Local Evolution of a Prenoon Drainage Plume

Abstract Plasmaspheric drainage plumes are typically observed in the postnoon sector during geomagnetic storms, but their local evolution in the prenoon sector remains poorly understood. Here, we report in situ observations from the Van Allen Probe, revealing multiple localized enhancements in cold electron density near apogee in the prenoon sector. These signatures are consistent with a spatiotemporally fluctuating narrow drainage plume intersecting the spacecraft trajectory. Global simulations that couple the plasmasphere, ring current, and magnetohydrodynamic models reproduce these density peaks and confirm their origin as a narrow plume evolved locally in the prenoon sector. The plume emerged from the sunward transformation of a pre‐existing dusk bulge, transported eastward by co‐rotation, under sustained enhanced convection. This work provides the first combined observational and modeling evidence of a prenoon drainage plume, and highlights its potential impact on magnetospheric dynamics, including lowering the dayside reconnection rate, plasma wave generation, and wave–particle interactions.