Electron butterfly distribution modulation by magnetosonic waves

Geophysical Research Letters RESEARCH LETTER 10.1002/2016GL068161 Key Points: • Unlike Berstein-mode waves, the observed waves are not bound by neighboring harmonics of the proton gyrofrequency • Electron pitch angle distribution is modulated by the magnetosonic wave intensity • Bounce resonance with magnetosonic waves is the mechanism behind our event’s electron pitch angle distribution as shown by our simulation Correspondence to: A. A. Maldonado, aam131130@utdallas.edu Citation: Maldonado, A. A., L. Chen, S. G. Claudepierre, J. Bortnik, R. M. Thorne, and H. Spence (2016), Electron butterfly distribution modulation by magnetosonic waves, Geo- phys. Res. Lett., 43, 3051–3059, doi:10.1002/2016GL068161. Received 9 FEB 2016 Accepted 15 MAR 2016 Accepted article online 21 MAR 2016 Published online 14 APR 2016 Electron butterfly distribution modulation by magnetosonic waves Armando A. Maldonado 1 , Lunjin Chen 1 , Seth G. Claudepierre 2 , Jacob Bortnik 3 , Richard M. Thorne 3 , and Harlan Spence 4 1 Department of Physics, University of Texas at Dallas, Richardson, Texas, USA, 2 Space Science Department, The Aerospace Corporation, El Segundo, California, USA, 3 Atmospheric and Oceanic Sciences Department, UCLA, Los Angeles, California, USA, 4 NASA Goddard Space Flight Center, Greenbelt, Maryland, USA Abstract The butterfly pitch angle distribution is observed as a dip in an otherwise normal distribution of electrons centered about 𝛼 eq = 90 ∘ . 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. 1. Introduction Particles trapped in the Van Allen radiation belts undergo three quasiperiodic motions, from fastest to slowest: gyromotion around the field line, bounce motion between two mirroring points on the field line, and azimuthal drift around the Earth. Each motion is associated with an adiabatic invariant which is conserved for slow variation of the background magnetic field. Violation of the adiabatics can be caused by wave-particle interactions and can result in particles escaping through the magnetopause or along the field line and into the Earth’s upper atmosphere. The nature of interaction between fast magnetosonic waves and electrons are of particular interest in this paper. Fast magnetosonic waves are typically observed within a few degrees of the geomagnetic equator and are sometimes referred to as equatorial noise or ion Bernstein-mode waves. They are observed over a frequency range of a few hertz to a few hundred hertz confined between the proton gyrofrequency and the lower hybrid resonance frequency [Russell et al., 1969]. Magnetosonic waves are magnetically compressional-mode waves that propagate nearly perpendicular to the background magnetic field and are generated through cyclotron harmonic instability due to energetic proton ring distributions ( ∼ 10 keV) [Boardsen et al., 1992; Horne et al., 2000; Chen et al., 2010]. Their dominant magnetic component points along the field line and has an average amplitude of about 50 pT [Ma et al., 2013] but can occasionally be observed as high as 1 nT [Tsurutani et al., 2014]. Horne et al. [2007] found that magnetosonic waves are capable of accelerating electrons in the outer radiation belt by Landau interaction. ©2016. American Geophysical Union. All Rights Reserved. MALDONADO ET AL. During geomagnetic storms, electrons at all pitch angles are observed to be lost over a wide range of L shells, but the theory accounting for the loss of equatorially mirroring electrons ( 𝛼 eq =90 ∘ ) is incomplete [Shprits et al., 2008]. Electrons with mirror points exactly at the magnetic equator are exempt from gyroresonant interactions since they do not have a parallel velocity as required by the gyroresonant condition (except for ultrarelativistic energies). The electron butterfly pitch angle distribution is observed as a smooth dip in an otherwise normal distribution of electrons centered about 𝛼 eq = 90 ∘ . During storm times, this distribution has been attributed to global processes such as L shell splitting in tandem with other eﬀects, namely, magne- topause shadowing or strong positive radial flux gradients [Sibeck et al., 1987; Su et al., 2010]. This occurs when drifting from nightside toward dayside, particles with higher pitch angles will drift to outer equatorial radial BUTTERFLY DISTRIBUTION MODULATION