Contribution of EMIC and Chorus Waves to the Formation of the Three-Belt of Ultra-Relativistic Electrons

Abstract

The Earth's radiation belts are usually composed of an inner and an outer zone. Previous studies have reported a third belt of ultra-relativistic electrons, but the mechanism requires to be further clarified. Here we report a very interesting feature of $3.4-6.3$ MeV electrons during two storms in October 2018. During the October 7 storm the electrons in the region $L\approx 3.5-5.0$ were quickly scattered by EMIC waves, leading to a storage ring left between $L\approx 3.0-3.5$. Over the following days, due to the chorus-driven acceleration, electron fluxes significantly increased in the region $L>4$, consequently allowing for a three-belt structure. During the October 13 storm the electrons in the region $L>5$ were swept away by EMIC waves, leaving a narrow outer radiation belt confined within the region $L\approx 4-5$ for several days. Numerical calculations demonstrate that the EMIC-driven loss and chorus-induced acceleration can account for the formation and evolution of the third-belt morphology.