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Astron. Astrophys. 317, 942-948 (1997)

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1. Introduction

Small-scale magnetic flux tubes are an essential ingredient for the structure and the dynamics of the solar atmosphere. They constitute a substantial part of the magnetic flux which emerges from the solar surface. They are almost ubiquitous, found not only in active regions but also in the quiet Sun at downdrafts (Stenflo 1973). Small-scale magnetic elements are also intimately related to bright structures and highly dynamic processes in the chromosphere and corona of the Sun (the reader is referred to Ulmschneider et al. (1991), Solanki (1993, 1995), or Stenflo (1994) and to further references there). They possess several hundred mT fields (e.g. Beckers & Schr"oter 1968) and have diameters at the base of the solar photosphere ([FORMULA] = 1) of at most a few 100 km, i.e. at the lower limit of what modern solar telescopes can resolve. Much of the properties of small-scale flux tubes are derived, by indirect ways, from observations with low spatial resolution (e.g. Solanki & Stenflo 1984, 1985). Spectropolarimetric observations with high spatial resolution are rare. Fleck & Deubner (1991) presented the analysis of a time sequence of Stokes V profiles with high spatial resolution with emphasis on the temporal evolution of flux tubes. Keller & von der Lühe (1992) could confirm the smallness of magnetic elements, by combining broad-band polarimetry with speckle methods. Volkmer et al. (1995), performing two-dimensional spectropolarimetry with both high spatial and high wavelength resolution, could follow the horizontal migration of flux tubes and found for the first time short-period (100 s) oscillations in small-scale elements.

Very recently, Martnez Pillet et al. (1996) performed an analysis of full Stokes vector data with a spatial resolution of about [FORMULA]  from active regions on the Sun obtained with the Advanced Stokes Polarimeter at Sacramento Peak Observatory. By means of an inversion code, they derived such parameters as magnetic field strength, field orientation with respect to the solar surface, and filling factors in small-scale magnetic features. They also put much emphasis on the centre to limb variation of field strength, the velocity of the magnetic plasma, and the Stokes V, Q, and U profile asymmetries.

In this contribution, we continue the efforts by Amer & Kneer (1993) and present spectropolarimetric observations with high spatial resolution. Although spatial averaging is not avoidable due to limited telescopic resolution and to seeing it is substantially reduced compared with e.g. data from the Fourier Transform Spectrometer. High resolution observations give detailed views on the state and dynamic behaviour of the magnetized plasma and may be used for a comparison with the radiation calculated from models of flux tubes.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998