ECMI resonance in AKR revisited: Hyperbolic resonance, harmonics, and wave–wave interaction

Abstract

Recapitulation of the resonance condition for the fundamental and higher electron cyclotron harmonics in the electron cyclotron maser instability (ECMI) enables radiation below and confirms the possibility of radiation in a narrow band above harmonics n > 1. Near n = 1 resonance on the confined lower X-mode branch, amplification is supported by the decrease in phase and group speeds. Confined slow large-amplitude quasi-electrostatic X-modes non-linearly modulate the plasma to form cavitons until self-trapped inside them at a further increasing wavenumber. They undergo wave–wave interaction, enabling escape into free space in the second harmonic band below n = 2. At a sufficiently large parallel wavenumber (oblique propagation), the fundamental resonance n = 1 is hyperbolic, a possibility so far missed but vital for an effective ECMI in the upward current region. Here, the resonance hyperbola favorably fits the loss-cone boundary, the presumably important ECMI upward-current source-electron distribution, to stimulate ECMI growth at available auroral electron energies.

Electron cyclotron maser instability by evolving fast electron beams in the flare loops

“…It may undergo resonance mode conversion or nonlinear wave-wave coupling to become an escapable radiation wave. Baumjohann and Treumann (2023) investigated the excitation of X mode, involving nonlinear wave-wave interaction and favored for its large expected growth rates. However, the issue of how the X1 mode can escape from the plasma is quite a different subject which deserves a completely separate investigation, and this is beyond the scope of the current article's main focus.…”