Bicoherence Analysis of Oxygen Ion Cyclotron Harmonic Waves Observed by Van Allen Probes.

Bicoherence analysis is statistically performed for the oxygen ion cyclotron harmonic (OCH) waves observed by Van Allen Probes. While OCH waves and so‐called electromagnetic ion cyclotron harmonics have indistinguishable spectral characteristics in observation, they are believed to be excited by different mechanisms with the latter being suggested to arise from nonlinear wave‐wave coupling. Our result reveals that most of the OCH wave events have small bicoherence indices, so they are unlikely excited by wave‐wave coupling. The result is not much affected by the parameter choice in the bicoherence analysis, although increasing the subinterval length and/or reducing the subinterval overlap increases the chance of getting large bicoherence index values. Furthermore, the examination of two special events with large bicoherence indices shows that the observed phase relationship among different wave components does not align with the expectation from wave‐wave coupling. The events cannot be convincingly confirmed to be generated through wave‐wave coupling. Plain Language Summary: This paper presents a statistical bicoherence analysis study of the oxygen ion cyclotron harmonic (OCH) waves observed by Van Allen Probes. OCH waves have apparent features similar to electromagnetic ion cyclotron (EMIC) harmonics in observation. Previous studies have suggested that OCH waves are excited through the oxygen ion Bernstein instability but EMIC harmonics are generated by nonlinear wave‐wave coupling. Bicoherence analysis is commonly employed for assessing wave‐wave coupling. The results of our bicoherence analysis indicate that most of the OCH events have bicoherence indices less than 0.5, so they are unlikely generated through wave‐wave coupling. The study also demonstrates that the parameter choice in bicoherence analysis affects the calculated index values, but the index values remain mostly less than 0.5 for the various reasonable parameter choices tested. In addition, a detailed analysis of the phase relationship among various electric and magnetic wave components in two specific OCH events reveals that, despite their large bicoherence indices, the observed phase relationship does not align with what would be expected if the waves are generated from wave‐wave coupling. This reminds us that having a large bicoherence index is a necessary but not sufficient condition to prove that waves are generated by wave‐wave coupling. Key Points: Most of the oxygen ion cyclotron harmonic (OCH) waves observed by Van Allen Probes have small bicoherence indicesIncreasing the subinterval length and/or reducing the subinterval overlap increases the chance of getting large bicoherence index valuesExamination of two special OCH wave events with large bicoherence indices does not support them being generated from wave‐wave coupling [ABSTRACT FROM AUTHOR]