Gap formation around <em>Ω</em>_e/2 and generation of low-band whistler waves by Landau-resonant electrons in the magnetosphere: Predictions from dispersion theory

Abstract

In this paper we show that two significant phenomena of magnetospheric chorus emission can be explained by the participation of beam‐like electron structures, created by Landau‐resonant interaction with growing oblique whistler waves. The first concerns the widely observed spectral gap near half the electron cyclotron frequency Ω e; the second is related to the observation of very obliquely propagating lower‐band waves that cannot be directly generated by temperature anisotropy. Concerning the gap, kinetic dispersion theory reveals that interference of the beam‐related cyclotron mode ω~Ωe‐kVb with the conventional whistler mode leads to mode splitting and the appearance of a ‘forbidden’ area in the ω‐k space. Thereby the beam velocity V b appears as an essential parameter. It is directly related to the phase velocity of the most unstable whistler wave mode, which is close to V Ae/2 for sufficiently hot electrons (V Ae is the electron Alfven velocity). To clarify the second point, we show that Landau‐resonant beams with V b < V Ae/2, which arise in cold plasmas from unstable upper‐band waves, are able to generate lower‐band whistler mode waves at very oblique propagation (θ ≥ 60°). Our studies demonstrate the important role of Landau‐resonant electrons in nonlinear whistler wave generation in the magnetosphere.

Ionosphere Feedback to Electron Scattering by Equatorial Whistler Mode Waves

“…Oblique whistler mode waves are damped by Landau‐resonant, suprathermal (20–1,000 eV) electrons, possessing a distribution with strong gradients in parallel velocities (see, e.g., Artemyev et al, 2016, and references therein). However, the presence of an additional field‐aligned electron population can significantly reduce this Landau damping and allow for oblique whistler mode wave generation (Gao et al, 2016; W. Li, Mourenas, et al, 2016; Mourenas et al, 2015; Sauer et al, 2020).…”

“…It depicts linear growth rate solutions for two cases, both including a primary distribution of transversely anisotropic electrons: Case F 0 does not include a field-aligned electron population, whereas case F 1 includes a low-density electron beam that forms a plateau in parallel velocities at ∼100-1,000 eV (note that there is no strong positive gradient with F 1 / v || > 0, but only F 1 / v || ≈ 0). The whistler mode wave growth rate calculated for distribution F 1 shows growth at large wave normal angles (very oblique whistler mode waves are generated near the resonance cone because of cyclotron instability and reduced Landau damping, see Mourenas et al, 2015;Sauer et al, 2020). Therefore, a realistic enhancement of the field-aligned electron population in the whistler mode wave source region can drive the generation of very oblique, quasielectrostatic whistler mode waves, which are indeed often observed in the inner magnetosphere (see statistics of such wave observations and their properties in Agapitov et al, 2013Agapitov et al, , 2018Gao et al, 2016;W.…”

Gap Formation Around 0.5Ωe in the Whistler‐Mode Waves Due To the Plateau‐Like Shape in the Parallel Electron Distribution: 2D PIC Simulations

“…Based on a theoretical analysis, Sauer et al. (2020) have indicated that the plateau electrons can cause the damping around 0.5Ω e in whistler‐mode waves. Nevertheless, they haven't talked about how the plateau electrons are produced and how the energy is transferred around 0.5Ω e are still unknown.…”

Electron Resonant Interaction With Whistler Waves Around Foreshock Transients and the Bow Shock Behind the Terminator

“…Electromagnetic whistler waves, with frequencies between lower‐hybrid frequencies and electron cyclotron frequencies, are commonly observed in planetary magnetospheres and in the solar wind. These waves are driven by various types of electron anisotropy: thermal anisotropy (e.g., C. Kennel, 1966; Sagdeev & Shafranov, 1961), flow anisotropy (e.g., Sauer et al., 2020), or heat flux (Gary & Feldman, 1977; Tong et al., 2019; Vasko et al., 2020). This type of waves plays an important role in electron scattering and heating that are commonly described within the quasi‐linear approximation (C. F. Kennel & Engelmann, 1966; Trakhtengerts, 1966; Vedenov et al., 1962).…”