[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-15] Effects of the dynamics of cold plasma on the excitation of internally driven ULF waves by ring current ions based on the magnetosphere-ionosphere coupled model

*Tomotsugu Yamakawa¹, Kanako Seki¹, Takanobu Amano¹, Yoshizumi Miyoshi², Naoko Takahashi³, Aoi Nakamizo³, Kazuhiro Yamamoto¹ (1.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:Magnetosphere-Ionosphere coupling, ULF waves, ring current, plasmasphere, drift-bounce resonance

Internally driven ULF waves are electromagnetic pulsations, which can be generated by ring current ions associated with the injection from the magnetotail during substorms. The excitation mechanism and global distribution of ULF waves are keys to understand the dynamic variation of the outer radiation belt, since ULF waves in the Pc5 range (1.6-6.7 mHz) can drive radial transport of radiation belt electrons [e.g. Elkington et al., 2003]. Theoretically, Southwood [1976] proposed the drift-bounce resonance as a candidate excitation mechanism. Previous spacecraft observations suggested both the drift resonance and drift-bounce resonance excitations of ULF waves [e.g. Dai et al., 2013; Takahashi et al., 2018]. Recently, Yamakawa et al. [2022] could reproduce two types of internally driven ULF waves on the dayside and the duskside based on the magnetosphere-ionosphere coupled model. The coupled model could reproduce standing Alfven waves with a long duration. However, this simulation was performed under the condition of constant density and how the dynamics of cold plasma affects the excitation of ULF waves is far from understood.
We investigated the excitation of internally driven ULF waves based on the magnetosphere-ionosphere coupled model between GEMSIS-RC [Amano et al., 2011] and GEMSIS-POT [Nakamizo et al., 2012]. GEMSIS-RC model solves 5-D drift-kinetic equation for the PSD of ions and Maxwell equations self-consistently. GEMSIS-POT is a 2-D potential solver in the ionosphere. In this study, we developed the simulation code for updating the density of cold plasma in GEMSIS-RC model in order to include the dynamics of cold plasma. We compared simulation results between two cases; Case a (constant density, [Yamakawa et al. [2022]) and Case b (including the dynamics of cold plasma).
Simulation results have shown the excitation of two types of Pc5 ULF waves for both cases. First, we find the drift resonance excitation of Pc5 waves on the dayside. They are driven by the positive energy gradient of the PSD of ions with the energy of 50-120 keV. Second, Pc5 waves are generated by the drift-bounce resonance on the duskside. They are driven by inward gradient of ion PSD. Unlike in Case a, localized eastward propagating Pc4 waves are excited by the drift resonance on the dawnside associated with the azimuthal density gradient in Case b, which are driven by outward gradient of PSD about 10 keV. We also detect fundamental Pc4-Pc5 waves near the plasmapause on the nightside in the drift resonance with 100-150 keV ions in Case b. We will report on the effects of the plasmapause on the instability of ion PSD and the excitation of ULF waves.