THIN THREADS OF SOLAR FILAMENTS 1. Introduction Solar filaments ...

Solar Physics (2005) 226: 239–254 DOI: 10.1007/s11207-005-6876-3

C

Springer 2005

THIN THREADS OF SOLAR FILAMENTS YONG LIN1 , ODDBJØRN ENGVOLD1 , LUC ROUPPE VAN DER VOORT1,2 , JUN ELIN WIIK1 and THOMAS E. BERGER3 1 Institute

of Theoretical Astrophysics, University of Oslo, P.O. Box 1029, Blindern, N-0315 Oslo, Norway 2 Center of Mathematics for Applications, University of Oslo, P.O. Box 1053 Blindern, N-0316 Oslo, Norway 3 Lockheed Martin Corp., 3251 Hanover St., Palo Alto, CA 94304, U.S.A. (e-mail: [email protected])

(Received 8 October 2004; accepted 18 November 2004)

Abstract. High-resolution images obtained in Hα with the new Swedish Solar Telescope at La Palma, Spain, have been used for studies of fine-scale threads in solar filaments. The widths of the thin threads are ≤0.3 arc sec. The fact that the width of the thinnest threads is comparable to the diffraction limit of the telescope of about 0.14 arc sec, at the wavelength of Hα, suggests that even thinner threads may exist. Assuming that the threads represent thin magnetic strings, we conclude that only a small fraction of these are filled with observable absorbing plasma, at a given time. The absorbing plasma is continuously flowing along the thread structures at velocities 15 ± 10 km s−1 , which suggests that the flows must be field-aligned. In one case where a bundle of thin threads appears to be rooted in the nearby photosphere, we find that the individual threads connects with intergranular, dark lanes in the photosphere. We do not find signs of typical network fields at the ‘roots’ of the fine threads, as normally evidenced by bright points in associated G-band images. It is suggested that filament threads are rooted in relatively weak magnetic fields.

1. Introduction Solar filaments provide, indirectly, information about the magnetic fields in the central regions of the large complex of filament channels consisting of magnetic arcades that bridge regions of opposite magnetic polarity in the photosphere. The small-scale nature of these magnetic fields is evidenced from high-resolution observations showing numerous fine threads in prominences beyond the limb of the Sun (Dunn, 1960; Menzel and Wolbach, 1960a,b; Engvold, 1976; de Boer, Stellmacher, and Wiehr, 1998). It is well-established observationally that the entire filament bodies are composed of numerous fine threads (Engvold, 2001; Martin, 2001), that run largely horizontal and at angles of 20–25◦ relative to the long axis of the filaments and filaments channels themselves (cf., Tandberg-Hanssen, 1995; and references therein). At semi-regular positions along the filaments, bundles of the threads (barbs) divert from the main body, into photospheric regions within the channel (Martin, 1998). The filament plasma is seen to flow continuously along the filament body, as well as in and out of the barbs (Schmieder, Raadu, and Wiik, 1991; Zirker, Engvold,

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and Martin, 1998; Lin, Engvold, and Wiik, 2003) at velocities of 10 km s−1 , and higher, which are assumed to be directed along the magnetic fields. The current overall picture of the magnetic topology of solar filaments is that they are magnetically linked to the photosphere via their barbs, through which mass may be transported into and out of the filaments. Some studies have indicated that filament barbs tend to be rooted in minority polarity fields, or in mixed-polarity regions where flux cancellation also is taking place (Martin and Echols, 1994; Priest, van Ballegooijen, and MacKay, 1996; Wang, 2001). The true nature of the fine threads is yet poorly understood. Unsettled questions concern, for example, how the overall magnetic fields connect with the surroundings, the dimensions and lifetime of the individual thin magnetic flux strings, the source and filling fraction of the cool (∼104 K) absorbing plasma within, and how the flows are generated. It is the objective of this study to determine the characteristics of filament threads, based on high spatial resolution Hα filtergrams showing filaments. These data are supplemented by simultaneous G-band images showing photospheric granulation and magnetic bright points. 2. Observations, Instruments and Data Reduction 2.1. T HE

