Trunk‐Like Ion Spectral Structures: Manifestation of Azimuthal Ion Dispersion in the Inner Magnetosphere

In the Earth's inner magnetosphere, the complex interplay of ion transport, acceleration, and loss processes often manifests in spacecraft observations as distinct ion spectral structures. An intriguing type of such structures, the trunk‐like structures named for their resemblance to elephant trunks in the energy‐time spectrograms of heavy ions below several keV, is characterized by enhanced fluxes at progressively lower energies toward Earth. An outstanding puzzle from Van Allen Probes observations is that these structures appear predominantly during inbound, rather than outbound, spacecraft passes. This strong inbound‐outbound asymmetry indicates that the ions in the trunk‐like structures are not dispersed solely in the radial direction; azimuthal dispersion must also be involved. We show that such dispersion arises naturally from the energy‐dependent gradient‐curvature drift motion of the ions following their sudden injection into the inner magnetosphere. Despite its simplicity, this mechanism captures the essential signatures of many observed trunk‐like structures, although additional processes may still contribute. Moreover, the same azimuthal dispersion mechanism may also account for the formation of multiple nose‐like structures observed during the outbound passes. These findings highlight how a seemingly complicated observational puzzle can be resolved with a straightforward physical explanation, enhancing our understanding of particle dynamics in the inner magnetosphere.