬁeld near the equatorial plane [Kasahara et al., 1994; Chen and Thorne, 2012], and the waves inside the plasmapause may become naturally trapped within the plasmasphere [Ma et al., 2014b]. ©2015. American Geophysical Union. All Rights Reserved. MA ET AL. The energetic electron populations in the Earth’s outer radiation belt can experience diffusive scattering by various plasma waves [Kennel and Engelmann, 1966; Lyons, 1974a, 1974b]. Two important magnetospheric waves that cause local electron acceleration in the radiation belts are whistler-mode chorus and magnetosonic waves [Thorne, 2010]. The Van Allen Probes observations and their related simulation studies have conﬁrmed that whistler-mode chorus waves provide sufﬁcient local heating of energetic electrons in the heart of radiation belts during geomagnetic storm periods [Reeves et al., 2013; Thorne et al., 2013; Foster et al., 2014; Li et al., 2014b]. On the other hand, intense magnetosonic wave events are observed by the spacecraft in the inner magnetosphere and could have a potential role in electron acceleration via Landau resonance and transit-time scattering effects [Horne et al., 2007; Bortnik and Thorne, 2010; Li et al., 2014a]. During Landau resonant ELECTRON SCATTERING BY MS WAVES