G. Ruediger, M. Kueker, R. S. Schnerr
The quasilinear mean-field theory for driven MHD turbulence leads to the
result that the observed cross helicity >\vec{u}\cdot \vec{b}> may directly
yield the magnetic eddy diffusivity \eta_T of the quit Sun. In order to model
the cross helicity at the solar surface, magnetoconvection under the presence
of a vertical large-scale magnetic field is simulated with the nonlinear MHD
code NIRVANA. The very robust result of the calculations is that < u_z
b_z>\simeq 2 <\vec{u}\cdot \vec{b}> independent of the applied magnetic field
amplitude. The correlation coefficient for the cross helicity is about 10%. Of
similar robustness is the finding that the rms value of the magnetic
perturbations exceeds the mean-field amplitude (only) by a factor of five. The
characteristic helicity speed u_\eta as the ratio of the eddy diffusivity and
the density scale height for an isothermal sound velocity of 6.6 km/s proves to
be 1.1 km/s. This value well coincides with empirical results obtained from the
data of the HINODE satellite and the Swedish 1-m Solar Telescope (SST)
providing the cross helicity component < u_z b_z>. Both simulations and
observations thus lead to a numerical value of \eta_T \simeq 1.1 x 10^12 cm^2
/s as characteristic for the surface of the quiet Sun.
View original:
http://arxiv.org/abs/1202.1429
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