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Abstract
Abstract. The numerical simulation of the nonlinear evolution of the parallel propagating Alfvén waves in a radially expanding plasma is performed by using a kinetic-fluid model (the Vlasov–MHD model). In our study, both the nonlinear evolution of the Alfvén waves and the radial evolution of the velocity distribution function (VDF) are treated simultaneously. On the other hand, important ion kinetic effects such as ion cyclotron damping and instabilities driven by the non-equilibrium ion velocity distributions are not included in the present model. The results indicate that the steepened Alfvén wave packets outwardly accelerate ions, which can be observed as the beam components in the interplanetary space. The energy of imposed Alfvén waves is converted into the longitudinal fluctuations by the nonlinear steepening and the nonlinear Landau damping. The wave shoaling due to the inhomogeneity of the phase velocity is also observed.
Highlights
It is well known that nonlinear Alfvén waves are ubiquitously observed in the solar wind (Bruno and Carbone, 2013)
It was shown by Araneda et al (2008) and Matteini et al (2010) that the protons trapped by the envelope modulation due to the parametric instabilities of Alfvén waves can be observed as the beam protons in the velocity space (Araneda et al, 2008)
The results indicate that the steepened Alfvén wave packets outwardly accelerate ions, which can be observed as the beam components in the interplanetary space
Summary
It is well known that nonlinear Alfvén waves are ubiquitously observed in the solar wind (Bruno and Carbone, 2013). The nonlinear damping of low-frequency, quasi-parallel propagating Alfvén waves is of great interest. It is known that the proton beams are frequently observed in solar wind plasmas (Marsch, 2006) It was shown by Araneda et al (2008) and Matteini et al (2010) that the protons trapped by the envelope modulation (or corresponding longitudinal fluctuations) due to the parametric instabilities of Alfvén waves can be observed as the beam protons in the velocity space (Araneda et al, 2008). The radial inhomogeneity of the plasma density and the magnetic field may not be negligible to discuss the nonlinear evolution of low-frequency Alfvén waves in coronal plasmas, since the radial change of the nonlinearity and the non-WKB effects such as the reflection may occur (e.g., Chandran et al, 2011)
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