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The ultrarelativistic electrons (E>similar to 3MeV) in the outer radiation belt received limited attention in the past due to sparse measurements. Nowadays, the Van Allen Probes measurements of ultrarelativistic electrons with high energy resolution provide an unprecedented opportunity to study the dynamics of this population. In this study, using data from the Van Allen Probes, we report significant flux enhancements of ultrarelativistic electrons with energies up to 7.7MeV during a small to moderate geomagnetic storm. The underlying physical mechanisms are investigated by analyzing and simulating the evolution of electron phase space density. The results suggest that during this storm, the acceleration mechanism for ultrarelativistic electrons in the outer belt is energy-dependent: local acceleration plays the most important role in the flux enhancements of similar to 3-5MeV electrons, while inward radial diffusion is the main acceleration mechanism for similar to 7MeV electrons at the center of the outer radiation belt.

Plain Language Summary The Earth's radiation belt electrons exhibit frequent flux variations under the effect of various physical mechanisms. The inward radial diffusion and local acceleration by whistler mode waves are recognized to be the most important acceleration mechanisms for outer belt electrons, while their effectiveness and relative importance are still under considerable debate. In the past decades, intensive studies have been performed on the acceleration of energetic and relativistic electrons (hundreds of keV to similar to 2MeV). However, ultrarelativistic electrons (>similar to 3MeV) received limited attention due to sparse measurements. The Van Allen Probes provide high-resolution measurements of ultrarelativistic electrons and thus an unprecedented opportunity to study the dynamics of this population. In this study, using data from Van Allen Probes, we report significant flux enhancements of ultrarelativistic electrons with energies up to 7.7MeV during a small to moderate storm. The analysis and simulation of electron phase space density evolution indicate that the acceleration mechanism for ultrarelativistic electrons during this storm is energy-dependent: local acceleration plays the most important role for similar to 3-5MeV electron acceleration, while the inward radial diffusion is the most important mechanism causing similar to 7MeV electron acceleration at the center of outer belt.