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

6. Evolution of chromospheric fine structures

We observed the evolution of the quiescent filament channel during 10 hours on September 25 1996, so that it is possible to investigate the evolution of the 3-D distribution of the dipped field lines. We correlate horizontal motions as well as some emergence of magnetic polarities with the displacement of the feet and the evolution of the H fine structures.

6.1. Quasi-static evolution

At a first sight, it may appear irrelevant to use the reconstruction of magnetic configurations which are in equilibrium to model an evolution. However, as long as it does not enter an eruptive phase, the filament evolution is mainly driven by the motions of photospheric magnetic fields. These polarities move with a typical speed of 0.1 to 1 km s^-1, which is very small compared to the Alfvén velocity in the corona (of the order of thousands of km s^-1) and even in prominences (a few hundreds km s^-1). Consequently, the global magnetic field is likely to evolve quasi-statically, hypothesis well justified by the observed velocities (see Sect. 7).

The twisted flux-tube which defines the main body of the filament is not expected to evolve significantly, since this part of the filament was quiescent long before and after the day of September 25. Moreover, the flux and the horizontal extension of the observed inhomogenuous bipolar background field in the magnetograms do not change significantly during this day. Consequently, the parameters , f, , and can be kept constant. From the observations we have no direct way to estimate the evolution of the plasma parameters a and H so we kept them constant. Using this conservative approach we can miss some evolution in the model, but it has the advantage that the computed dip evolution is only based on a clear observational ground: the evolution of the photospheric magnetic polarities.

6.2. Evolution of the filament channel on September 25

In this section we extrapolate the magnetic field from modified SOHO/MDI magnetograms. We compare the 3-D distribution of dipped field lines with H dark features observed with the MSDP. The observing times are given in Table 1. The results are presented on Fig. 3a-h. All the fields of view are 125 Mm125 Mm, or 172"172". Then we describe the evolving morphologies of several features. We will always refer to the time of the MSDP (H images).

Fig. 3a-h. Evolution of the filament channel on September 25 1996. a , c , e and g  show the filament channel observed with the MSDP in H at 08:43 UT, 12:14 UT, 15:57 UT and 17:04 UT respectively. b , d , f and h  show the 3-D distribution of the dipped field lines computed in lmhs (using the parameters given in Table 2) from the modified SOHO/MDI magnetogram obtained at 07:40 UT, 12:53 UT, 15:59 UT and 17:35 UT, respectively. The drawing convention is the same as in Fig. 1b,c.

6.2.1. The main body of the filament

The straight body of the filament moves away from the strong positive magnetic polarity which is present close to the photospheric inversion line, in the vicinity of N1 and N2 (see Sect. 4.2.2). It has the same polarity as the background field on the right-hand side of the inversion line (i.e. positive polarity). As this polarity decreases in flux and moves away from the main inversion line (see Fig. 4), the global distribution of dips becomes more straight along the inversion line as does the filament body in H from 08:43 UT to 17:04 UT (see Fig. 3). However, one cannot strictly associate the changes to only one polarity, they are also partly due to the evolution of the neighboring polarities.

Fig. 4a-d. Co-aligned magnetograms of the filament channel on September 25 1996. b -d  show the filament channel observed with SOHO/MDI at 07:40 UT, 12:53 UT, 15:59 UT and 17:35 UT, respectively. The box highlights the evolution of the parasitic polarities in the vicinity of the filament foot F1. Note the change in the position and strength of N1 and N2.

On the other hand, where there is a parasitic polarity, the twisted flux-tube as well as the low-altitude portions of neighboring field lines are locally bent towards the photosphere, forming new dips (e.g. the group of positive polarities at the top-left side of the filament in Fig. 3a,b). The evolution of these polarities close to the photospheric inversion line is closely related to the changing of the shape of the filament body.

6.2.2. Correlation between the feet and the parasitic polarities

In this evolution, the best modeled foot is the largest one which is at the middle-right of the filament (F1). At 08:43 UT, F1 is not well formed (see Fig. 3a,b). The two negative polarities which form it are far from one another (see Fig. 4a): N1, which is very close to the inversion line (only leading to a weak perturbation of the filament body), and the other one N2, is at approximately 20 Mm on the right (it forms an isolated dip pattern). By 12:14 UT, both polarities have moved towards each other (see Fig. 4b), leading to a large and quasi-continuous dip pattern which forms the lateral foot F1 (see Fig. 3c,d). From 15:57 UT to 17:04 UT the polarity N2 gradually moves to the north (see Fig. 4c,d). This leads to a displacement of F1 which gets more and more perpendicular to the filament (Fig. 3e-h).

Some other large and dark lateral extensions appear at different times, which have the characteristics of lateral feet. A careful look at them reveals that, even if they are not as well modeled as the foot F1, their observed main shape and evolution in H are recovered from the lmhs extrapolations (see Fig. 3). From this study the location and evolution of the lateral feet can now be correlated to those of parasitic polarities.

6.2.3. The evolution of the "M-shaped" fine structure

We leave to the reader a detailed comparison between all the observed dark features with the distribution of dips (as there are many to look at, and as we do not want to describe all these in details), taking into account the problems expressed in Appendix B.

We only describe below the evolution of one of the groups of dark fibrils which is well observed and fairly well modeled. This dark feature was previously named as S4 in Fig. 1a,b at 12:14 UT. It shows an "M-shaped" dark pattern at 08:43 UT (see Fig. 3a and the Box 4 in Fig. 5). S4 is formed by a group of weak parasitic polarities. Their maximum vertical field is approximately equal to 10 G at 08:43 UT, and they form a set of very low lying dips which appear below 2 Mm (see Fig. 2b,d). The dip pattern of S4 is not continuous, so it is not easy to recognize the M-shape, though a careful look at the orientation and location of the computed dips shows an agreement with the observed "M-shaped" structure in H. The H feature S4 evolves with time, and it is slightly shifted upwards of the represented field of view by 12:14 UT (Fig. 3c). For the later observing times, the magnetic flux of the corresponding polarities is more dispersed, though they still form some new isolated dips. This "M-shaped" fine structure is replaced by a set of H fibrils nearly parallel to the filament. The computed dips follow this trend (see Figs. 3e-h).

Fig. 5. a  is a zoom on the filament channel observed in H with the MSDP at 08:43 UT. b  is the corresponding field of view of the model taken from Fig. 3b. c  shows the H Dopplergram deduced from the MSDP data at 08:43 UT. Black (resp. white) refers to blueshifts (resp. redshifts). Dopplershifts are derived by the line bisector method at H 0.25 Å. d  shows the co-aligned SOHO/MDI magnetogram obtained at 07:40 UT. The field of views are 106 Mm73 Mm or 147"100".

© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999
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