Electron energization by static and moving series of double-layers in the Earth's magnetosphere

Current models for the energization of radiation belt electrons by whistler-mode chorus waves up to the observed energies of around multi-MeV require seed electron populations with energies of tens of keV. The origin of this seed population remains an open key question in magnetospheric physics. Electrostatic double layers (DLs), frequently observed along magnetic field lines by the Van Allen Probes, have been proposed as a candidate mechanism capable of accelerating electrons from eVs to keVs energies through cumulative parallel potential. However, spacecraft time-series measurements integrate over spatially distributed structures, and the actual energy gain experienced by a single electron during one field-line traversal has not been rigorously quantified. We develop a fully relativistic, test-particle simulation model in a dipolar magnetic field to investigate electron energization by an ensemble of DLs. We demonstrate that an individual electron can be efficiently accelerated from 20~eV to approximately 15~keV in a single equatorial traversal through step-wise interactions with successive DLs, placing its energy within the range required for seed populations of chorus-driven relativistic acceleration. We find that for counter-propagating DL series, the net energization decreases systematically with increasing DL speed. The analysis further shows that the main parameter controlling energization is the ratio of the propagation speed of the DL to the electron parallel velocity. These results establish that spacecraft-inferred cumulative potentials represent upper bounds on single-particle energy gain, and that moving DL series provide a viable single-particle pathway toward the tens-of-keV energy range required for subsequent chorus-driven relativistic acceleration.