Multi-Scale Density Structures in the Plasmaspheric Plume During a Geomagnetic Storm

Evolution of large-scale and fine-scale plasmaspheric plume density structures was examined using space-ground coordinated observations of a plume during the 7-8 September 2015 storm. The large-scale plasmaspheric plume density at Van Allen Probes A was roughly proportional to the total electron content (TEC) along the satellite footprint, indicating that TEC distribution represents the large-scale plume density distribution in the magnetosphere. The plasmaspheric plume contained fine-scale density structures and subauroral polarization streams (SAPS) velocity fluctuations. High-resolution TEC data support the interpretation that the fine-scale plume structures were blobs with similar to 300 km size and similar to 500-800 m/s in the ionosphere (similar to 3,000 km size and similar to 5-8 km/s speed in the magnetosphere), emerging at the plume base and drifting to the plume. The short-baseline Global Navigation Satellite System receivers detected smaller-scale (similar to 10 km in the ionosphere, similar to 100 km in the magnetosphere) TEC gradients and their sunward drift. Fine-scale density structures were associated with enhanced phase scintillation index. Velocity fluctuations were found to be spatial structures of fine-scale SAPS flows that drifted sunward with density irregularities down to similar to 10 s of meter-scale. Fine-scale density structures followed a power law with a slope of similar to-5/3, and smaller-scale density structures developed slower than the larger-scale structures. We suggest that turbulent SAPS flows created fine-scale density structures and their cascading to smaller scales. We also found that the plume fine-scale density structures were associated with whistler-mode intensity modulation, and localized electron precipitation in the plume. Structured precipitation in the plume may contribute to ionospheric heating, SAPS velocity reduction, and conductance enhancements.