Parametric analysis of pitch angle scattering and losses of relativistic electrons by oblique EMIC waves

Abstract

This study analyzes the effects of electromagnetic ion cyclotron (EMIC) waves on relativistic electron scattering and losses in the Earth’s outer radiation belt. EMIC emissions are commonly observed in the inner magnetosphere and are known to reach high amplitudes, causing significant pitch angle changes in primarily >1 MeV electrons via cyclotron resonance interactions. We run test-particle simulations of electrons streaming through helium band waves with different amplitudes and wave normal angles and assess the sensitivity of advective and diffusive scattering behaviors to these two parameters, including the possibility of very oblique propagation. The numerical analysis confirms the importance of harmonic resonances for oblique waves, and the very oblique waves are observed to efficiently scatter both co-streaming and counter-streaming electrons. However, strong finite Larmor radius effects limit the scattering efficiency at high pitch angles. Recently discussed force-bunching effects and associated strong positive advection at low pitch angles are, surprisingly, shown to cause no decrease in the phase space density of precipitating electrons, and it is demonstrated that the transport of electrons into the loss cone balances out the scattering out of the loss cone. In the case of high-amplitude obliquely propagating waves, weak but non-negligible losses are detected well below the minimum resonance energy, and we identify them as the result of non-linear fractional resonances. Simulations and theoretical analysis suggest that these resonances might contribute to subrelativistic electron precipitation but are likely to be overshadowed by non-resonant effects.

Properties of Intense H‐Band Electromagnetic Ion Cyclotron Waves: Implications for Quasi‐Linear, Nonlinear, and Nonresonant Wave‐Particle Interactions

“…For electrons with pitch‐angles below 10°, force bunching occurs for almost ∼10% of the observed waves, with an average wave‐packet peak amplitude of ∼0.5 nT. Most of such intense wave packets are not too short, β ∼ 50, and, thus, force bunching will compete with phase trapping and scattering for determining the magnitude of electron precipitation (see discussion in Bortnik et al., 2022; Grach et al., 2022; Hanzelka, Li, & Ma, 2023).…”

“…When the electron pitch‐angle is small enough, another nonlinear regime, the so called force bunching , becomes important, and could potentially prevent electron precipitation by advecting such electrons toward larger pitch‐angles (see discussion in Albert et al., 2022; Bortnik et al., 2022; Grach & Demekhov, 2020, and references therein). Force bunching plays an important role in the formation of precipitation fluxes (see Grach et al., 2021, 2022; Hanzelka, Li, & Ma, 2023), and can be effective for short packets, when trapping doesn't occur even for | S | < 1 because of the short packet length (Grach et al., 2021, 2022). This effect can be characterized by the parameter y R (or δ = y R – 1), an analog of S , but derived for a system with small pitch‐angles: the force bunching regime corresponds to y R < 1 (see Albert et al., 2021; Artemyev et al., 2021).…”

“…Quite comprehensive parametric investigations of possible nonlinear resonant effects for electron interactions with EMIC waves can be found in Kubota et al. (2015), Kubota and Omura (2017), Grach and Demekhov (2020), Hanzelka, Li, and Ma (2023). Here, we discuss the two most important nonlinear processes: phase bunching and phase trapping.…”

“…Electron phase trapping is a nonlinear resonant effect occurring when the wavefield can lock electrons around the resonance on a time‐scale of a fraction of the electron bounce period, that may change their pitch‐angle and energy. Such phase trapping by EMIC waves results in a decrease in pitch‐angle, and constitutes one of the main mechanisms responsible for fast electron precipitation by intense EMIC waves (Grach et al., 2021; Hanzelka, Li, & Ma, 2023; Kubota & Omura, 2017). Such trapping is nonlocal, that is, the efficiency of the associated electron transport in pitch‐angle and energy depends on the shape/size of EMIC wave‐packets and their coherence (see discussion of this effect for various waves types in Z.…”

On the Spatial and Temporal Evolution of EMIC Wave‐Driven Relativistic Electron Precipitation: Magnetically Conjugate Observations From the Van Allen Probes and CALET

“…Some of the primary causes of MeV electron precipitation are wave‐particle interactions, through which the pitch angle of trapped electrons can be scattered into the loss cone (see Blum & Breneman, 2020, and references within). Electromagnetic ion cyclotron (EMIC) waves have been shown to be able to scatter energetic electrons through both resonant and nonresonant interactions, driving strong precipitation particularly at MeV energies (e.g., Albert & Bortnik, 2009; Chen et al., 2016; Grach et al., 2022; Hanzelka et al., 2023; Thorne & Kennel, 1971). Initially demonstrated theoretically, a growing number of studies have provided observational confirmation that some precipitation is driven by EMIC waves.…”