Figuers 4 and 5 Datasets

We report the recent observation of a density duct structure in the equatorial plasmasphere, identified using data from the NASA Van Allen Probes (RBSP). The structure consists of a low-density duct located above a high-density duct, forming a "double duct" system capable of simultaneously trapping and guiding multiple whistler-mode waves at distinct frequencies. Simulations based on the electron-MHD model confirm that such density double ducts enable efficient confinement and propagation of whistler-mode waves along the background magnetic field.