Universality and Diversity of Solar Winds Driven by Nonlinear
Low-Frequency Alfvén Waves from the Photosphere -Fast/Slow Winds and
Disappearance of Solar Winds-
Takeru Suzuki Submitted: 2005-10-31 15:26
(abridged) We investigate how the properties of the corona and solar wind
in the open coronal holes depend on the properties of the magnetic fields and
their footpoint motions at the surface, by perfoming 1D MHD simulations from
the photosphere to 0.3 or 0.1AU.
We impose low-frequency (<0.05Hz) 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,
and super-radial expansion of the cross section, f_max.
We find that a transonic solar wind is the universal consequence.
The atmosphere is also stably heated up to >106K
by the dissipation of the Alfvén waves through compressive-wave generation
and wave reflection in the case of the sufficient wave input with
photospheric amplitude, <dv> > 0.7km s-1.
The density, and accordingly the mass flux, of solar winds show a quite
sensitive dependence on <dv> because of an unstable aspect of the heating
by the nonlinear Alfvén waves. A case with <dv>=0.4km s-1 gives ~50 times
smaller mass flux than the fiducial case for the fast wind with <dv>=0.7km s-1;
solar wind almost disappears only if <dv> becomes half.
We also find that the solar wind speed has a positive correlation with
B/f_max, which is consistent with recent observations.
We finally 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> and B/f_max.
Our simulations naturally explain the observed (i) anticorrelation of the
solar wind speed and the coronal temperature and (ii) larger amplitude of
the Alfvénic fluctuations in the fast winds.
In Appendix, we also explain our implementation of the outgoing boundary
condition of the MHD waves with some numerical tests.
Authors: Takeru K. Suzuki & Shu-ichiro Inutsuka
Publication Status: Submitted for publication in JGR, available on astro-ph/0511006
Last Modified: 2005-10-31 15:30