Solar winds driven by nonlinear low-frequency Alfvén waves from the photosphere: Parametric study for fast/slow winds and disappearance of solar winds

摘要

We investigate how properties of the corona and solar wind in open coronal holes depend on properties of magnetic fields and their footpoint motions at the surface. We perform one-dimensional magnetohydrodynamical (MHD) simulations for the heating and the acceleration in coronal holes by low-frequency Alfvén waves from the photosphere to 0.3 or 0.1 AU. We impose low-frequency (0.05 Hz) transverse fluctuations of the field lines at the photosphere with various amplitude, spectrum, and polarization in the open flux tubes with different photospheric field strength, B r,0, and superradial expansion of the cross section, f max. We find that transonic solar winds are universal consequences. The atmosphere is also stably heated up to 106 K by the dissipation of the Alfvén waves through compressive-wave generation and wave reflection in the cases of the sufficient wave input with photospheric amplitude, dv ,0 0.7 km s~-1. The density, and accordingly the mass flux, of solar winds show a quite sensitive dependence on dv ,0 because of an unstable aspect of the heating by the nonlinear Alfvén waves. A case with < dv ⊥0> = 0.4 km s~-1 gives 50 times smaller mass flux than the fiducial case for the fast wind with < dv ⊥0 >= 0.7 km s~-1; solar wind virtually disappears only if dv ,0 becomes 1/2. We also find that the solar wind speed has a positive correlation with B r,0/f max, which is consist ent with recent observations by Kojima et al. On the basis of these findings, we show that both fast and slow solar winds can be explained by the single process, the dissipation of the low-frequency Alfvén waves, with different sets of dv ,0 and B r,0/f max. Our simulations naturally explain the observed (1) anticorrelation of the solar wind speed and the coronal temperature and (2) larger amplitude of Alfvénic fluctuations in the fast wind. In Appendix A, we also explain our implementation of the outgoing boundary condition of the MHD waves with some numerical tests.