TARGET FILAMENTS

Three sections of two large filaments on the solar disk were observed on August 25, 26 and 27, 2003 with the new Swedish 1-m Solar Telescope (SST) (Scharmer et al., 2003a) at La Palma, Spain. Figure 1 shows the locations of the two targets. The filament in the southern hemisphere, located close to the disk center, was observed on both August 25 and 26. The two fields of view that cover part of the filament are denoted, respectively, F1/F2 and F4. The observed part of the filament section in the northern hemisphere is referred to as F3. See Table I for further details. A highresolution frame of the central part of F3 is shown in the lower panel of Figure 1. TABLE I Details of the observations with the SST. Filament

Observational period

Position

Data

F1

25 Aug. 2003 16:01 –16:21UT

S14 W08

Hα, G-cont and Ca II H

F2

26 Aug. 2003 08:40 –09:00UT

S17 W00

Hα, G-cont and Ca II H

F3

27 Aug. 2003 09:58 –10:53UT

N22 E18

Hα, G-band and Ca II H

F4

26 Aug. 2003 07:53 –08:38UT

S25 E06

Hα

Filtergrams at four different wavelengths were used, i.e., Hα, G-band, nearby continuum ˚ (G-cont) and Ca II H 3965A.

THIN THREADS OF SOLAR FILAMENTS

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Figure 1. Upper panel: Full-disk image of the Sun on August 26, 2003 (courtesy Catania Astrophysical Observatory). The white rectangles indicate the locations of the four filament sections (see also Table I). Note that F1 and F2 are the same target but observed on two consecutive dates. Lower panel: High-resolution image of central part of F3 obtained with the SST on August 27, 2003.

2.2. INSTRUMENT

AND DATA PROCESSING

All images used in this study were obtained with the new Swedish 1-m Solar Telescope. The SST optical system consists of a 0.97 m clear aperture lens with a Schupmann corrector and a re-imaging adaptive optics (AO) system (Scharmer et al., 2003b) that includes a tip-tilt mirror and a 37-electrode bimorph mirror. The bimorph mirror is deformable and controlled by a computer which processes the micro-lens images from a 37-element Shack-Hartmann wavefront sensor. The SST has repeatedly shown to be capable of reaching the diffraction limit, which corresponds to 0.14 arc sec for Hα and 0.09 arc sec for the G-band.

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The data contains filtergrams that are recorded simultaneously with four separate cameras. Hα filtergrams, covering an area of 115 × 77 arc sec2 , were obtained with the Solar Optical Universal Polarimeter (SOUP) (Title and Rosenberg, 1981) system of the Lockheed Martin Solar and Astrophysics Laboratory. The three other cameras ˚ nearby continuum (G-cont, at that recorded, respectively, the G-band (4305A), ˚ ˚ 4364A), and in the wing of Ca II H line (3965A), all covering an area of 83×84 arc s2 FOV centered on the corresponding images from the SOUP filter. The camera pixel size corresponds to 0.075 arc sec for Hα and 0.041 arc sec for the other cameras. Real-time frame selection was used for all four cameras to select and store only the three sharpest images within selectable sampling periods which was set to 5 s for the SOUP camera and to 15 s for the other three. An unforeseen wavelength drift in the SOUP filter caused some frames of the Hα time series to be slightly off-band. Some of the sharp off-band images could, however, be used to ensure correct registration of images recorded with different cameras. Standard flat-field, dark-current and diurnal field rotation corrections, were applied to all images. Post processing of the images included correction for the theoretical telescope modulation transfer function (MTF). A so-called Multi-Frame Blind De-convolution (MFBD) (Löfdahl, 2002) technique for image reconstruction, was used for images recorded during exceptionally good seeing. This was the case for some images of the August 27 filament (see the lower panel of Figure 1). 2.3. CALCULATIONS

