EDP Sciences
Free access
Volume 494, Number 3, February II 2009
Page(s) 1091 - 1106
Section The Sun
DOI http://dx.doi.org/10.1051/0004-6361:200810448
Published online 20 November 2008
A&A 494, 1091-1106 (2009)
DOI: 10.1051/0004-6361:200810448

Dynamics of small-scale magnetic fields on the Sun: observations and numerical simulations

N. Bello González1, L. Yelles Chaouche2, O. Okunev1, 3, and F. Kneer1

1  Institut für Astrophysik, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
    e-mail: [nazaret,olok,kneer]@astro.physik.uni-goettingen.de
2  Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany
    e-mail: yelles@mps.mpg.de
3  Central Astronomical Observatory of the Russian Academy of Sciences, Pulkovskoye chaussee 65/1, 196140 St. Petersburg, Russia

Received 23 June 2008 / Accepted 18 September 2008

Context. Small-scale magnetic fields play an important role in the structure and the dynamics of the solar atmosphere.
Aims. This study aims at revealing the evolution of magnetic fields, together with granular convection in the quiet Sun by means of observations with high spatial, spectral, and temporal resolution and of numerical MHD simulations.
Methods. Time sequences from quiet Sun disc centre were obtained with the upgraded “Göttingen” Fabry-Perot spectropolarimeter at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in the $\ion{Fe}{i}$ 6173 Å line. The data were reconstructed with speckle methods. For comparison with the observations, numerical simulations of granular magnetoconvection were carried out with the MURaM code. The intensities and Stokes vectors emerging from the simulation box were degraded in wavelength, spatial co-ordinates, and noise to the quality of the observations.
Results. The noise in the observed magnetograms from the centre-of-gravity method is $\sigma_B\approx2$ G, yielding a polarimetric sensitivity of $3 \times 10^{15}$ Mx, at a cadence of 23 s with 0$\farcs$33 spatial resolution in a field of view of ${\sim}33\arcsec\times 25\arcsec$. Many of the observed V profiles in network and internetwork (IN) areas exhibit strong asymmetries that indicate strong magnetoconvection. The temporal evolutions of IN structures and of a bright point (BP), as seen in broadband and line-minimum images, in Dopplergrams, and in magnetograms, are presented. The magnetic field structure in the numerical MHD simulations is even more complex than seen in the observations. Correspondingly, the emergent Stokes profiles are often very abnormal. The degradation yields a reduction of the intrinsic field strength to the “observed” one by a factor 4–5. The spectral resolution of the spectrometer is adequate, yet the limitation in spatial resolution and by noise filtering swamps the details seen in the non-degraded simulations. A BP was not found in the simulations, presumably because BP's are not so common and the simulated box is quiet small: only 6 Mm wide in both horizontal directions.
Conclusions. The combination of high-resolution observations with numerical simulation is a highly valuable means for studying small-scale magnetic fields on the Sun. Two-dimensional, low-noise data with spectral resolution as good as achieved here and with spatial resolution of 0$\farcs$1 and better are needed to better understand this important part of the solar magnetism.

Key words: Sun: magnetic fields -- techniques: high angular resolution -- techniques: polarimetric -- methods: numerical -- methods: observational

© ESO 2009