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Astron. Astrophys. 343, 983-989 (1999)

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1. Introduction

Dynamic phenomena on the Sun take place in three spatial dimensions. It is thus of interest to follow the processes, apart from their horizontal evolution, also along the height co-ordinate in the solar atmosphere. The present investigation deals with the variation of the granulation pattern with height in the atmosphere. This gives information on the penetration of the convective overshoot into the stably stratified photosphere.

Rodríguez Hidalgo et al. (1992) and Wilken et al. (1997, see also references in these papers), among others, used the centre-to-limb variation of high resolution images to study the height dependence of the granular phenomenon. The latter authors found, on the one hand, that the coarse granular structure fades away towards the limb. On the other hand, small-scale brightenings, presumably the bright rims of granules (de Boer et al. 1992), are still seen close to the limb, thus at larger heights in the atmosphere. Wilken et al. (1997) concluded that this observation favours the model calculations of granulation by Steffen et al. (1994) in which the bright rims are caused by penetration of hot material due to local underpressure.

Observations of the granular dynamics with slit-spectrographs (see e.g. Kneer et al. 1980, Durrant & Nesis 1982, Nesis et al. 1999, Nesis 1985, Komm et al. 1990, 1991a, 1991b, among many others) allow us to probe many atmospheric heights simultaneously by means of various formation heights of different spectroscopic features, albeit only along one horizontal co-ordinate. One conclusion with regard to the granular intensity fluctuations of many of the above studies is that the granular pattern reaches heights of only 150 km (above [FORMULA]). Higher up secondary processes are excited.

In the present paper we combine two-dimensional, high spatial resolution imaging with spectroscopy by means of a Fabry-Perot interferometer (FPI). At present, this still asks for rather involved image restoration. Therefore, we concentrate here on the outline of the methods and on images of the granulation taken in the wings of a strong Fraunhofer line. The results will enable us to directly see the granulation at different atmospheric heights and to follow, in two-dimensional images, its disappearance. The following Sect. 2 describes the instrumental setup and the observations and Sect. 3 the data analysis. We discuss temperature response functions in Sect. 4, in order to associate the images to certain "formation heights". Sect. 5 discusses the results and Sect. 6 concludes the paper with an outlook.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999