The cross helicity at the solar surface by simulations and observations

Details The cross helicity at the solar surface by simulations and observations The cross helicity at the solar surface by simulations and observations

2012-02-07

arxiv.org/abs/1202.1429v2

Astronomy and Astrophysics

Astrophysics

Solar and Stellar Astrophysics

6 pages, 6 figures

Scientific paper

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 quiet 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.

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