SOLAR FILAMENTS AND PHOTOSPHERIC ... - Springer Link

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Solar Physics (2005) 227: 283–297

Springer 2005

SOLAR FILAMENTS AND PHOTOSPHERIC NETWORK YONG LIN1 , JUN ELIN WIIK1 , ODDBJØRN ENGVOLD1 , LUC ROUPPE VAN DER VOORT1,2 and ZOE A. FRANK3 1 Institute

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

(Received 10 November 2004; accepted 18 January 2005)

Abstract. The locations of barbs of quiescent solar filaments are compared with the photospheric/chromospheric network, which thereby serves as a proxy of regions with enhanced concentrations of magnetic flux. The study covers quiet regions, where also the photospheric network as represented by flow converging regions, i.e., supergranular cell boundaries, contain largely weak magnetic fields. It is shown that close to 65% of the observed end points of barbs falls within the network boundaries. The remaining fraction points into the inner areas of the network cells. This confirms earlier findings (Lin et al., Solar Physics, 2004) that quiescent filaments are basically connected with weaker magnetic fields in the photosphere below.

1. Introduction Solar filaments (called prominences when observed beyond the limb) result from particular magnetic configurations in the central regions of filament channels. The fine thread structure and rapid flowing of the cool (∼104 K) plasma indicate that filaments are contained within arches that are low relative to the coronal loops that straddle these filament arches. It is not yet well understood how the magnetic fields of filaments are rooted and connected with the photosphere below. It is assumed by some authors (cf., Martin, 1998a) that the sub-structures at semi-regular spacing diverting to either side and away from the main filament body, the so-called barbs, represent magnetic feet that point into the photosphere. In an earlier study Sýkora (1968) noted distinct maxima in the distributions of distances between filament feet (barbs) corresponding to, respectively, one and two supergranular cell diameters. Plocienak and Rompolt (1973) used spectroheliograms in CaII K showing the chromospheric network, and in Hα showing filaments, and found that a great majority of the feet connected with corners between three or more supergranular cells, i.e., in so-called down drafts. Martin and Echols (1994) found that barbs of an active region filament seemed to be rooted in minority polarities on either side of the filament channel. A most interesting model prediction for an observed filament was made by Aulanier, Srivastava and Martin (2000). Using

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an observed magnetogram they were able to reproduce the overall appearance of the corresponding filament structure by applying a three-dimensional magnetohydrostatic model (cf. Aulanier et al., 1999). Their result implies a relation between barbs and observable small parasitic polarities, often along the edge of supergranulations in the photosphere. However, earlier studies have also failed to detect magnetic signals associated with barbs of quiescent filaments (cf., Engvold, 1998). A theoretical model by Priest, van Ballegooijen, and MacKay (1996) explained filament barbs as a result of magnetic flux emergence and reconnection governed by supergranular flows. A clearer picture of how filament barbs appear and develop in relation to photospheric network may therefore shed new light on how and why they are formed. It is the objective of this study to find how filaments and their barbs, relate to the network field below, which in turn is controlling the distribution of small-scale magnetic flux. In the present study we use simultaneous, co-spatial time series of Hα images containing filaments and high-resolution white-light images of photospheric granulation. The flow cell structure is derived from the motion of the photospheric granulation measured by a Local Correlation Tracking (LCT) technique (November and Simon, 1988).

2. Data 2.1. THE

TARGET FILAMENTS

All together, six filament sections have been studied, see Table I. Filaments F1–F3 were observed with the Richard B. Dunn Solar Telescope (DST) at Sacramento Peak, National Solar Observatory (NSO) in May 2000. The other three filaments (F4–F6) were observed with the new Swedish 1-m Solar Telescope (SST) (Scharmer

TABLE I Details of the observations with the DST/NSO and the SST. Filaments

Telescope

Observational period

Position

Data

F1 F2 F3 F4 F5 F6

DST DST DST SST SST SST

3 May 2000, 14:20−14:33 UT 6 May 2000, 15:07−15:38 UT 7 May 2000, 16:09−16:31 UT 25 Aug 2003, 16:01−16:21 UT 26 Aug 2003, 08:40−09:00 UT 27 Aug 2003, 09:58−10:53 UT

N14 W1.4 S30 E13.5 S24 W11 S14 W08 S17 W00 N22 E18

Hα and WL Hα and WL Hα and WL Hα, G-cont, CaII H Hα, G-cont, CaII H Hα, G-band, CaII H

For the DST May 2000 observational run, we used Hα and white-light (WL) filtergrams. For the SST August 2003 run, four filtergrams were used; Hα, G-band, nearby continuum (G-cont) and ˚ CaII H 3965 A.

