Electron Acceleration by Magnetosonic Waves in the Deep Inner Belt (L = 1.5–2) Region During Geomagnetic Storm of August 2018.

The relativistic electron acceleration in the inner radiation belt have received little attention in the past due to sparse measurements. The 90° minimum pitch angle distributions of electrons in extremely low L‐shells during storms are speculated to be preferentially heated by fast magnetosonic waves. The high‐quality measurements of relativistic electrons by instruments onboard Van Allen Probes and ZH–1 provide a great opportunity to investigate the dynamics of relativistic electrons in the inner radiation belt. At extremely low L‐shells (L < $< $ 2), a weak flux enhancement (increased by 2–3 times) of 100s keV electrons and the corresponding formation of butterfly PADs are observed during the major geomagnetic storm (minimum Dst ≈ −179 nT) occurred in August 2018. Simultaneously the magnetosonic waves, accompanied with weak hiss waves, were observed also in such low L‐shells. Combining with a numerical simulation of wave and particle interaction model, magnetosonic waves are thought to play an important role in electron acceleration and formation of butterfly PADs during this storm, which is the first direct observation in such low L‐shells. Key Points: Electron evolution was investigated in extremely low L‐shells (L < $< $ 2) during a storm based on ZH–1 and Van Allen Probes joint observationsFormation of 90° minimum pitch angle distributions and a weak flux enhancement of 100s keV electrons appear at storm timeThe magnetosonic wave is shown to be heating the electrons in the deep inner belt [ABSTRACT FROM AUTHOR]