Sub-packet structures in EMIC triggered emissions observed by the THEMIS probes
Thursday, 18 December 2014
We report observations of electromagnetic ion cyclotron (EMIC) rising tone emissions comprising sub-packet structures, using waveform data obtained by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes. EMIC rising tone emissions have recently attracted much attention because of their strong nonlinear interaction with energetic particles in the inner magnetosphere. We investigate two typical cases in detail, one case with a continuous single rising tone and the other characterized by a patchy emission with a broadband frequency. The degree of polarization of each sub-packet is generally higher than 0.8 with a left-handed polarization, and the wave direction of the sub-packets is typically field-aligned. We find that the observed frequency sweep rates and amplitude growth rates are consistent with estimations based on nonlinear wave growth theory. In addition, we compare the observed time duration of each sub-packet structure with the theoretical trapping time for second-order cyclotron resonance. These show reasonable agreement, indicating that individual sub-packet elements are continually generated through a triggering process which generates an element with the theoretically predicted optimum amplitude.