ON THE TEMPERATURE AND VELOCITY THROUGH THE PHOTOSPHERE OF A SUNSPOT PENUMBRA

We investigate the structure in depth of a sunspot penumbra by means o f the inversion code of the radiative transfer equation proposed by Ru iz Cobo & del Toro Iniesta (1992), applied to a set of filtergrams of a sunspot, scanning the Fe I line at 5576.1 Angstrom, with a sampling interval of 30 m Angstrom, from - 120 to 120 m Angstrom from line cent er (data previously analyzed by Title et al. 1993). The temperature st ructure of this penumbra is obtained for each of the 801 pixels select ed (0''32 x 0''32). On average, the temperatures seem to decrease as w e move inward, but the differences are of the order of the rms values (approximate to 100-200 K) at a given distance to sunspot center. The outer parts of the penumbra have also a bigger curvature in the T vers us log tau(5) relation than the inner parts. We realize, however, that these differences might be influenced by possible stray light effects . Compared to the quiet Sun, penumbral temperatures are cooler at deep layers and hotter at high layers. A mean penumbral model atmosphere i s presented. The asymmetries observed in the intensity profile (the li ne is magnetically insensitive) are deduced to be produced by strong g radients of the line-of-sight velocity that sharply vary spatially alo ng slices of almost constant distance to sunspot center. These variati ons suggest that such gradients are not only needed to explain the bro adband circular polarization observed in sunspots (see Sanchez Almeida & Lites 1992) but are a main characteristic of the fine-scale penumbr a. The results are compatible with an Evershed flow present everywhere , but its gradient with depth turns out to vary so that the flow seems to be mainly concentrated in some penumbral fibrils when studied thro ugh Dopplergrams. Finally, as by-products of this study, we put constr aints to the practical usefulness of the Eddington-Barbier relation, a nd we explain the values of the Fourier Dopplergrams to be carrying in formation of layers around the centroid of the generalized response fu nction of Dopplergrams to velocity fluctuations.