OF THE PHOTOSPHERIC FLOW CELL STRUCTURE

The supergranular and mesogranular flows within the observed photospheric areas were calculated by use of the so-called local correlation tracking (LCT) technique (November and Simon, 1988; Yi and Molowny, 1995). This technique is thoroughly described and discussed in the paper by Shine, Simon, and Hurlburt (2000) and in several papers cited therein. Before the LCT could be applied, the G-band and G-cont images were aligned to remove image jitter and then were de-stretched to correct image warping. A ‘sub-sonic’ Fourier filter was subsequently used to suppress p-mode intensity oscillations by attenuating modulations with horizontal speeds above 4 km s−1 . The high quality of the co-aligned G-continuum and G-band images permitted calculation of flow cell maps from only 5 min long series of images. The derived flow velocities vary somewhat with the size of the correlation window, which for an area of 2 × 2 arc sec2 gives velocities typically in the range from 200 to 1000 m s−1 . Further details are presented in Lin et al. (2005). 3. Widths, Lengths and Lifetimes of Thin Threads A number of sharp Hα filtergrams, such as the example shown in Figure 2, were used to measure the thickness and area density of threads in quiescent filaments. With the intent of finding the width of thin threads, we only selected thin threads


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Figure 2. The thin thread width (FWHM) measurement both for the filament threads in F1 (upper panel) and for the fibrils of F3 at the chromosphere and the filament channel (lower panel). The lower panel shows one off-band Hα image, where the contour of the corresponding Hα filament is superimposed (cf., the lower panel of Figure 1).

that were relatively well resolved. For reference and comparison we made similar measurements of thin, sharp fibrils observed in the chromosphere (see the off-band Hα image of the area F3 displayed in Figure 2). The width of the threads/fibrils is given as the full width at half minimum (FWHM) in the perpendicular direction. The distribution of widths (Figure 3) of the filament threads ranges from 0.2 to 0.6 arc sec, with an average of about 0.3 arc sec. For comparison, chromospheric fibrils have similar thickness (0.3 arc sec), but a slightly narrower distribution range (0.2–0.4 arc sec). The finest resolved threads are about 0.16 arc sec wide, and

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Figure 3. The distribution of measured thin thread widths (FWHM), both for the fibrils at the chromosphere (left panel) and for the filament threads (right panel).

close to the resolution limit of the SST. This suggests that even thinner structures are likely to exist. Individual threads are occasionally observed, but most often they appear in bundles. In regions with relatively dense threads, we resolve one or two threads per arc sec. This is consistent with the measured average width (0.3 arc sec) of resolved thin threads. Some regions of the filaments show no clear thread structure. This could be the result of the higher number density of threads such that individual threads merge into a uniform surface in the 2-D image plane. The length of visible Hα threads is found to vary from about 5–20 arc sec. The observable length of a given thread outlines the part of the magnetic string containing, momentarily, sufficiently dense, cold material. The magnetic string itself is probably much longer. We sometimes see threads that are detached from the filament main body, which probably are cases when parts of the threads situated closer to the filament itself are too faint to be detected. One such case is indicated by a white arrow in the upper panel of Figure 5. Regarding the lifetime of filament threads one must differentiate between the true lifetime of the magnetic string and the visibility of absorbing ‘cool’ plasma. The threads appear highly time variable since the absorbing parts come and go, possibly due to rapid cooling and heating of the plasma, and flowing in and out of view. Some threads can be followed for only a few minutes before they become unrecognizable, while some can be followed up to 20 min. 4. Thread Dynamics 4.1. MOVEMENTS

OF INDIVIDUAL THREADS

From time lapse movies of images in the Hα line center, we see that the threads move relative to each other and change positions. The lower five panels of Figure 4


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Figure 4. Upper panel: A total of seven slices (white lines) are selected from filament F2. Lower panel: A five frame “movie” shows the sideways motion of one thread in the region close to slice L2.

