The Structure and Microstructure of Rising-Tone Chorus With Frequencies Crossing at f ∼ 0.5 fce

Abstract

Intense, midnight-to-dawn sector, near-equatorial, chorus rising tones which cross frequencies of (Formula presented.) have been analyzed to determine their structures and possible substructures. Upper band ((Formula presented.)) chorus and “gap” ((Formula presented.)) chorus are examined in detail for the first time. It is found that upper band chorus and gap chorus are composed of the same structure as lower band ((Formula presented.)) chorus: they are composed of short-duration subelements, which are monochromatic with (Formula presented.). These findings have strong implications for the chorus element generation mechanism. Following Kennel and Petschek (1966, https://doi.org/10.1029/JZ071i001p00001) the overall chorus riser is most likely generated by anisotropic ((Formula presented.) > 1) ∼10–100 keV substorm-injected electrons. Assuming cyclotron resonance, the upper band chorus is generated by the low energy portion of the electron spectrum. The often-present gap at (Formula presented.) is related to Landau/cyclotron damping. This however is not the end of the story. There is another type of two-frequency chorus (called Type 2) for which the lower band is not well connected to the upper band. A Type 2 chorus reported previously by Fu et al. (2014, https://doi.org/10.1002/2014JA020364) has also been studied in detail. Both the lower band and upper band are composed of subelements which are monochromatic. Such a similar fine structure for the different type of chorus may imply a similar generation mechanism, for which the difference between them is just the energy range of resonant energetic electrons. One mechanism discussed here, generation by phase bunched electrons, will be tested in the near future. © 2022. American Geophysical Union. All Rights Reserved.