Hybrid simulations of parallel and oblique electromagnetic alpha/proton instabilities in the solar wind Q. M. Lu School of Earth and Space Science, Univ. of Science and Technology of China, Hefei, Anhui , P.R. China Abstract Hybrid simulations are performed to investigate the nonlinear evolution of the parallel and oblique electromagnetic alpha/proton instabilities in the fast solar wind. Both the magnetosonic and Alfven waves are unstable for oblique alpha/proton instabilities, and the threshold of the Alfven waves is lower than that of the magnetosonic waves. The influences of the temperature anisotropy of alpha particles and temperature anisotropy of protons on both the magnetosonic waves and Alfven waves are considered in the simulations. Both the magnetosonic and Alfven waves can be significantly enhanced when the temperature anisotropy of alpha particles is less than 1, while the temperature anisotropy of protons will reduce these two modes. Introduction Simulation Model Results Summary Among the minor ions in the fast solar wind, alpha particles are the most common species with an average abundance around 5% and they flow faster than the core protons with the relative drift velocity around the local Alfven speed. Because of the small concentrations of alpha particles and other minor ions in the solar wind, their contributions to electromagnetic waves are often ignored. However, observations have shown significant deceleration of alpha particles in the fast solar wind, and the contributions of alpha particles to electromagnetic instabilities have recently paid attentions. Similar to proton/proton instabilities discussed above, both magnetosonic and oblique Alfven waves can be excited by alpha/proton instabilities. Using cold plasma theory, the velocity threshold of the alpha/proton magnetosonic instabilities is larger than the local Alfven speed. However, with the linear Vlasov theory Li and Habbal [2000] showed that the threshold can be significantly reduced to be smaller than the Alfven speed when the temperature anisotropy of alpha particles is less than 1. The nonlinear left-hand polarization waves are also found to have effect on the magnetosonic waves. With the linear Vlasov theory and two-dimensional hybrid simulations Gary et al. [2000] also studied the electromagnetic alpha/proton instabilities, and they found that both magnetosonic and Alfven modes with properties similar to that of proton/proton instabilities. The magnetosonic waves are dominated for large plasma beta, while the oblique Alfven waves operate when plasma beta is small. In this paper, with one- dimensional (1-D) simulations we investigate the nonlinear evolution of parallel and oblique alpha/proton instabilities in the solar wind, considering the influences of the temperatures anisotropies of protons and alpha particles. Both protons and alpha particles satisfy bi-Maxwellian velocity distribution with a drift speed parallel to the background magnetic field, and they have same thermal velocity in the direction parallel to the ambient magnetic field. The drift speed is 1.76 times of the local Alfven speed. Periodic boundary conditions for the particles and fields are used in the simulations. The hybrid simulations are performed in the x direction. The parallel proton plasma beta is The simulations are performed in the center-of-mass frame. In this paper, the nonlinear evolution of the parallel and oblique alpha/proton instabilities is investigated. Consistent with the linear Vlasov theory, in our simulations both the magnetosonic and oblique Alfven are excited, and the oblique Alfven waves can decelerate the alpha particles into much smaller average velocity. The temperature anisotropies of proton and alpha particles on the alpha/proton instabilities are also considered in this paper. When the temperature anisotropy of the alpha particles is 0.6, the amplitudes of both the magnetosonic waves and oblique Alfven waves are enhanced, and can decelerate alpha particles more effectively. The temperature anisotropy of protons will reduce the amplitudes of both the magnetosonic waves and oblique Alfven waves, and thus decelerates alpha particles with less efficiency. Fig. 3: The time evolution of (a) the relative velocity between protons and alpha particles, (b) the temperature anisotropy of the alpha particles, and (c) the temperature anisotropy of protons for cases (A), (B) and (C), respectively. The deceleration of alpha particles can be thought to consist of two stages. The first stage is due to the scattering of the magnetosonic waves, and the second is due to the oblique Alfven waves. Fig. 2: the time evolution of the amplitude of (a) the magnetosonic waves and (b) the Alfven waves for cases (A), (B) and (C). For oblique alpha/proton instabilities, both magnetosonic and Alfven waves can be excited. The inverse temperature anisotropy of alpha particle enhance both the magnetosonic and Alfven waves, while the temperature anisotropy of protons tends to reduce them. Fig. 1 The linear Vlasov theory predicted that parallel alpha/proton instabilities can be excited magnetosonic waves, while oblique alpha/proton instabilities can excited both magnetosonic and Alfven waves. In this paper with hybrid simulations we investigate the nonlinear evolution of parallel and oblique alpha/proton instabilities, and the effects of the temperature anisotropies of protons and alpha particles are considered. Three sets of parameters are used: in case (A) both the proton temperature anisotropy and temperature asnisotropy of alpha particles are 1, (B) the proton temperature anisotropy is 1, the temperature anisotropy is 0.6, and (C) the proton temperature anisotropy is 2, the temperature anisotropy is 1. 1.Parallel alpha/proton instabilities 2. Oblique alpha/proton instabilities with angle 30 0 Fig. 1: The time evolution of (a) the amplitude of the fluctuating magnetic field, (b) the relative velocity between protons and alpha particles, (c) the temperature anisotropy of alpha particles, and (d) the temperature anisotropy of protons. In the figure, the solid, dash and dot lines represent cases (A), (B) and (C). For parallel alpha/proton instabilities, only the magnetosonic waves can be excited, and it can scatter alpha particle and decelerate them. The inverse temperature anisotropy of alpha particle enhance the waves, while the temperature anisotropy of protons reduce them. References Li, X., and S. R. Habbal, JGR, 105, 7583, 2000 Gary, S. P. et al., GRL, 27, 1355, 2000.