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

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5. The influence of the plasma effects

The effects of pressure and gravity were not included in the previous work presented in Paper II, though the results were globally satisfactory. We now carry out a linear force-free field (lfff ) extrapolation of the magnetic field (i.e. [FORMULA]) in the filament channel of the modified magnetogram, keeping constant all the parameters of the model (see Table 2), except a which is reduced to zero, to satisfy the lfff equations. The aim of this lfff extrapolation is to investigate up to what level the plasma effects are important for the magnetic configurations.

5.1. Comparison between lfff and lmhs extrapolations

The modeled lfff filament is represented on Fig. 1c. It can be compared to the lmhs filament (see Fig. 1b) and to the observed H[FORMULA] filament (see Fig. 1a). Without plasma effects, the distribution of dips in the body of the filament, the deformation of the twisted flux-tube, as well as the two components for the large foot (F1) are very close to the lmhs configuration. However, the angle between the field lines and the photospheric inversion line are much smaller in the lfff case. Such small angles are neither consistent with the observed H[FORMULA] fine structures in the body of our filament at 12:14 UT, nor with typical Hanle effect measurements of the magnetic field inside prominences (e.g. Bommier & Leroy, 1998 and references therein).

The effects of pressure and gravity are more important in the case of the isolated dips located in the filament channel. The lfff extrapolation only shows very small and dispersed features associated with dipped field lines, while these are much larger in the lmhs case (compare Fig. 1c and Fig. 1b). Moreover, some of the isolated dips which appear in lmhs are not present at all in lfff . As a consequence, there are less observed dark H[FORMULA] features that can be correlated with computed dips in lfff than in lmhs .

5.2. The importance of pressure and gravity

From this comparison between lfff and lmhs extrapolations, it can be finally concluded that the influence of pressure and gravity is of secondary importance with respect to the purely magnetic field configuration (in accordance with the conclusions of Aulanier et al., 1998b). However it is noteworthy that the plasma effects are still important in the sense that they slightly modify the magnetic configuration, leading to a better match with observations.

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999