Emissions of second‐harmonic auroral kilometric radiation

Second‐harmonic auroral kilometric radiation (AKR) is discussed within the context of the cyclotron maser theory by Wu and Lee. The results are in good agreement with the ISIS 1 observation reported by Benson. The major conclusions are as follows: (1) when ω e /Ω e < 0.3 (where ω e is the electron plasma frequency and Ω e is the electron gyrofrequency), the fundamental AKR prevails over the second‐harmonic AKR; (2) when ω e /Ω e ≳ 0.3, the fundamental AKR may become weaker than the second‐harmonic AKR; (3) when ω e /Ω e > 0.3, the fundamental AKR may diminish; and (4) the second harmonic AKR with frequencies below 2Ω e can propagate downward.