Whistler‐Mode Waves in the Density Double Ducts

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.