show an example of a moving thread. The velocities of these relative movements, as determined with the time-slice technique (Lin, Engvold, and Wiik, 2003), are typically 2–3 km s−1 . Such motions are in agreement with the line-of-sight velocity fluctuations in a large prominence recorded by Zirker and Koutchmy (1990, 1991). It is probable that the observed relative movement of individual threads is a consequence of their anchoring in a dynamic photosphere. 4.2. FLOWS

WITHIN THREADS

In addition to the continuous movements of the threads themselves, we also see moving material within the threads. Again, by applying the earlier mentioned timeslice technique, we determine the speed of these flows in the plane of the sky. A total of 15 threads selected from two filaments have been studied, out of which nine are indicated in Figure 5. The lower panel of this figure shows the time slice diagram for one of the threads. The motion of a dark blob is seen in this plot with a speed of

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Figure 5. Nine threads (T1–T9) are selected from the filament section F4 (see Figure 1) observed on Aug 26, 2003 (upper panel). The white arrow in the bottom left corner points to one thread that appears to be detached from the main body. Only the Hα time-slice diagram for thread T3 is shown in the lower panel. The duration is about 5.6 min.

nearly 30 km s−1 . The average velocity perpendicular to the line of sight is found to be about 15 ± 10 km s−1 , which is higher than the typical speed reported by Lin, Engvold, and Wiik (2003). Such differences may represent real variation from one filament to another, but it may also be an effect of spatial resolution. Higher spatial resolution gives rise to less smearing and averaging over several fine structures. 5. Photospheric Associations of Filament Barbs and Threads 5.1. B ARBS

AND SUPERGRANULAR NETWORK

The feet of the characteristic prominence arches are located in supergranulation cell boundaries, according to Rompolt and Bogdan (1986) and Forbes and Malherbe (1986). Lin et al. (2005) compare series of Hα images of filaments with maps


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showing the corresponding supergranular network immediately below, as determined by the LCT technique (see Section 2.3). Lin et al. (2005) conclude that barbs do not show a preference to the supergranulation cell boundaries, but most often point to regions within these cells. 5.2. THREADS

AND PHOTOSPHERIC FLOWS

In several filaments we observe ‘curtain’-like Hα structures, again consisting of numerous thin threads, where an apparently smooth lower edge gives the visual impression of being ’rooted’ in the chromosphere. One such case is illustrated in the upper panel of Figure 2, at the end of the F1 barb close to L7, where we find that the curtain edge moves slowly in the same direction as the local photospheric flow and with approximately the same speed. This suggests that the ‘curtain’ is connecting with the photosphere and that its motion is thereby influenced by the photospheric flows. Due to variable seeing conditions, we are not able to follow the fine structures in this ‘curtain’ region for more than a few frames at a time. The 3-D orientation of filament barbs and threads is not readily interpreted from 2-D filtergrams. Therefore, one needs additional information for selection of locations where threads possibly are rooted in the photosphere. From a 54-min-long time series of the August 27 filament (F3 in Figure 1), obtained during very good seeing conditions, we see an isolated bundle of threads associated with a filament barb, which displays impulsive sideway motions at velocities ranging from 5 to 13 km s−1 . Since this occurrence corresponds to a localized slightly faster-thanaverage motion in the photosphere, we argue that these threads again are very likely anchored in the photosphere. 5.3. FILAMENT

THREADS AND PHOTOSPHERIC GRANULATION

Since the high-resolution Hα images are co-aligned with images recorded simultaneously in the G-band, G-continuum and Ca II H line wing, one may investigate where the apparent ends of the thin threads seen in Hα are located in the corresponding images of photospheric granulation. Figure 6 shows how the tips of the resolved threads (white crosses) are located relative to photospheric granulation recorded with the G-band camera. The great majority of these thin threads appears to end in or close to dark intergranular lanes. In order to determine whether or not the number of ‘hits’ differs from a random distribution of points, we made identical measurements with three random tests. In each test 100 points were randomly distributed over a region of one G-band image. All three realizations gave similar results, of which the average percentages are given in Table II. We conclude from this that the percentage of thread tips that appears to connect with dark, intergranular lanes, is significantly greater than predicted from pure coincidence.

