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During 26-29 March 2017 magnetic storm, the Arase satellite observed typical ultra low frequency (ULF) waves and acceleration of relativistic electrons. We simulate the global distribution of these ULF waves using CRCM with BATSRUS global magnetospheric magnetohydrodynamic model. The simulation can qualitatively reproduce the ULF waves observed by Arase at frequencies of 2-3 mHz. However, the simulated ULF wave power is 1-2 orders of magnitude smaller than the observation. The simulated ULF wave activity has a good correlation with the solar-wind dynamic pressure variation, while the wave activity on the ground is enhanced even during the recovery phase, possibly due to the Kelvin-Helmholtz instability and/or substorms. We also study the 3-6 April 2017 magnetic storm, in which low ULF wave activity and weak acceleration of relativistic electrons are seen. We suggest that the existence of ULF waves plays an important role in accelerating electrons up to relativistic energies.

Plain Language Summary The Earth has the region of space that is controlled by the Earth's magnetic field, called the magnetosphere. When the temporary disturbance of the magnetosphere (magnetic storm) occurs, extremely high-energy electrons (a few MeV) accelerate around the Earth. It is considered that periodic waves with a frequency of 1-20 mHz, known as ultra low frequency (ULF) waves, play an important role in accelerating electrons up to relativistic energies. On 27 March 2017, Japanese satellite, called as "Arase," successfully observed ULF waves. We focus on this ULF waves and investigate the global activity of ULF waves using both numerical simulations and observations. The numerical simulation using in this study qualitatively reproduced the enhancement of ULF waves with a frequency range of 1.6-6.7 mHz, but its intensity is smaller than the Arase satellite observation. Comparing to ground observations, we found that the global ULF wave activity estimated by the numerical simulation is strongly affected by the dynamic pressure enhancement caused by a stream of charged particles from the Sun. In addition, we suggest that the ULF wave activity might affect the acceleration characteristics of high-energy electrons around the Earth.