Probing quiet Sun magnetism using MURaM simulations and Hinode/SP results: support for a local dynamo

¹ Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany e-mail: danilovic@mps.mpg.de
² Astronomical Observatory, Volgina 7, 11160 Belgrade 74, Serbia
³ School of Space Research, Kyung Hee University, Yougin, Gyeonggi 446-701, Korea

Received: 30 September 2009
Accepted: 23 December 2009

Abstract

Context. Owing to the limited spatial resolution and the weak polarization signal coming from the quietest regions on the Sun, the organization of the magnetic field on the smallest scales is largely unknown.

Aims. We obtain information about the magnetic flux present in the quiet Sun by comparing radiative MHD simulations with observations, with particular emphasis on the role of surface dynamo action.

Methods. We synthesized Stokes profiles on the basis of the MHD simulation results. The profiles are degraded by taking the properties of the spectropolarimeter (SP) into account onboard the Hinode satellite. We used simulation runs with different magnetic Reynolds numbers (R_m) and observations at different heliocentric angles with different levels of noise.

Results. Simulations with an imposed mixed-polarity field and R_m below the threshold for dynamo action reproduce the observed vertical flux density, but do not display a high enough horizontal flux density. Surface dynamo simulations at the highest R_m feasible at the moment yield a ratio of the horizontal and vertical flux density consistent with observational results, but the overall amplitudes are too low. Based on the properties of the local dynamo simulations, a tentative scaling of the magnetic field strength by a factor 2–3 reproduces the signal observed in the internetwork regions.

Conclusions. We find agreement with observations at different heliocentric angles. The mean field strength in internetwork implied by our analysis is roughly 170 G at the optical depth unity. Our study shows that surface dynamo could be responsible for most of the magnetic flux in the quiet Sun outside the network, given that the extrapolation to higher R_m is valid.