Evolution of density structures in the inner magnetosphere using coordinated Van Allen probe observations

The plasmasphere contains rich small-scale structure that plays a key role in regulating wave propagation and mass transport in the inner magnetosphere, yet the evolution of plasmaspheric density ducts remains poorly understood. We use joint observations from the Van Allen Probes to investigate how density structures change over time by identifying matched structures observed by both spacecraft during the years 2013 and 2014. For each structure, we quantify changes in density, width, and location, and assess whether the structure satisfies theoretical criteria for whistler-mode ducting. We identify 167 density structures and find that structures are usually largest and most dynamic near the plasmapause and are associated with slightly increased levels of geomagnetic activity. Although some structures exhibit minimal evolution over the spacecraft separation interval, many undergo substantial changes in both density and width, with no clear systematic trends, suggesting largely stochastic evolution. The motion of these density structures includes both outward and inward radial shifts. The vast majority of structures remain capable of ducting whistler-mode waves throughout the observation period. These results provide new insights on plasmaspheric substructure evolution and highlight the importance of multi-point measurements for understanding ionosphere-magnetosphere coupling.