Origin of the Dawnside Field-Aligned-Electron Depletion

Determine the cause of the localized dawnside region at approximately 4 less than or equal to L less than or equal to 8–9 that is typically devoid of field-aligned electrons throughout the 30–200 eV energy band, including whether it results from the drift paths accessing the region, the absence of the field-aligned-electron source mechanism there, or both.

Background

The THEMIS statistical maps identify a localized dawnside region at approximately L = 4–8–9 in which the measured electron distributions exhibit transverse anisotropy across the entire 30–200 eV band and generally lack field-aligned electrons. This depletion is distinct from the stronger field-aligned-electron populations found in the plasmasphere and in the duskside and premidnight outer magnetosphere.

The region broadens in magnetic local time and L-shell with increasing energy and appears as a pronounced reduction in the occurrence probability of moderate-to-high field-aligned anisotropy. The paper suggests two possible explanations—limited access by electron drift paths or inactivity of the field-aligned-electron source mechanism—but does not determine which explanation, or combination of explanations, is responsible.

References

Contributing to this asymmetry, and of particular interest here, is a localized region on the dawnside in which only transverse anisotropies are observed over the entire 30--200~eV band, indicated by the dark blue in Figures \ref{fig:minmaxanisotropy_lo-high_e}j and \ref{fig:minmaxanisotropy_lo-high_e}k, where ${\sim$0.67--0.96. This region is typically, though not always, devoid of FAL electrons across the full band, indicated by the values ${ \lesssim 1$ and ${ \lesssim 1$ in the lower and median quartile maps, but not in the upper quartile map. The cause is not established here. This may be a reflection of the drift paths that have access to this region, the inaction of the FAL electron source mechanism there, or both.

— A Survey of Electron Anisotropies in the Magnetosphere: Dependence on Solar Wind Pressure and Kp  (2609.28745 - Hull et al., 23 Sep 2026) in Section 3, subsection “Field-aligned anisotropies” (discussion of Figures 4j–4k); reiterated in Section 7, Summary and Conclusions, item 3