4:15 PM - 4:30 PM
[PEM15-04] Hybrid simulation of the instabilities driven by pickup ions
Keywords:pickup ion, instability, hybrid simulation
Neutral particles ionized by charge exchange or photoionization are subject to the electric and magnetic fields of the solar wind. This phenomenon is called pickup. These ionized particles are called pickup ions, and they move with the solar wind. Because of the gyromotion of the pickup ions, they form a ring-like distribution around the solar wind velocity in the velocity space. Since the ring distribution is unstable and excites various waves, it is known that pickup ions will evolve into a shell distribution by pitch angle scattering. The origins of pickup ions are neutral hydrogen in interstellar gas and heavy atoms and molecules emitted from planets and comets.
These pickup-ion-driven waves have been well studied, especially for a pickup proton ring distribution (Min et al. 2017), and three unstable modes are known. The AIC (Alfven-ion-cyclotron) mode grows in the direction parallel to the magnetic field. In many cases, this wave will eventually become dominant. However, it is known that it does not grow due to cyclotron damping by the solar wind protons. In contrast, the mirror mode grows in the oblique direction to the magnetic field, and it is well known for its characteristic of having zero real frequency. The IB (ion-Bernstein) mode also grows in the oblique direction, but they have finite real frequency and shorter wavelength than the mirror mode.
In this presentation, we report the results of a two-dimensional hybrid simulation of instabilities driven by a ring distribution, the purpose of this study is to investigate the mass dependence of the pickup ions for which only a few simulation results have been reported. The initial distribution of pickup ions is assumed to be a ring distribution with the parameters chosen based on the previous study (Min et al. 2017). For the case of pickup protons, the simulation results were in good agreement with the linear growth rate presented in (Min et al. 2017). After saturation of linear modes, the power gradually shifted to the long wavelength, and finally the long wavelength AIC became dominant. When the mass of the pickup ion is four times larger than that of the proton (He2+), the linear growth rate of AIC is not much different from that of the proton, while the Mirror mode and IB are greatly reduced. The simulation results confirmed the linear theory. We found that the nonlinear evolution exhibits transfer of wave energy to longer wavelength than seen in the case of pickup protons. This suggests that the dependence of the simulation box size needs to be investigated.
These instabilities are also known to be important in the downstream of shock waves. We also plan to study the dynamics of shock waves in the presence of pickup ions, focusing on the mass dependence of the pickup ions.
These pickup-ion-driven waves have been well studied, especially for a pickup proton ring distribution (Min et al. 2017), and three unstable modes are known. The AIC (Alfven-ion-cyclotron) mode grows in the direction parallel to the magnetic field. In many cases, this wave will eventually become dominant. However, it is known that it does not grow due to cyclotron damping by the solar wind protons. In contrast, the mirror mode grows in the oblique direction to the magnetic field, and it is well known for its characteristic of having zero real frequency. The IB (ion-Bernstein) mode also grows in the oblique direction, but they have finite real frequency and shorter wavelength than the mirror mode.
In this presentation, we report the results of a two-dimensional hybrid simulation of instabilities driven by a ring distribution, the purpose of this study is to investigate the mass dependence of the pickup ions for which only a few simulation results have been reported. The initial distribution of pickup ions is assumed to be a ring distribution with the parameters chosen based on the previous study (Min et al. 2017). For the case of pickup protons, the simulation results were in good agreement with the linear growth rate presented in (Min et al. 2017). After saturation of linear modes, the power gradually shifted to the long wavelength, and finally the long wavelength AIC became dominant. When the mass of the pickup ion is four times larger than that of the proton (He2+), the linear growth rate of AIC is not much different from that of the proton, while the Mirror mode and IB are greatly reduced. The simulation results confirmed the linear theory. We found that the nonlinear evolution exhibits transfer of wave energy to longer wavelength than seen in the case of pickup protons. This suggests that the dependence of the simulation box size needs to be investigated.
These instabilities are also known to be important in the downstream of shock waves. We also plan to study the dynamics of shock waves in the presence of pickup ions, focusing on the mass dependence of the pickup ions.