Fine-Scale Structure in Plasmaspheric Plumes From GEO to the Magnetopause: Observations by Magnetospheric Multiscale

Plasmaspheric fine-scale structure (FSS) comprises density irregularities below 0.1–0.2 Earth radii in size. In this paper, we investigate FSS within dayside plasmaspheric plumes as they convect sunward from geosynchronous orbit to the magnetopause. We perform a statistical study of Magnetospheric Multiscale ion data, analyzing 39,018 ion moments from 122 plume events. We find FSS grows inside sunward-moving dayside plumes, increasing exponentially with 8–11 hr timescale. Spatially, FSS becomes concentrated in the outer duskside region where ion drift paths converge toward the magnetopause. We also investigate basic properties of plume ions. We confirm that plume ion temperature increases with distance, and find that for most plumes light ion densities are correlated to each other. In older plumes (18% of our database) with preferential heating of ${He}^{+}$, mesoscale and fine-scale $O^{+}$ structures grow more strongly correlated with protons, and light-ion correlation decreases. The average plume ion bulk flow is sunward and consistent with $14\%$ penetration of the solar wind electric field. From Fourier analysis, FSS scale sizes extend down to the lower limit of the instrumental sampling range, with evidence of structures below that limit but too small to measure. Plume events exhibit discrete peaks in the Fourier spectrum, but the specific peak structure changes with event. As dayside plumes age, global FSS spectral power migrates radially outward, and shifts to smaller spatial scales. Power-law fitting of density spectra suggests that turbulence is involved in generating FSS, possibly aided by convective elongation of existing structure and the gradient-drift instability.