Fast Diffusion of Ultrarelativistic Electrons in the Outer Radiation Belt: 17 March 2015 Storm Event

Inward radial diffusion driven by ULF waves has long been known to be capable of accelerating radiation belt electrons to very high energies within the heart of the belts, but more recent work has shown that radial diffusion values can be highly event‐specific, and mean values or empirical models may not capture the full significance of radial diffusion to acceleration events. Here we present an event of fast inward radial diffusion, occurring during a period following the geomagnetic storm of 17 March 2015. Ultrarelativistic electrons up to ∼8 MeV are accelerated in the absence of intense higher‐frequency plasma waves, indicating an acceleration event in the core of the outer belt driven primarily or entirely by ULF wave‐driven diffusion. We examine this fast diffusion rate along with derived radial diffusion coefficients using particle and fields instruments on the Van Allen Probes spacecraft mission. Plain Language Summary Large increases in the amount of electrons within the Earth's radiation belts can happen quite suddenly and are related to the effects of the Sun's solar wind. These changes are important since these particles can be damaging to communications and technology satellites that orbit close to Earth, at times disrupting GPS and cell phone signals or causing greater disturbances down at ground level. There are two primary mechanisms that cause the increase in high‐energy electrons that we observe with scientific satellites. This study highlights a space weather event, following the intense geomagnetic storm of March 2015, in which we have evidence of one specific type of acceleration mechanism called inward radial diffusion, and no evidence of a competing mechanism. This shows that enhancements can be caused by the one mechanism alone, which is still an open question in radiation belt physics. If we know definitively that intense enhancements can result from inward radial diffusion alone, this helps inform and improve our physics‐based forecast and prediction models of space weather. Key Points Fast radial diffusion of ultrarelativistic electrons is observed days after storm main phase Event‐specific radial diffusion can be orders of magnitude higher than statistical values ULF‐wave driven acceleration can account for intense particle enhancement observed in inner magnetosphere