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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 theLrange 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. 

Abstract During active geomagnetic periods both electrons and protons in the outer radiation belt have been frequently observed to penetrate to lowL(<4). Previous studies have demonstrated systematic differences in the deep penetration of the two species of particles, most notably that the penetration of protons is observed less frequently than for electrons of the same energies. A recent study by Mei et al. (2023,https://doi.org/10.1029/2022GL101921) showed that the time‐varying convection electric field contributes to the deeper penetration of low‐energy electrons and that a radial diffusion‐convection model can be used to reproduce the storm‐time penetration of lower‐energy electrons to lowerL. In this study, we analyze and provide physical explanations for the different behaviors of electrons and protons in terms of their penetration depth to lowL. A radial diffusion‐convection model is applied for the two species with coefficients that are adjusted according to the mass‐dependent relativistic effects on electron and proton drift velocity, and the different loss mechanisms included for each species. Electromagnetic ion cyclotron (EMIC) wave scattering losses for 100s of keV protons during a specific event are modeled and quantified; the results suggest that EMIC waves interacting with protons of lower energies than electrons can contribute to prevent the inward transport of the protons. 

Abstract Ultra‐low frequency (ULF) waves radially diffuse hundreds‐keV to few‐MeV electrons in the magnetosphere, as the range of drift frequencies of such electrons overlaps with the wave frequencies, leading to resonant interactions. Theoretically this process is described by analytic expressions of the resonant interactions between electrons and ULF wave modes in a background magnetic field. However, most expressions of the radial diffusion rates are derived for equatorially mirroring electrons and are based on estimates of the power of ULF waves that are obtained either from spacecraft close to the equatorial plane or from the ground but mapped to the equatorial plane. Based on recent statistical in situ observations, it was found that the wave power of magnetic fluctuations is significantly enhanced away from the magnetic equator. In this study, the distribution of the wave amplitudes as a function of magnetic latitude is compared against models simulating the natural modes of oscillation of magnetospheric field lines, with which they are found to be consistent. Energetic electrons are subsequently traced in 3D model fields that include a latitudinal dependence that is similar to measurements and to the natural modes of oscillation. Particle tracing simulations show a significant dependence of the radial transport of relativistic electrons on pitch angle, with off‐equatorial electrons experiencing considerably higher radial transport, as they interact with ULF wave fluctuations of higher amplitude than equatorial electrons. These findings point to the need for incorporating pitch‐angle‐dependent radial diffusion coefficients in global radiation belt models. 

Abstract Deep penetration of outer radiation belt electrons to lowL(<3.5) has long been recognized as an energy‐dependent phenomenon but with limited understanding. The Van Allen Probes measurements have clearly shown energy‐dependent electron penetration during geomagnetically active times, with lower energy electrons penetrating to lowerL. This study aims to improve our ability to model this phenomenon by quantitatively considering radial transport due to large‐scale azimuthal electric fields (E‐fields) as an energy‐dependent convection term added to a radial diffusion Fokker‐Planck equation. We use a modified Volland‐Stern model to represent the enhanced convection field at lowerLto match the observations of storm time values ofE‐field. We model 10–400 MeV/G electron phase space density with an energy‐dependent radial diffusion coefficient and this convection term and show that the model reproduces the observed deep penetrations well, suggesting that time‐variant azimuthalE‐fields contribute preferentially to the deep penetration of lower‐energy electrons. 

Abstract Understanding local loss processes in Earth’s radiation belts is critical to understanding their overall structure. Electromagnetic ion cyclotron waves can cause rapid loss of multi‐MeV electrons in the radiation belts. These loss effects have been observed at a range ofL* values, recently as low asL* = 3.5. Here, we present a case study of an event where a local minimum develops in multi‐MeV electron phase space density (PSD) nearL* = 3.5 and evaluate the possibility of electromagnetic ion cyclotron (EMIC) waves in contributing to the observed loss feature. Signatures of EMIC waves are shown including rapid local loss and pitch angle bite outs. Analysis of the wave power spectral density during the event shows EMIC wave occurrence at higherL* values. Using representative wave parameters, we calculate minimum resonant energies, diffusion coefficients, and simulate the evolution of electron PSD during this event. From these results, we find that O+ band EMIC waves could be contributing to the local loss feature during this event. O+ band EMIC waves are uncommon, but do occur in theseL* ranges, and therefore may be a significant driver of radiation belt dynamics under certain preconditioning of the radiation belts.