We study space environment surrounding the Earth (geospace) using a large scale computer simulations. Concerning the generation mechanism of whistler-mode chorus emissions and triggered emissions through interaction with energetic electrons, we have confirmed the nonlinear wave growth theory and the saturation mechanism at the optimum wave amplitude [1], the nonlinear theory has also been confirmed by a recent spacecraft observation [2]. The nonlinear theory has been extended for interpretation of falling-tone emissions [3]. As a numerical model of chorus emissions, we have constructed a numerical model of a chorus emission evolving in space and time [4]. We have made a review of the theoretical achievements related to whistler-mode chorus emissions[5]. Electromagnetic Ion cyclotron (EMIC) waves also observed in the same inner magnetosphere as chorus emissions. Through interaction with energetic protons, EMIC triggered emissions are generated, inducing proton precipitation [6]. We also studied interaction of relativistic electrons with the EMIC triggered emissions, which cause very effective precipitation of relativistic electrons [7].
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