Lower-Band “Monochromatic” Chorus Riser Subelement/Wave Packet Observations

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Abstract
Three lower-band (f < 0.5 fce) chorus riser elements detected in the dayside generation region were studied in detail using the Van Allen Probe data. Two subelements/wave packets within each riser were examined for their wave “frequency” constancy within seven consecutive wave cycles. The seven wave cycles contained the maximum amplitudes of the subelements/packets. Maximum variance B1 zero crossings were used for the identification of wave cycle start and stop times. It is found that the frequency is constant to within ~3\% (one standard deviation), with no evidence of upward frequency sweeping over the seven cycles. Continuous wavelet power spectra for the duration of the seven cycles confirm this conclusion. The implication is that a chorus riser element is composed of coherent approximately “monochromatic” steps instead of a gradual sweep in frequency over the whole element. There was no upward frequency stepping where the wave amplitude was the largest, contrary to the sideband theory prediction. It is shown that a chorus riser involves instability of cyclotron resonant energetic electrons from ~6 to ~40 keV at L = 5.8, that is, essentially the whole substorm electron energy spectrum. The above findings may have important consequences for possible wave generation mechanisms. Some new ideas for mechanisms are suggested in conclusion.
Notes
Year of Publication
2020
Journal
Journal of Geophysical Research: Space Physics
Volume
125
Number
Number of Pages
e2020JA028090
Date Published
09/2020
URL
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2020JA028090
DOI
https://doi.org/10.1029/2020JA028090