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Astron. Astrophys. 342, 867-880 (1999)

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1. Introduction

The magnetic field plays a key role in all the processes involved in the corona as in prominences because the plasma beta is low (in the range of [FORMULA] to [FORMULA]). It channels both the plasma motions and the thermal conduction. It provides support against gravity of the prominence plasma one hundred times denser than the coronal plasma. This primary role is not always recognized at its right level because the majority of observations are focused on the plasma, though Hanle effect measurements of the magnetic field inside prominences have been obtained as well (see Bommier & Leroy, 1998 and references therein). Furthermore, the importance of the magnetic field is less evident in prominences than in other phenomena since usually the prominence plasma does not directly visualize field lines as it does in arch-filament systems, surges and coronal loops.

On one hand 3-D models of the magnetic configurations are difficult to build, since the configurations are strongly non-potential and the electric current distribution is unknown. On the other hand it is challenging to understand the presence of filaments in the corona with their characteristic feet that reach the photosphere (see Démoulin, 1998). This problem also has a much larger impact on solar physics in general: it brings valuable informations on the global solar magnetic field with direct implications on dynamo theories (e.g. Berger 1998) and on the solar activity (like the launch of CMEs, see e.g. Low 1996). Consequently, knowing the typical magnetic configurations for filaments leads to consider them as local tracers of the coronal magnetic configuration. Filament observations combined with their associated longitudinal photospheric magnetic fields bring important constraints on the determination of the associated 3-D magnetic configuration.

In previous papers (Aulanier & Démoulin 1999, hereafter Paper I, and Aulanier et al., 1998, hereafter Paper II), we have shown that a twisted flux-tube is the most probable magnetic configuration supporting prominences. The model interprets many observations in a natural way (in particular the magnetic measurements in prominences and the chirality properties). Prominence feet appear as a direct consequence of the parasitic polarities present in the filament channel. In particular, we showed that the prominence lateral feet appear naturally, above secondary photospheric magnetic inversion lines and we described the morphological change of feet as the parasitic polarities evolve. We also related the topological properties of the deduced magnetic configuration (i.e. the separatrices) to chromospheric brightenings.

The purpose of this paper is to extend the previous results of Paper II. In this paper, a detailed consideration of observations give more precise constraints to the model, which is improved here. The set of observations related to the studied filament channel (H[FORMULA] data and photospheric magnetograms) is described in Sect. 2. Then in Sect. 3, we explain the method used to compute the magnetic field in the corona; it was developed by Low (1992) and now applied to observations. The present method differs from Paper II where we used magnetic charges to describe the magnetogram while here we use directly the magnetograms for the field computation. Moreover, the present model takes into account the effects of gravity and of plasma pressure, while in Paper II the magnetic field was modeled in the force-free approximation. We then describe in Sec. 4 the results of the reconstruction of the 3-D magnetic field constrained by one of the magnetograms, and compare the distribution of magnetic dips in 3-D with the morphology of the H[FORMULA] filament and its surrounding fibrils. In Sect. 5 we investigate the importance of pressure and gravity on the computed magnetic configuration. In Sect. 6 we follow the evolution of the main body of the filament, as well as one of its lateral feet and one of its surrounding group of dark fibrils, assuming a quasi-static evolution of the magnetic field. We confirm what was only suggested in Paper II, i.e. that the evolution of the H[FORMULA] dark fine structures in the filament channel is driven by the photospheric evolution of parasitic polarities. The observed vertical flow pattern obtained from H[FORMULA] Dopplershifts is then described in Sect. 7. It shows that the quasi-static assumption is justified by the observations. The Dopplershifts are then discussed in the context of the computed magnetic configuration. We summarize the results in Sect. 8, and we put them in a more general context.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999