Parametric instability in the solar wind: numerical study of the nonlinear evolution

摘要

A possible mechanism to explain the decrease of the Alfvenic correlation with the distance from the sun, observed in fast speed streams in the solar wind, is the parametric instability. Starting from a circularly polarized Alfven wave, this instability naturally produces backward propagating waves and compressive fluctuations, by destroying the initial correlation of the wave. However, the applicability of this mechanism to the solar wind is debatable, since it works better when the plasma beta is much lower than 1 and the initial wave is monochromatic. To elucidate better this phenomenon, we numerically simulated the propagation of a turbulent spectrum of Alfven waves on a uniform background magnetic "field by using a pseudo-spectral, one dimensional, MHD code. We used values of the plasma beta about one and very high values of the physical diffusivities, by using "artificial" numerical diffusivities to ensure the stability of the numerical scheme. We found that, even under such conditions, the initial alfvenic correlation of the waves is progressively destroyed. At the saturation of the instability, the forward and inward propagating waves have comparable energies in the spectra, at the larger scales, while the forward propagating fluctuations dominate the spectrum at smaller scales. The two spectra tend to approach each to the other at subsequent times. A tentative comparison of these results with the observations of the alfven waves present in the solar wind was carried out, with good qualitative agreement.