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The sunspot penumbra comprises numerous thin, radially elongated filamentsthat are central for heat transport within the penumbra, but whose structure isstill not clear. To investigate the fine-scale structure of these filaments, weperform a depth-dependent inversion of spectropolarimetric data of a sunspotvery close to solar disk center obtained by Hinode (SOT/SP). We have used arecently developed spatially coupled 2D inversion scheme which allows us toanalyze the fine structure of individual penumbral filaments up to thediffraction limit of the telescope. Filaments of different sizes in all partsof penumbra display very similar magnetic field strengths, inclinations andvelocity patterns. The similarities allowed us to average all these filamentsand to extract the physical properties common to all of them. This averagefilament shows upflows associated with an upward pointing field at its inner,umbral end and along its axis, downflows along the lateral edge and strongdownflows in the outer end associated with a nearly vertical, strong anddownward pointing field. The upflowing plasma is significantly hotter than thedownflowing plasma. The hot, tear-shaped head of the averaged filament can beassociated with a penumbral grain. The central part of the filament showsnearly horizontal fields with strengths of ~1kG. The field above the filamentconverges, whereas a diverging trend is seen in the deepest layers near thehead of the filament. We put forward a unified observational picture of asunspot penumbral filament. It is consistent with such a filament being amagneto-convective cell, in line with recent MHD simulations. The uniformity ofits properties over the penumbra sets constraints on penumbral models andsimulations. The complex and inhomogeneous structure of the filament provides anatural explanation for a number of long-running controversies in theliterature.

Structure of sunspot penumbral filaments: a remarkable uniformity of properties