Plasmasphere is an upward extension of the ionosphere at low and middle latitudes along the closed magnetic field lines rooted in the Earth, where the trapped whistler-mode waves in the frequency range from tens to thousands of Hertz are named as plasmaspheric hiss. Since its discovery, plasmaspheric hiss has been considered an important plasma wave to drive the precipitation loss of the Van Allen radiation belt electrons. However, the origin of plasmaspheric hiss has long been under debate. There have been two classes of sources proposed for the plasmaspheric hiss: (1) background plasma noises inside the plasmasphere and (2) other plasma waves outside the plasmasphere. In 2012, the Van Allen Probes mission capable of measuring an enormous range of particles, fields and waves was launched, providing an unprecedented opportunity to investigate the origin of plasmaspheric hiss. Here we briefly review the progress on the internal sources of plasmaspheric hiss during the recent five years. It has been suggested the background plasma noises can be amplified to the plasmaspheric hiss by a combination of linear and nonlinear instabilities of energetic electrons. During geomagnetically active times, energetic electrons are transported from the magnetotail into the plasmaspheric body and plume. The linear growth rate of whistler-mode waves destabilized by these energetic electrons exhibits an analogous frequency-dependence to the observed hiss power spectral density. When the seed wave amplitude exceeds a threshold, the nonlinear instability can act and then amplify the seed wave to the observable level. Such internal sources could exist over a broad range of magnetic local times, radial distances, and other structures of the plasmasphere.
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