Characteristics of “Zebra Stripes” of Relativistic Electrons Unveiled by CIRBE/REPTile-2 Measurements and Test Particle Simulations

Abstract

Drift periodic echoes of electrons in the inner belt appear as structured bands in energy spectrograms, also known as “zebra stripes”. Such phenomenon is normally observed at energies from 10s of keV to ∼250 keV. We report multiple series of zebra stripes of relativistic electrons observed by the recent Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. The high energy resolution measurements taken by the REPTile-2 (Relativistic Electron and Proton Telescope integrated little experiment-2) instrument onboard CIRBE show that zebra stripes of radiation belt electrons can be observed from 300 keV to >1 MeV, crossing the L range from 1.18 to >3, from quiet times to storm times. Through test particle simulations, we show that a prompt electric field with a peak amplitude ∼5 mV/m in near-Earth space can trigger 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 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 will have a shorter lifetime due to more perturbations there, including energy and pitch-angle diffusion, which diminish the structure. This study demonstrates the important role of electric fields in the dynamics of relativistic electrons and contributes to the understanding of the mechanisms creating and diminishing zebra stripes.