Nonlinear Scattering of 90° Pitch Angle Electrons in the Outer Radiation Belt by Large‐Amplitude EMIC Waves

Abstract

Electromagnetic ion cyclotron (EMIC) waves can cause relativistic electron scattering and atmospheric precipitation, primarily via cyclotron resonant interactions in the Earth's radiation belts. However, the conventional quasilinear resonance theory suggests that the cyclotron resonance condition is not satisfied for 90° pitch angle (PA) electrons, which constitute the majority of electrons in the outer radiation belt, such that scattering mainly affects low‐PA electrons. In contrast to this theory, using test particle calculations, we demonstrate that even exactly 90° PA electrons can be significantly scattered by large‐amplitude EMIC waves. The finite wave force results in the parallel transport of 90° PA electrons away from the equator, corresponding to intrinsically nonresonant scattering. This can lead to parallel velocity that meets cyclotron resonance conditions as local PA deviates from 90°. Different types of resonance are identified depending on the wave normal angle, that is, first‐ and second‐order resonances for parallel and oblique waves, respectively.

Effects of Nonlinear Resonance Broadening on Interactions Between Electrons and Whistler Mode Waves

“…With the increasing observations of intense whistler mode waves (Agapitov et al, 2018; Kellogg et al, 2011; Zhang et al, 2018, 2019), the roles of nonlinear effects on radiation belt electrons dynamics get more and more attention. Many previous studies focused on the coherent nonlinear effects such as phase‐trapping and phase‐bunching on electron acceleration and precipitation (Artemyev et al, 2014, 2016; Gan et al, 2020; Lee et al, 2020; Omura et al, 2015). Here, we investigate nonlinear effects caused by large amplitude but still incoherent waves and test the validation of KM nonlinear resonance broadening theory.…”

Physics of Earth’s Radiation Belts

“…But for larger amplitudes the small oscillation may hit the bounce resonance and scatter the electron to off-equatorial motion. Lee et al (2020) performed test-particle simulations of 5-MeV electrons with an initial α eq = 90 • and different phase angles η integrating Eq. (6.47) in a dipolar B 0 (λ).…”

Sensitive Dependence of Ultrarelativistic Electron Precipitation on EMIC Wave Frequency

“…Additionally, Cao et al (2017) considered the bounce resonant scattering of near-equatorially mirroring electrons by H + -band EMIC waves (Roberts & Schulz, 1968) to more field-aligned pitch angles, where the cyclotron resonance can further scatter electrons into the loss cone. Most recently, D. Lee et al (2020) demonstrated that even electrons with a pitch angle of exactly 90° can be directly scattered by large-amplitude EMIC waves, which can eventually result in cyclotron resonant interactions leading to significant pitch angle changes. This process can take place without Landau or bounce resonance.…”

“…The EMIC wave‐electron interaction has also been examined via the nonlinear test particle approach (Albert & Bortnik, 2009; Lee et al., 2020, 2018; Lemons et al., 2009; Liu et al., 2012, 2010; Zhu et al., 2020), and the validity of the quasilinear diffusion theory has been examined with test particle simulations (Fu, Ni, Tao, et al., 2019; Su et al, 2012, 2013). Other aspects of the EMIC wave‐electron interaction that have been examined previously include oblique wave effects on relativistic electron scattering (Lee et al., 2018, 2020; G. Wang et al., 2017), precipitation by EMIC rising tone emission (Grach & Demekhov, 2020; Kubota et al., 2015; Kubota & Omura, 2016; Omura & Zhao, 2012, 2013), and nonresonant scattering (Chen et al., 2016). The typical electron resonance energy with EMIC waves is ∼MeV (Lorentzen et al., 2000), but lower resonance energy may be possible (Capannolo et al., 2018; Denton et al., 2019; Ukhorskiy et al., 2010; X. Zhang et al., 2019).…”

“…Following the early work by Lyons and Thorne (1972), who formulated specifically the bounce‐averaged diffusion coefficients based on quasilinear resonant diffusion theory (C. F. Kennel & Engelmann, 1966), efforts in theory and simulations have continued until recently, yielding more extensive and realistic estimations of the quasilinear resonance diffusion coefficients (Albert, 2003; Kang et al., 2015, 2016; Kersten et al., 2014; Jordanova et al., 2008; W. Li et al., 2007; Ni et al., 2015, 2018; Shprits et al., 2009, 2013; Summers & Thorne, 2003). The EMIC wave‐electron interaction has also been examined via the nonlinear test particle approach (Albert & Bortnik, 2009; Lee et al., 2020, 2018; Lemons et al., 2009; Liu et al., 2012, 2010; Zhu et al., 2020), and the validity of the quasilinear diffusion theory has been examined with test particle simulations (Fu, Ni, Tao, et al., 2019; Su et al, 2012, 2013). Other aspects of the EMIC wave‐electron interaction that have been examined previously include oblique wave effects on relativistic electron scattering (Lee et al., 2018, 2020; G. Wang et al., 2017), precipitation by EMIC rising tone emission (Grach & Demekhov, 2020; Kubota et al., 2015; Kubota & Omura, 2016; Omura & Zhao, 2012, 2013), and nonresonant scattering (Chen et al., 2016).…”