Ultrawideband Rising‐Tone Chorus Waves Observed Inside the Oscillating Plasmapause

Abstract

AbstractPrevious 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 0.1fce (fce is the electron gyrofrequency) are observed mostly outside the plasmapause. Here we observed ultrawideband rising‐tone chorus waves with frequencies extending to lower hybrid resonance frequency (fLHR ~ 101 Hz) in a dawnside high‐density region (magnetic local time [MLT] ~ 7 hr and Ne ~ 75 cm−3) inside the oscillating plasmapause. The ultrawideband chorus waves have also typical two‐band structures separated by a power gap at 0.5fce, but their lowest frequency (fLHR ~ 0.023fce) in the high‐density region is much smaller than that of the normal chorus waves (>0.1fce) in the low‐density trough (Ne < 40 cm−3). The Poynting fluxes of the waves indicate that the ultrawideband chorus waves are excited near the magnetic equator. By comparing the linear wave growth rate to the nonlinear growth rate, we found that the ultrawideband chorus waves are probably amplified through the nonlinear excitation mechanism.