Cross helicity at the solar surface by simulations and observations

• A result of the quasilinear mean-field theory for driven magnetohydrodynamic (MHD) turbulence is that the observed cross helicity < u . 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)> similar or equal to 2 < u.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 kms(-1) prove to be 1 kms(-1) for weak fields. This value coincides well 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 the numerical value of eta(T) similar or equal to 10(12) cm(2)/s as characteristic for the surface of the quiet Sun.

Subject headings

NATURVETENSKAP  -- Fysik -- Astronomi, astrofysik och kosmologi (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Astronomy, Astrophysics and Cosmology (hsv//eng)

Keyword

convection

magnetohydrodynamics (MHD)

Sun: granulation

Sun: surface magnetism

Publication and Content Type

ref (subject category)

art (subject category)