Ultra-wideband rising-tone chorus waves observed inside the oscillating plasmapause

Previous observations indicate that there are mainly unstructured plasmaspheric hiss in the high-density plasmasphere, whereas structured whistler-mode chorus waves with the lowest frequency of 0.1f_ce (f_ce is the electron gyrofrequency) are observed mostly outside the plasmapause. Here, we observed ultra-wideband rising-tone chorus waves with frequencies extending to lower hybrid resonance frequency (f_LHR 101 Hz) in a dawnside high-density region (MLT 7 h, and N_e 75 cm^-3) inside the oscillating plasmapause. The ultra-wideband chorus waves have also typical two-band structures separated by a power gap at 0.5f_ce, but their lowest frequency (f_LHR 0.023f_ce) in the high-density region is much smaller than that of the normal chorus waves (> 0.1f_ce) in the low-density trough (N_e< 40 cm^-3). The Poynting fluxes of the waves indicate that the ultra-wideband chorus waves are excited near the magnetic equator. By comparing the linear wave growth rate to the nonlinear growth rate, we found that the ultra-wideband chorus waves are probably amplified through the nonlinear excitation mechanism.