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A magnetic conjunction between Van Allen Probes spacecraft and the Balloon Array for Radiation-belt Relativistic Electron Losses (BARREL) reveals the simultaneous occurrence of broadband Alfvenic fluctuations and multi-timescale modulation of enhanced atmospheric X-ray bremsstrahlung emission. The properties of the Alfvenic fluctuations are used to build a model for pitch angle scattering in the outer radiation belt on electron gyro-radii scale field structures. It is shown that this scattering may lead to the transport of electrons into the loss cone over an energy range from hundreds of keV to multi-MeV on diffusive timescales on the order of hours. This process may account for modulation of atmospheric X-ray fluxes observed from balloons and constitute a significant loss process for the radiation belts.

Plain Language Summary Energetic particle populations in Earth's near-space environment pose existential risks to spacecraft supporting our modern way of life. Understanding the enigmatic rapid variability of these populations is required to mitigate these risks. Coordinated observation from National Aeronautics and Space Administration (NASA)'s Van Allen Probes and Balloon Array for Radiation-belt Relativistic Electron Losses has discovered a new mechanism through which energetic electrons in Earth's outer radiation belt may be depleted. This mechanism arises through the action of fine-scale oscillations in Earth's magnetic field that are driven to large amplitudes in space weather events known as geomagnetic storms. These oscillations, known as kinetic Alfven waves, disrupt the orbit of energetic electrons around the Earth so that they precipitate into Earth's atmosphere. Estimates based on observations and theory suggest this mechanism is a significant driver of radiation belt depletion.