Nonlinear damping of chorus emissions at local half cyclotron frequencies observed by Geotail at L > 9

We have performed a statistical analysis of upper band and lower band chorus emissions observed by the Geotail spacecraft. Chorus elements are generated near the magnetic equator (or the high-latitude minimum-B pockets in the outer magnetosphere) as discrete emissions covering a frequency range of 0.1-0.65 f(ce0), where f(ce0) is the cyclotron frequency in the generation region, with wave vectors parallel to the ambient magnetic field. As the emissions propagate away from the generation region, it is possible that they undergo the nonlinear damping at half the local cyclotron frequency 0.5 f(ce) because of the quasi-parallel propagation. The nonlinear damping can separate the emissions into the upper band and lower band chorus emissions. We have found that the lower cutoff of the upper band emissions coincides with 0.5 f(ce), while the upper cutoff of the lower band emissions represents 0.5 f(ce0). When 0.5 f(ce) exceeds 0.65 f(ce0), the chorus emission is observed as a lower band-only emission.