Relationship Between Finger‐Like Ion Spectral Structures and Plasmaspheric Drainage Plumes: Observations and Simulations

Abstract Finger‐like ion spectral structures in energy‐time spectrograms, characterized by nearly simultaneous flux enhancements from eV to keV, are consistently observed by the Van Allen Probes within the density trough between the plasmasphere and its drainage plume. Test‐particle simulations demonstrate that these features arise from variations in magnetospheric convection, which redistribute both plasmaspheric ions and those injected from the nightside. Under enhanced convection, the plasmasphere is eroded, forming drainage plumes in the afternoon sector, while injected ions penetrate inward to populate the region outside the newly formed plasmapause. As convection subsides, the system corotates eastward with slight distortion, enabling injected ions to accumulate in the density trough and produce the observed finger‐like structures. The established spatial correspondence also suggests that low‐energy ion signatures may serve as practical indicators of plasmapause and plume boundaries when direct cold‐plasma measurements are unavailable.