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et al., 2003a) of the Royal Swedish Academy of Sciences in August 2003 at La Palma, Spain. Figure 1 shows the positions of the target filament sections on the full-disk images obtained on May 6, 2000 and August 25, 26, 2003, respectively. All filaments, including their barbs are well formed on the days observed. According to the rule of chirality (Martin, 1998b), Filaments 1 and 6 in the northern hemisphere have right-bearing barbs. The other three filaments are left-bearing. 2.2. OBSERVATIONS

AND DATA PROCESSING

2.2.1. Observations with the Dunn Solar Telescope of the NSO The study of the filaments located in areas F1–F3 (Figure 1) uses Hα filtergrams and white-light (WL) filtergrams (a broad-band filter in the yellow wavelength region) from DST/NSO and magnetograms from the Michelson Doppler Imager (MDI) on board SOHO (Scherrer et al., 1995). With the Universal Birefringent Filter (UBF) mounted on the DST, all three ˚ and filament sections were recorded in this sequence: Hα line center, Hα − 0.3 A ˚ The WL filtergrams were made simultaneously. The UBF and the Hα + 0.3 A. WL both use a 1024 × 1024 pixel CCD camera that covers a 184 arcsec × 184 arcsec field of view. For the Hα time series the cadence is about 13 s for each of the wavelength positions. The WL series have a cadence of 4.3 s. The Hα and the WL images are corrected for flat field and dark current. The Dopplergrams are derived from the line shift of the reconstructed Gaussian profile using intensities of the Hα line center and the two wings. The averaged profile from a selected chromospheric area is adopted as line center zero shift. Using the images of a standard “target” grid taken by the UBF and the WL cameras, the Hα images and the Doppler maps are aligned with the WL images. The corresponding three full-disk magnetograms on May 3, 6 and 7 are obtained from the web site of MDI (http://soi.stanford.edu/magnetic/index5.html). They are taken close in time to the three target filaments. Spatial registration of the Hα filtergrams with magnetograms from the Kitt Peak Vacuum Telescope/NSO were done ˚ images from the same observatory. via intermediate registration with HeI 10,830 A Subsequently, the Hα filtergrams are also co-aligned with the MDI magnetograms. During our short observational periods, the magnetic field is relatively stable. We thus obtain, for each of the three filaments, series of images: Hα (line center, ˚ Dopplergram, WL, and one magnetogram. These have all been co-aligned. ±0.3 A), 2.2.2. Observations with the Swedish 1-m Solar Telescope The filaments indicated by the frames denoted F4, F5 and F6 (Figure 1) have been observed with four different narrow-band filters: Hα with the Solar Optical Universal Polarimeter (SOUP) (Title and Rosenberg, 1981) system from Lockheed ˚ filtergram system, Martin Solar and Astrophysics Laboratory, a G-band 4305 A ˚ ˚ H-line filtergram a G-continuum 4364 A filtergram system and a CaII 3965 A

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Figure 1. Positions of the six target filament sections (Table I) are indicated on the full-disk images of the Sun on May 6, 2000 (upper frame) and August 26, 2003 (lower left frame) (courtesy Observatoire de Paris, Meudon). Note that F4 and F5 are the same target observed on two different days. The filament observed on August 25, 2003, containing F4 and F5 sections, is also shown in the lower right panel (courtesy the Big Bear Solar Observatory).


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system. The SOUP filter is equipped with 1534 × 1032 pixel CCD camera which provides 115 arcsec × 77 arcsec field of view (FOV). All the other three systems use 2029 × 2047 pixel CCD cameras, which correspond to a FOV approximately 83 arcsec × 84 arcsec. An Adaptive Optics (AO) system (Scharmer et al., 2003b) served in combination with an agile tip/tilt mirror to partly compensate and correct for, respectively, seeing and residual motions due to telescope jitter. A real-time frame selection served, furthermore, to store only the sharpest images recorded within pre-set intervals of 5–10 s. The G-band, G-continuum and CaII H have slightly smaller FOV but better spatial resolution compared with Hα images. Some off-band Hα images were also present in the Hα series due to an unforeseen wavelength drift in the SOUP filter. However, some of the sharp off-band images could be used for alignment to the G-band images. 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) technique for image reconstruction (Löfdahl, 2002), was used for images recorded during exceptionally good seeing. This was the case for some images of the August 27 filament (F6) (see also Lin et al., 2004).

