Nonlinear Alfvén Waves in the Radially Expanding Solar Wind Plasmas

Abstract

The anti-sunward propagating nonlinear Alfven waves are ubiquitously observed in the solar wind plasmas. The low-frequency, quasi-parallel propagating Alfven waves are robust for not only the linear collisionless damping due to the low-frequency and the small propagation angle[1], but also the turbulent perpendicular cascading among the counter propagating waves due to the high cross helicity[2]. In the homogeneous plasmas, the nonlinear collisionless damping and steepening (parallel cascading) may play an important role in the dissipation of the Alfven waves[3]. On the other hand, the inhomogeneity of the background plasmas and the background magnetic field are not negligible in the vicinity of the sun (inner heliosphere). In this study, a modified derivative nonlinear Schrodinger equation is derived from the expanding box model[4] by using the reductive perturbation method, which is different from the one used in Buti et al[5]. The effects of the inhomogeneity in the nonlinear evolution of the solar wind Alfven waves are discussed by using the model.