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Figure 6. From top to bottom: A small area of the barb region (cf., the lower panel of Figure 1) shown in Hα, G-band, two different 5-min flow maps combined with G-band, and Ca II H images. The white plus signs mark the positions of the Hα filament tips. The numbers indicate the frame number in the image series. North is up. The dominant direction of the motions of these Hα tips is towards the north.

Compared with the flow maps (cf. the third row of Figure 6), some Hα tips are even situated at the junction of several flow vectors with different directions. In other words, the Hα filament foot points seem to be confined to expanding granule boundaries. From the upper panels of Figure 6, one sees that the dominant direction of motion of the Hα tips is towards the north. Figure 7 shows the time variation of the motions of some rather long lasting foot points in this direction. The solid lines are the linear fits. As time-slice diagrams, the slopes of these lines give the velocities of

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TABLE II The percentage of the observed footpoints and random points falling in the intergranular lanes, close to the lane (less than 0.1 arc sec away from the boundary of the lane) and in the granular cell.

Tips of filament threads Random points

In the lane (%)

Close to the lane (%)

In the granule (%)

72 45

20 18

8 37

Data is from the barb area of F3 in G-band. The boundaries of the lanes are roughly outlined by the mean intensity of individual G-band images.

Figure 7. The northward motions in the plane of the sky of some visible Hα footpoints in the August 27 filament (F3) (cf., Figure 6). The solid lines are the linear fits.

the northward motions, which are in the range from 4–9 km s−1 . One must keep in mind that the derived mean velocity of about 6.7 km s−1 is slightly lower than the true velocity, since we have ignored a (lower) velocity component perpendicular to the larger, northward velocity. Although the Hα image series of filaments F1 and F2 are less sharp compared with the images of filament F3, one finds the similar correspondence between the Hα filament tips and the lanes in their barb regions. Some examples are shown in Figure 8. 6. Magnetic Fields in the Photosphere and Filament Threads Since we do not have temporal high-resolution magnetograms, we use G-band images as proxies for the magnetic field. Berger and Title (2001) conclude from observations that all G-band bright points, when properly distinguished from granulation brightening, are magnetic in nature. Schüssler et al. (2003) reach the

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Figure 8. Hα, G-continuum images of the small areas of filament F2 (below the L1 region shown in Figure 4). The white plus signs mark the positions of the Hα filament tips. The numbers indicate the frame number in the image series.

same conclusion via radiative MHD simulations. Also, images recorded in the Ca II H line wing showing similar brightenings may serve as proxies for magnetic flux. Neither one of the two proxies provide information about the magnetic polarity or field strength. Although not all magnetic flux concentrations turn out to be bright in the G-band images, in quiet regions, such as filaments, the occurrence of magnetic elements without discernible associated bright points is rare (Shelyag et al., 2004). Figure 9 shows the distribution of bright points in the Ca II H line wing in the barb areas of the filaments observed on, respectively, August 25, 26 and 27, 2003 (F1–F3). The current data shows that there is no obvious correlations between filament features and Ca II H bright points in these data. It has been shown above that thin filament threads appear to be rooted in intergranular lanes. Figures 6 and 8, demonstrate, however, that the ends of the threads are not associated with observable bright points that can be seen in G-band images. This may imply that the small-scale filament threads are not associated with magnetic fields in the photosphere, or, alternatively, that the corresponding fields are very weak and therefore give rise to very faint excess emission. Given the wellestablished magnetic character of solar filaments the first hypothesis appears rather unlikely. Berger and Title (2001) studied the statistical correlation between G-band brightenings and measured magnetic flux density. Given that the magnetic flux elements could be smaller than the resolution area it has yet to be decided whether an apparent correlation between the brightness of bright points and the field strength is due to a true variation of magnetic field strength or to a variation in number density of unresolved magnetic elements within the resolution area. Also, an earlier study and comparison of magnetic flux density and filigree brightness by Yi (1992) showed a correlation between the two. According to the numerical models by Schüssler et al. (2003), magnetic flux concentration in lateral pressure balance


