Statistical Study of Plasmaspheric Plumes based on THEMIS observations

We present a statistical survey of 488 detached plasmaspheric plume crossings identified by THEMIS from 2011 to 2022, characterizing their spatial distribution, densities and morphological properties. Plumes occur primarily at L = 5 – 8.5 and those on the duskside exhibit the highest densities, occurrence rates, and radial widths (∆L) among all MLT sectors. Dayside plumes extend to larger L than those in other MLT sectors, indicating a sunward-wrapping geometry originating from the duskside. The electron densities of plumes typically concentrate in the range of 60 – 90 cm⁻³. Plume densities decrease outward from L = 5 – 6, indicating progressive dilution with radial extension. Both positive gradients (Gpos ) and negative gradients (Gneg ) decrease systematically with L while showing a pronounced dawn–dusk asymmetry, with stronger gradients on the dawnside and weaker gradients on the duskside for L > 6. The radial widths of most plumes are within the range of ∆L = 0.2 – 1.0. Plume electron density increases with ∆L, suggesting that narrower plume is accompanied by a faster reduction in electron density. These results provide a comprehensive statistical framework for understanding plume formation, evolution, and their role in magnetospheric dynamics.