On the “Zebra Stripes” of Relativistic Electrons Unveiled by CIRBE/REPTile‐2 Measurements and Test Particle Simulations

We report multiple series of zebra stripes (aka drift echoes) of relativistic electrons, measured by the REPTile‐2 (Relativistic Electron and Proton Telescope integrated little experiment‐2) Instrument onboard CIRBE (Colorado Inner Radiation Belt Experiment) CubeSat, which operated in a highly inclined low Earth orbit from April of 2023 to October of 2024. Thanks to the high energy resolution measurements, zebra stripes of 0.25–1.4 MeV electrons, appearing as structured bands in energy spectrograms, across the entire inner belt and part of the outer belt (L=1.18 to >3) have been frequently observed, from quiet times, moderate times, to active times. Through test particle simulations, we show that a prompt electric field with a peak amplitude ∼5 mV/m in near‐Earth space can lead such zebra stripes of relativistic electrons. Azimuthal inhomogeneity of electron distribution caused by the prompt electric field modulates the electron energy spectrum by energy‐dependent drift phases to form the zebra stripes. Though zebra stripes are observed in both belts, they tend to last longer and appear more frequently in the inner belt. Zebra stripes in the outer belt tend to have a shorter lifetime due to more perturbations, resulting in energy and pitch angle diffusion of the electrons, which diminish the structure. This study demonstrates the important role of electric fields, the exact causes of which are still under investigation, in the dynamics of relativistic electrons and contributes to the understanding of the mechanisms creating and diminishing zebra stripes.