Propagation of Whistler Waves Through Density Gradients in Earth’s Inner Magnetosphere

Whistlers are electromagnetic modes commonly observed in Earth’s magnetosphere, where their resonant interaction with electrons over a broad frequency range plays a key role in regulating radiation belt dynamics. In this case study, we analyze whistler waves propagating through regions characterized by different electron densities, namely the plasmatrough, a plasmaspheric plume, and the plasmasphere. Using high-resolution spectral measurements, wave parameters, and energy-flux data from Van Allen Probes EMFISIS suite, we characterize the evolution of the emissions as they propagate through these regions. The whistler waves exhibit higher amplitudes near the plume boundaries, where the presence of rising-tone elements indicates nonlinear wave growth. Chorus-like whistler emissions originating in the plasmatrough are reflected at the plume boundary with oblique wave-normal angles. Evidence of energy conversion between different whistler waves is also observed near the plume boundaries. These results provide new insight into the behavior of whistler waves across density gradients in the inner magnetosphere.