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Figure 9. These Ca II H images show the three filament barb regions in question. The contours of the corresponding Hα barbs are superimposed (cf., the upper panels of Figures 2 and 4 and the lower panel of Figure 1).

will result in lower gas density and thereby lead to a weakening of CH molecule absorption and increased atmospheric transparency. One may thereby reproduce the noted relation between brightness of G-band bright points and magnetic flux concentration. For further discussions on theoretical models for G-band bright points, see Spruit (1976), Sánchez Almeida et al. (2001), Rutten et al. (2001), Schüssler et al. (2003) and Shelyag et al. (2004). From above we conclude that the absence of G-band bright points at the assumed ‘roots’ of filament threads may be due to a low magnetic flux density at these locations. Such a conclusion would agree with the fact that the measured strength of magnetic fields in quiescent solar filaments is typically less than 20 G (Leroy, 1988; Bommier et al., 1994).

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7. Discussion and Conclusions Solar filaments are dynamic entities constituted from numerous thin magnetic strings, which become observable in Hα when and where they are filled with sufficiently dense, cool absorbing plasma. New high resolution images in Hα showing solar filaments and simultaneous observations in G-band, G-continuum and Ca II H proved suitable for a study of the characteristics of filament smallscale structures, their connection and possible interactions with the photosphere below. In one example where a number of fine threads appears to be rooted in the photosphere, it is found that these threads are anchored in the dark intergranular lanes, which normally also harbor the magnetic network flux. The absence of observable bright points at the apparent roots of these threads may, however, be the result of a substantially weak magnetic flux density at these locations. This seems to be in agreement with the result from a recent spectro-polarimetric study of a large quiescent filament by Zong et al. (2003), who measure magnetic field strengths of 20 G close to two barb endpoints. Both the observed continuous, relative movements of the threads, at speeds about 2–3 km s−1 and the occasional impulsive motion of apparent ‘footpoints’ with higher velocity (∼7 km s−1 ) are, conceivably, a consequence of the threads being rooted in the dynamic photosphere. Based on G-band filtergrams, Berger et al. (1998) measured the speeds of magnetic elements. They found that the majority of bright point speeds are low (∼0.3 km s−1 ), but a fraction has speeds up to 3–4 km s−1 . These faster bright points are mostly found in quiet regions. Their motions may reflect the magnetic element motions in the intergranular lanes below the Hα filament, although no observable bright points are detected at the ‘footpoints’ of filament threads. The higher velocities Berger et al. (1998) found are comparable with the speed of the sideway motions of threads or even the Hα ‘footpoints’ speed mentioned above. The ever present field-aligned flowing of the cool plasma is a fundamental property of solar filaments. These flows may hold the answer to how the cool, dense plasma enters and leaves the low-lying, flat magnetic flux loops that constitute filaments in the central regions of the filament channels. The origin of the flow, or rather the underlying acceleration mechanism, ought to be pursued further, theoretically as well as observationally. There exists as of now, no fully developed model for filaments and prominences that accounts for the flows and magnetic topology emerging from the present study. Numerical models that have been developed by Antiochos, MacNeice, and Spicer (2000) and Karpen et al. (2001), where local condensations forms in magnetic loops as a consequence of foot-point heating, are promising in terms of explaining some of the spatial and dynamic character of filament threads.


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Acknowledgements LRvdV’s research is funded by the European Commission’s Human Potential Programme through the European Solar Magnetism Network (contract HPRN-CT2002-00313). The Swedish 1-m Solar Telescope is operated on the island of La Palma by the Institute for Solar Physics of the Royal Swedish Academy of Sciences in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrof´ısica de Canarias. We thank the staff of the SST for their invaluable support with the observations. Travel related to extensive discussions with US colleagues, Sara F. Martin and Jack B. Zirker, was in part supported by NASA grants NAG54180 and NAG5-10852 to Helio Research.

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