Cross helicity at the solar surface by simulations and observations
2012 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 546, A23- p.Article in journal (Refereed) Published
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.
Place, publisher, year, edition, pages
2012. Vol. 546, A23- p.
convection, magnetohydrodynamics (MHD), Sun: granulation, Sun: surface magnetism
Astronomy, Astrophysics and Cosmology
IdentifiersURN: urn:nbn:se:su:diva-82960DOI: 10.1051/0004-6361/201219268ISI: 000310349100023OAI: oai:DiVA.org:su-82960DiVA: diva2:576858