Flow geometry in a sunspot penumbra

We have measured the material How in the penumbra of a large symmetric sunspot during the passage of the spot across the solar meridian. The line-of-sight velocity field has been obtained from Doppler measurements in a Fe LI line using a filter spectrometer with a large field of view. From data sets taken on different days, i.e. at different view angles, we have reconstructed the magnitude and orientation of the penumbral How field in the deep photosphere. We find upflows near the inner and downflows at the outer boundary of the penumbra with nearly horizontal outflow in between. From our measurements we derive the following How geometry: narrow upflow channels rise at different penumbral radii, they bend outwards with a nearly horizontal outflow, and are finally tilled a few degrees downwards at the outer penumbral boundary, but still inside the spot. The flow reaches its maximum speed of about 3.5 km s(-1) in the outer part of the penumbra. Our findings are a significant step towards an understanding of the mass balance of the Evershed flow. The proposed geometry is consistent with recent numerical models of penumbral filaments.