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Abstract

Prior research has linked the complexity of the structured characteristics of the particles’ distributions in the Earth’s radiation belt to several competing mechanisms of acceleration, loss and transport, depending on the interactions with various magnetospheric waves and the dynamics of the plasmasphere driven by solar wind conditions.Despite the dawn-dusk asymmetry and the radial dependence of the global particle distribution, especially for electrons, has been reported to be associated with solar activities, other relevant particle distribution principles are still poorly recognized. Here we present an analytic theory to demonstrate that electrons with an initially asymmetric spatial distribution would form an Archimedean spiral distribution in the inner magnetosphere. Spectrograms of energetic electrons from Van Allen Probes have shown ubiquitous regular patterns of time-varying organized peaks and valleys in Earth's inner radiation belt, referred to as “zebra stripes”. As the manifestation of energy-dependent radial structure in the radiation belts on the spectrogram, zebra stripes can be accurately predicted by our theory of Archimedean spiral distribution. We use the Rice Convection Model (RCM) to reproduce the formation and development of the Archimedean spiral distribution and the corresponding zebra stripes in the inner radiation belt. The simulation results depict time-dependent structure and evolution of the zebra stripes, which perfectly matches the analytic theory and in good consistency with twin Van Allen Probes observations. Such evolution is a result of the gradient/curvature drift, whose angular velocity decreases with radial distance. Electric field plays a significant role in moving plasma to generate the initially asymmetric spatial distribution.