AUTHOR=Baumjohann W. , Treumann R. A. 

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

JOURNAL=Frontiers in Physics

VOLUME=11

YEAR=2023

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2023.1174557

DOI=10.3389/fphy.2023.1174557

ISSN=2296-424X

ABSTRACT=<p>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 <italic>n</italic> &gt; 1. Near <italic>n</italic> = 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 <italic>n</italic> = 2. At a sufficiently large parallel wavenumber (oblique propagation), the fundamental resonance <italic>n</italic> = 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.</p>