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

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 $3.4-6.3$ MeV electrons during two storms in October 2018. During the October 7 storm the electrons in the region L≈3.5−5.0 $L\approx 3.5-5.0$ were quickly scattered by EMIC waves, leading to a storage ring left between L≈3.0−3.5 $L\approx 3.0-3.5$. Over the following days, due to the chorus‐driven acceleration, electron fluxes significantly increased in the region L>4 $L > 4$, consequently allowing for a three‐belt structure. During the October 13 storm the electrons in the region L>5 $L > 5$ were swept away by EMIC waves, leaving a narrow outer radiation belt confined within the region L≈4−5 $L\approx 4-5$ 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. Plain Language Summary: The Earth's electron radiation belts are usually composed of two high‐flux zones called the inner and outer belts, which are separated by a low‐flux slot region. However, previous studies have found that ultra‐relativistic (kinetic energy Ek>3 ${E}_{\mathrm{k}} > 3$ MeV) electrons sometimes exhibit a three‐belt morphology. Although some theories have been proposed, the mechanism underlying this rare phenomenon still requires further elucidation. Here we report a very interesting feature of 3.4–6.3 MeV electrons during two geomagnetic storms in October 2018. Prior to the first storm on October 7, the outer belt was present between the magnetic L $L$‐shell 3.0–5.0. During the storm main phase, electrons in the region L≈3.5−5.0 $L\approx 3.5-5.0$ were quickly lost by electromagnetic ion cyclotron (EMIC) waves, leaving a high‐flux zone between L≈3.0−3.5 $L\approx 3.0-3.5$. Over the following days, electrons in the region L>4 $L > 4$ were accelerated by chorus waves, consequently allowing for a three‐belt structure. During the second storm on October 13, electrons in the region L>5 $L > 5$ were swept away by EMIC waves, leaving a narrow outer belt confined within the region L≈4−5 $L\approx 4-5$ 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. Key Points: Ultra‐relativistic electrons exhibited three‐belt morphologies during storms in October 2018, corresponding to enhanced EMIC/chorus wavesCalculations show that electron loss/acceleration timescales induced by EMIC/chorus are about 0.01/10 days, comparable to observationsThe combined role of electromagnetic ion cyclotron (EMIC) and chorus waves can explain the formation and evolution of the three‐belt morphology [ABSTRACT FROM AUTHOR]