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Astron. Astrophys. 361, 734-742 (2000)

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5. Conclusions

The uncombed penumbral model proposed by Solanki & Montavon (1993) provides a satisfactory framework to understand recent observational results. The inversions of Westendorp Plaza and co-workers nicely fit in the description of the unresolved structure of the penumbral magnetic field proposed by the uncombed model. Similarly, the model easily predicts the linear, quadratic and rms gradients analyzed by Sánchez Almeida (1998). This is possible thanks to the rather peculiar statistics of P generated in the uncombed model. This statistics is dominated by three possible values of the gradient, a very small P over most of the atmosphere and orders-of-magnitude larger P at the tube boundaries that can be either positive or negative. But we have to bear in mind that a description of the penumbral magnetic field in terms of horizontal flux tubes with smaller field strengths and a background magnetic field can only be a first order description of a real penumbra. A generalization of this model would include a highly structured magnetic field with probably no "background field". All field lines would be part of penumbral tubes that are participating in the convective process that makes penumbrae so bright (in spite of having field strengths that are only a factor 2 smaller than umbrae). This generalization can provide a natural way to explain one of the less satisfactory results of the uncombed model. We are referring to the fact that a model where one would concentrate all Evershed flow in the horizontal tubes but put no flow in the background field (as used in Schlichenmaier et al. 1998 simulations) is totally incompatible with observations. If this were the case, the center- and limb-side NCP curves would be almost the same but with different signs (as already pointed out by SM). This is in severe contrast with the observations. The difference between these two penumbral sides can only be explained by using a background field that carries a smaller (but significant) part of the Evershed flow. In our opinion, the results of this paper are showing nothing but what would be the signs of the correlations that a more elaborated uncombed model would have. For example, if those tubes that are rising (as part of the convective process) suffer a deceleration of the outward flow when they reach the upper part of the spectral line forming layer, a net effect like the one we have modeled will result. Similarly, one can think of descending tubes that suffer an acceleration of the outward flow. Evidence for hot rising and cold sinking plasma elements has been reported recently by Schlichenmaier & Schmidt (1999). Thus, a better understanding of how energy is transported in the presence of a horizontal field and the resulting magnetic field structuring is strongly needed. It is interesting to point out that at present spectroscopic resolutions (1 ") one can have, in the volume contributing to the observations, more than 20 tubes similar to those modeled here (we have used, in our volume, only around 5 of them).

From an observational point of view, a natural next step should be an analysis of the predictions of the uncombed model by using inversion techniques. The SIR code can be complemented with a physical scenario that accounts for the unresolved nature of the penumbral structure. This scenario can be, as a first step, an uncombed model with one tube and a background field. By least-square fitting of the observed profiles, one would obtain a great deal of information that in this work has been prescribed ad-hoc. Although, as we have stated before, a model with one tube and a background field may not be realistic, it would provide useful information about the correlations that prevail in the penumbrae of sunspots.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000