Statistical Study of Plasmaspheric Plumes Based on THEMIS Observations
doi: 10.11728/cjss2026.05.2025-0240 cstr: 32142.14.cjss.2025-0240
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Abstract: 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.
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Key words:
- Plasmasphere /
- Plasmaspheric plume /
- Spatial characteristics /
- Electron density gradients
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Figure 1. Example of plasmaspheric plume observed by THEMIS-A spacecraft on 29 December 2011 in GSE coordinates. (a) Spacecraft potential measured by EFI instrument. (b) Electron density derived from spacecraft potential. (c) L. (d) Spacecraft trajectory. The solid red line represents orbit during the plume. (e) Electron density as a function of L. The red line and dots represent the plume. The red shadows in (a) –(c) indicate the time interval of the detected plasmaspheric plume
Figure 2. Locations of plasmaspheric plume events observed by the THEMIS-A and THEMIS-E spacecraft from 2011 to 2022, projected onto the GSE XY plane. Each short line segment represents a portion of the spacecraft orbit during a plume crossing. Electron density is indicated by color. The geosynchronous orbit is indicated by a dashed circle
Figure 3. Number of plasmaspheric plume events as functions of L, MLT, Ne_max, Ne_mean, ∆L, ∆MLT
Figure 4. Occurrence rate of plume events in the four MLT sectors (a). Median values of L, Ne_max, Ne_mean, ∆L, ∆MLT for plume events in each MLT sector
Figure 5. Scatter plots of Ne_max (a) and Ne_mean (c) versus L for all plume events, and the corresponding median values of Ne_max (b) and Ne_mean (d) as a function of L
Figure 6. Electron density gradients of plasmaspheric plumes in MLT-L coordinates. (a) Mean value of $ {G}_{\text{pos}} $. (b) Mean value of $ {G}_{\text{neg}} $. Black dots indicate plume event locations
Figure 7. Distributions of plume electron densities. (a) Ne_max and (b) Ne_mean are as a function of ∆L. Each dot represents a single plume event
Table 1. Characteristics of plasmaspheric plumes
Name Symbol Mean value Median value Magnetic Local Time MLT 15.34 16.84 Radial distance L 6.56 6.45 Radial width ∆L 0.56 0.40 Peak electron density Ne_max 144.26 cm−3 120.32 cm−3 Mean electron density Ne_mean 81.34 cm−3 70.95 cm−3 
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