09:00 〜 10:30
[PEM17-P09] Development of a nonlinear gyrokinetic model of the magnetosphere-ionosphere coupling
キーワード：オーロラ、ジャイロ運動論、フィードバック不安定性
The auroral emission is known to result from collisions between neutral particles in the ionosphere and precipitating electrons from the magnetosphere. However, physical mechanisms behind a variety of auroral phenomenon have not been fully understood. For example, some fundamental questions, such as how the auroral electrons are accelerated and how the auroral structures develops, remain unresolved.
Since interactions between the magnetosphere and the ionosphere play important roles, numerous theoretical models of the magnetosphere-ionosphere (M-I) coupling system have been developed. Particularly, the feedback instability in the M-I coupling system has been discussed as a plausible model that can simultaneously address the issues mentioned above.
Since the original works in the 1970s [1,2], the feedback instability has been investigated with various models. In Refs. [3,4] the secondary growth of the Kelvin-Helmholtz instability in the M-I coupling system is investigated by nonlinear simulations using reduced magnetohydrodynamic (MHD) models, which elucidate spontaneous formation of auroral vortex structures. However, the parallel electric field, which can accelerate the auroral electrons along field lines, is not included self-consistently in the MHD models. Thus, we need to extend the M-I coupling model to include kinetic effects, which can produce and sustain the parallel electric field.
The gyrokinetic theory is relevant to describe the magnetospheric dynamics self-consistently including the parallel electric field in the low-frequency regime. In Ref. [5], the linear feedback instability is successfully formulated by means of the gyrokinetic equations for the magnetospheric plasma, where the particle acceleration by kinetic Alfven wave is confirmed in terms of the energy exchange rate through the wave-particle interaction. The dynamical evolution of the feedback M-I coupling is also simulated using a linearized gyrokinetic model [6]. However, to investigate the auroral vortex formation and the electron acceleration simultaneously, one needs to explore nonlinear evolutions of the feedback instability
In the study, therefore, we develop the gyrokinetic simulation model for the magnetosphere including nonlinear effects of the ExB convection and the parallel advection terms. For this purpose, the gyrokinetic simulation code GKV has been extended with a plug-in module for the M-I coupling. We will also discuss some details on the boundary condition for the M-I coupling.
[1] G. Atkinson, "Auroral arcs: Result of the interaction of a dynamic magnetosphere with the ionosphere," J. Geophys. Res. 75, 4746 (1970).
[2] T. Sato, "A theory of quiet auroral arcs," J. Geophys. Res. 83, 1042 (1978).
[3] T.-H. Watanabe, "Feedback instability in the magnetosphere-ionosphere coupling system: Revisited," Phys. Plasmas 17, 022904 (2010).
[4] T.-H. Watanabe, et al., "Generation of auroral turbulence through the magnetosphere–ionosphere coupling," New J. Phys. 18, 125010 (2016).
[5] T.-H. Watanabe, "A unified model of auroral arc growth and electron acceleration in the magnetosphere-ionosphere coupling," Geophys. Res. Lett. 41, 6071 (2014).
[6] S. Nishimura and R. Numata, "Linear stability analysis of feedback instability using gyrokinetic model of magnetosphere," J. Phys. Soc. Japan 90, 094901 (2021).
Since interactions between the magnetosphere and the ionosphere play important roles, numerous theoretical models of the magnetosphere-ionosphere (M-I) coupling system have been developed. Particularly, the feedback instability in the M-I coupling system has been discussed as a plausible model that can simultaneously address the issues mentioned above.
Since the original works in the 1970s [1,2], the feedback instability has been investigated with various models. In Refs. [3,4] the secondary growth of the Kelvin-Helmholtz instability in the M-I coupling system is investigated by nonlinear simulations using reduced magnetohydrodynamic (MHD) models, which elucidate spontaneous formation of auroral vortex structures. However, the parallel electric field, which can accelerate the auroral electrons along field lines, is not included self-consistently in the MHD models. Thus, we need to extend the M-I coupling model to include kinetic effects, which can produce and sustain the parallel electric field.
The gyrokinetic theory is relevant to describe the magnetospheric dynamics self-consistently including the parallel electric field in the low-frequency regime. In Ref. [5], the linear feedback instability is successfully formulated by means of the gyrokinetic equations for the magnetospheric plasma, where the particle acceleration by kinetic Alfven wave is confirmed in terms of the energy exchange rate through the wave-particle interaction. The dynamical evolution of the feedback M-I coupling is also simulated using a linearized gyrokinetic model [6]. However, to investigate the auroral vortex formation and the electron acceleration simultaneously, one needs to explore nonlinear evolutions of the feedback instability
In the study, therefore, we develop the gyrokinetic simulation model for the magnetosphere including nonlinear effects of the ExB convection and the parallel advection terms. For this purpose, the gyrokinetic simulation code GKV has been extended with a plug-in module for the M-I coupling. We will also discuss some details on the boundary condition for the M-I coupling.
[1] G. Atkinson, "Auroral arcs: Result of the interaction of a dynamic magnetosphere with the ionosphere," J. Geophys. Res. 75, 4746 (1970).
[2] T. Sato, "A theory of quiet auroral arcs," J. Geophys. Res. 83, 1042 (1978).
[3] T.-H. Watanabe, "Feedback instability in the magnetosphere-ionosphere coupling system: Revisited," Phys. Plasmas 17, 022904 (2010).
[4] T.-H. Watanabe, et al., "Generation of auroral turbulence through the magnetosphere–ionosphere coupling," New J. Phys. 18, 125010 (2016).
[5] T.-H. Watanabe, "A unified model of auroral arc growth and electron acceleration in the magnetosphere-ionosphere coupling," Geophys. Res. Lett. 41, 6071 (2014).
[6] S. Nishimura and R. Numata, "Linear stability analysis of feedback instability using gyrokinetic model of magnetosphere," J. Phys. Soc. Japan 90, 094901 (2021).