Statistical Study of Plasmaspheric Plumes Based on THEMIS Observations

A comprehensive statistical study of detached plasmaspheric plumes is presented, based on THEMIS observations from 2011 to 2022. Using electron densities derived from spacecraft potential measurements, we identify 488 detached plume crossings and investigate their spatial distribution, density characteristics, and morphological properties across the magnetosphere. The results show that plasmaspheric plumes occur primarily within L = 5 – 8.5, with the highest occurrence near L = 6 – 6.5. Additionally, plumes exhibit a pronounced dawn-dusk asymmetry, with significantly more events observed in the dusk sector than in the dawn sector, a trend that remains consistent even after accounting for the effects of orbital sampling, with the duskside showing the highest occurrence rate of plume crossings. In contrast, plumes observed on the dayside tend to extend to larger L than those in other Magnetic Local Time (MLT) sectors, reflecting the sunward wrapping of the entire plume structure as it extends from the duskside toward the dayside. The electron densities of plumes are typically concentrated in the range of 60 – 90 cm⁻³. Plume densities decrease outward from L = 5 – 6, indicating that plume plasma becomes progressively diluted as it extends outward. Density gradients within plumes are also quantified for the first time in a statistical sense. Both positive density gradients ($ {G}_{\text{pos}} $) and negative density gradients ($ {G}_{\text{neg}} $) generally decrease with increasing L and show clear dawn–dusk asymmetry, with stronger gradients on the dawnside and weaker gradients on the duskside for L > 6. Morphologically, most plumes have radial widths of ∆L = 0.2 – 1.0, and plumes on the duskside exhibit larger ∆L than those in other MLT sectors. In addition, plume electron density increases with ∆L, suggesting that plumes with smaller ∆L are accompanied by a faster reduction in electron density. These results provide a comprehensive statistical framework for understanding plume formation, evolution, and their potential roles in magnetospheric dynamics.