Fundamental and harmonic electron cyclotron maser emission

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Abstract
Auroral kilometric radiation has been attributed to an electron cyclotron maser instability and is known to have components at both the cyclotron frequency and its harmonics. Solar microwave spike bursts and similar bursts from flare stars are also thought to be maser emission at harmonics of the cyclotron frequency. Fundamental X mode growth is known to dominate when ωp/Ωe ≲ 0.3. At higher ωp/Ωe the dominant radiation from the maser depends on the prevailing plasma conditions. In particular, it is shown that the dominant radiation can be (1) fundamental Z or O mode radiation or second‐harmonic X mode radiation when 0.3 ≲ ωp/Ωe ≲ 1, (2) fundamental Z mode or second‐harmonic X mode radiation when , (3) second‐harmonic O mode or third‐harmonic X mode radiation when or (4) second‐harmonic Z mode radiation when . In these ranges of ωp/Ωe X mode growth at the relevant harmonic tends to be suppressed if the distribution of the energetic electrons only has a loss cone anisotropy, whereas it tends to dominate if the distribution is peaked at large pitch angles.