[P-EM11] Dynamics of the Inner Magnetospheric System

[PEM11-07] Excitation of two types of storm-time Pc5 ULF waves based on the magnetosphere-ionosphere coupled model

*Tomotsugu Yamakawa¹, Kanako Seki¹, Takanobu Amano¹, Yoshizumi Miyoshi², Naoko Takahashi³, Aoi Nakamizo³ (1.Department of Earth and Planetary Science, School of Science, The University of Tokyo, 2.Institute for Space-Earth Environmental Research, Nagoya University, 3.National Institute of Information and Communications Technology)

Keywords:ring current, ULF wave, drift-bounce resonance, drift-kinetic model, magnetosphere-ionosphere coupling

Storm-time Pc5 ULF waves are electromagnetic pulsations (1.67-6.67 mHz), which can be generated by ring current ions associated with the injection from the magnetotail during substorms. Since Pc5 waves can drive radial transport of radiation belt electrons [e.g. Elkington et al., 2003], the excitation and global distribution of ULF waves are keys to understand the dynamic variation in the inner magnetosphere. Theoretically, Southwood [1976] proposed that the drift-bounce resonance is a candidate excitation mechanism. Previous spacecraft observations have suggested the excitation of ULF waves through the resonance [e.g. Dai et al., 2013; Oimatsu et al., 2018]. Recently, Yamakawa et al. [2020] could reproduce the drift-bounce resonance excitation of ULF waves in the global drift-kinetic simulation in the inner magnetosphere. However, the model does not reproduce standing Alfven waves associated with the field line resonance and the amplitude of the excited ULF waves is too small compared to spacecraft observations. One possible reason is the damping of field fluctuations at the ionospheric boundary in the model.
In order to improve the ionospheric boundary condition, we have implemented Magnetosphere-Ionosphere coupling between GEMSIS-RC [Amano et al., 2011] and GEMSIS-POT model [Nakamizo et al., 2012]. GEMSIS-RC model solves 5-D drift-kinetic equation for the phase space density (PSD) of ions and Maxwell equations self-consistently in the inner magnetosphere. GEMSIS-POT is a global potential solver in the ionospheric. We use FAC from GEMSIS-RC as an input to GEMSIS-POT for the Region 2 current. The resultant electric field potential is then used as the ionospheric boundary condition of GEMSIS-RC. The coupled model enables us to simulate ion injection from the plasma sheet into the inner magnetosphere and reproduce the field line resonance excitation of Alfven waves.
Simulation results show the excitation of two types of Pc5 waves. First, we find the drift resonance excitation of Pc5 waves in the dayside. They are driven by positive energy gradient of the PSD at 50-100 keV. Another type of Pc5 is the second harmonic Pc5 waves in the duskside (drift-bounce resonance). We find that the excited waves are driven by inward gradient of the PSD. We will also report on shielding effects by the Region-2 FAC on the excitation of ULF waves and what determines the Pc5 properties such as wave frequency and the azimuthal waves number.