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- Ruediger, G., et al.
(författare)
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Cross helicity at the solar surface by simulations and observations
- 2012
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 546, s. A23-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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