Abstract
Abstract. We review our recent results of global one-dimensional (1-D) MHD simulations for the acceleration of solar and stellar winds. We impose transverse photospheric motions corresponding to the granulations, which generate outgoing Alfvén waves. We treat the propagation and dissipation of the Alfvén waves and consequent heating from the photosphere by dynamical simulations in a self-consistent manner. Nonlinear dissipation of Alfven waves becomes quite effective owing to the stratification of the atmosphere (the outward decrease of the density). We show that the coronal heating and the solar wind acceleration in the open magnetic field regions are natural consequence of the footpoint fluctuations of the magnetic fields at the surface (photosphere). We find that the properties of the solar wind sensitively depend on the fluctuation amplitudes at the solar surface because of the nonlinearity of the Alfvén waves, and that the wind speed at 1 AU is mainly controlled by the field strength and geometry of flux tubes. Based on these results, we point out that both fast and slow solar winds can be explained by the dissipation of nonlinear Alfvén waves in a unified manner. We also discuss winds from red giant stars driven by Alfvén waves, focusing on different aspects from the solar wind.
Highlights
The Alfven wave, generated by the granulations or other surface activities, is a promising candidate operating in the heating and acceleration of solar winds from coronal holes
The low-frequency Alfven waves are generated by the footpoint fluctuations of the magnetic field lines
We have shown that the dissipation of the low-frequency Alfven waves through the generation of the compressive waves and shocks is one of the solutions for the heating and acceleration of the plasma in the coronal holes
Summary
The Alfven wave, generated by the granulations or other surface activities, is a promising candidate operating in the heating and acceleration of solar winds from coronal holes. A number of attempts have been carried out to investigate nonlinear Alfven waves in stratified solar winds in 1-D (Lau and Siregar, 1996; Boynton and Torkelsson, 1996) and 2-D (Ofman and Davila, 1997, 1998; Grappin et al, 2002), and by multi-fluid treatments (Ofman, 2004) These studies nicely explain observations by suitable choices of perturbation amplitude at the inner boundary in the corona. To consistently treat the heating of the gas, the radiative loss needs to be properly taken into account
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