Quiet Sun magnetic fields observed by Hinode: Support for a local dynamo

Context. The Hinode mission has revealed copious amounts of horizontal flux covering the quiet Sun. Local dynamo action has been proposed to explain the presence of this flux. Aims. We sought to test whether the quiet Sun flux detected by Hinode is due to a local or the global dynamo by studying long-term variations in the polarisation signals detectable at the disc centre of the quiet Sun between November 2006 and May 2012, with particular emphasis on weak signals in the internetwork. Methods. The investigation focusses on line-integrated circular polarisation Vtot and linear polarisation LPtot profiles obtained from the Fe I 6302.5 Å absorption line in Hinode SOT/SP. Results. Both circular and linear polarisation signals show no overall variation in the fraction of selected pixels from 2006 until 2012. There is also no variation in the magnetic flux in this interval of time. The probability density functions (PDF) of the line-of-sight magnetic flux can be fitted with a power law from 1.17 × 1017 Mx to 8.53 × 1018 Mx with index α = −1.82 ± 0.02 in 2007. The variation of α’s across all years does not exceed a significance of 1σ. Linearly polarised features are also fitted with a power law, with index α = −2.60 ± 0.06 in 2007. Indices derived from linear polarisation PDFs of other years also show no significant variation. Conclusions. Our results show that the ubiquitous horizontal polarisation on the edges of bright granules seen by Hinode are invariant during the minimum of cycle 23. This supports the notion that the weak circular and linear polarisation is primarily caused by an independent local dynamo.