Theories of Growth and Propagation of Parallel Whistler-Mode Chorus Emissions: A Review

Abstract

The significant role of nonlinear wave–particle interactions in the macrodynamics and microdynamics of the Earth’s outer radiation belt has long been recognised. Electron dropouts during magnetic storms, microbursts in atmospheric electron precipitation, and pulsating auroras are all associated with the rapid scattering of energetic electrons by the whistler-mode chorus, a structured electromagnetic emission known to reach amplitudes of about $$1\%$$ 1 % of the ambient magnetic field. Despite the decades of experimental and theoretical investigations of chorus and the recent progress achieved through numerical simulations, there is no definitive theory of the chorus formation mechanism, not even in the simple case of parallel (one-dimensional) propagation. Here we follow the evolution of these theories from their beginnings in the 1960s to the current state, including newly emerging self-consistent excitation models. A critical review of the unique features of each approach is provided, taking into account the most recent spacecraft observations of the fine structure of chorus. Conflicting interpretations of the role of resonant electron current and magnetic field inhomogeneity are discussed. We also discuss the interplay between nonlinear growth and microscale propagation effects and identify future theoretical and observational challenges stemming from the two-dimensional aspects of chorus propagation.

Evolution of Chorus Subpackets in the Earth's Magnetosphere

“…Among all spectra, the rising tone elements (with frequencies increasing with time) are the most frequently observed (Gao et al., 2014; Li et al., 2011). The chirping of rising tone elements is suggested to be triggered by the resonant current attributed by energetic electrons due to nonlinear effects (Hanzelka & Santolík, 2023; Nunn, 1974; Omura, 2021; Omura et al., 2008; Roux & Pellat, 1978; Vomvoridis et al., 1982; Zonca et al., 2022). The corresponding sweep rate, which is a key parameter depicting the rapidity of the frequency chirping, has been extensively studied in both observations (Cully et al., 2011; Macúšová et al., 2010; Tao et al., 2012) and simulations (H. Chen et al., 2022; Katoh & Omura, 2011; Ke et al., 2020).…”

Energy Transfer Between Various Electron Populations Via Resonant Interaction With Whistler Mode Wave

“…Although a systematic investigation of nonlinear resonant interaction between whistler waves and energetic electrons started in 1960s from the case of constant frequency waves, which was largely stimulated by experiments with VLF transmitter at Siple station (Helliwell & Katsufrakis, 1974), most of the research in this area is currently related to chorus emissions. The review papers by Hanzelka and Santolík (2023) and Tao et al (2019) deal with parallel propagating emissions, but contain many references related to general case of oblique propagation. Comparison of various simulations of chorus emissions performed with different codes may be found in Zhang et al (2021).…”