3. The Photospheric Flow Cell Pattern 3.1. CALCULATION

OF FLOW VECTORS

The photospheric flow vectors are obtained via the motion of photospheric granulation cells that can be derived with the now well-established LCT technique. Since the typical flow velocity on the Sun’s surface is ≤0.5 km s−1 , the displacement after one minute is ≤30 km, which is less than, or comparable to, the pixel size of DST and SST images. However, November and Simon (1988) have shown that cross correlation techniques can achieve an accuracy as good as 20 m s−1 . The optimum time separation between correlated image pairs is found to be ∼50 s. Therefore, we set the separation time of 52 s for the three DST filaments (F1–F3) and 81 s, 50 s and 50 s for the three SST filaments (F4–F6). Before performing the cross correlation, all images in each of the series are aligned with each other in order to remove telescope jitter and rotation. Each image is ‘de-stretched’ by correlating it with its adjacent image in order to compensate for small, local seeing distortions. 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 . After applying the cross correlation between each image pair, we obtain several flow map sequences. For the DST flow maps, we take the average of the good samples to further suppress the seeing noise. The same averaging procedure is used for the time series obtained with the SST.

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Figure 2. Distribution of flow velocities in the photosphere from observations with the DST in May 2000 (upper panels) and with the SST in August 2003 (lower panels). The derived velocities are generally less than 1 km s−1 .

However, these observations were recorded with much higher spatial resolution. One could therefore include all LCT frames in the averages. Finally, one mean flow map is obtained for each filament. The distributions of the measured flow speeds are shown in Figure 2. As can be seen, the horizontal flow velocities at the photosphere are typically less than 1 km s−1 , in agreement with the flow measurements by other authors (e.g., Krijger, Roudier and Rieutord, 2002). However, the flow velocities derived from the high resolution SST 2003 data have more points in the high-speed tail compared with those derived from the DST 2000 WL images. The spatial resolution in the observations obtained in the year 2000 is ≥0.5 arcsec, as compared with a resolution ranging from 0.1 to 0.3 arcsec for the 2003 observations (Lin, 2004). We degraded the G-band images of F6 (pixel resolution is 0.042 arcsec) to the level of the DST WL images (pixel resolution is 0.18 arcsec). The derived velocity distribution of the flow fields is then reduced and become identical to the distribution shown in the upper panels of Figure 2. 3.2. VISUALIZATION

OF THE CELL STRUCTURES

In order to bring out more clearly the cell structure of the photospheric flows one introduces a uniform grid of test particles (often referred to as ‘corks’) that are allowed to move with the local flow vector (Simon et al., 1988). The flows are simulated by calculating the motion of the ‘corks’ after a number of sufficiently small time steps. The resulting maps outline the regions of convergence,


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which thereby show the boundaries of the flow cells. Flow cell maps resulting from a few hours of flowing start to delineate the characteristic flow cell structure. A number of smaller cells are also noticed within the larger, typical supergranular cells. The applied technique of numerical flowing of test particles tends to outline more clearly the smaller cells in the early phases of the flowing, whereas only the larger cells remain after several hours of flowing. A comparison of the flow fields derived from three consecutive 15 min long time series shows that also the smaller cells remain for at least one hour with only minor changes. Hence, the small cell must be equally real features which also may harbor concentrations of photospheric magnetic flux. A comparison of the flow cell structure derived from the high-resolution G-band images of 27 August 2003, and from the same time series smeared to a resolution of ∼0.5 arcsec (see above), yields nearly identical cell structures. This implies that the derived flow cell maps are not affected by a reduction in resolution from 0.1 arcsec to 0.5 arcsec.

3.3. FLOW

CELL BOUNDARIES AND MAGNETIC FLUX

The general correspondence between supergranular cells and the distribution of magnetic flux in solar active regions is well known (Foukal, 2004). It is the aim of this study to use the derived photospheric flow cell pattern as a reference and proxy for the distribution of magnetic flux, which then can be compared with the apparent end points of filament barbs. The converging areas representing the boundaries of the flow cells become very sharply defined by the particle flowing technique. The actual distribution of magnetic flux concentrations constitutes rather a few arcsec wide band along these cell boundaries. The good correspondence between magnetic flux concentrations represented by chromospheric brightness elements seen in near-ultraviolet images from TRACE and areas of flow convergence is nicely demonstrated by Krijger, Roudier and Rieutord (2002). The width of these narrow bands of magnetic flux appears to be typically ≥5 arcsec. In quiet regions one sees, in general, a lot less observable magnetic flux, which therefore provide a rather fragmented outline of the network. Figure 3 gives one such example showing the flow cell structure superimposed on a corresponding CaII H wing image. The small bright structures correspond to the location of magnetic flux element. In the following when we use flow cell boundaries as reference to locations of concentrations of magnetic flux, we accept deviations up to ±3 arcsec. The fact that magnetic flux, via various proxies, like chromospheric bright points, is detected at very few places along flow cell boundaries in the quiet-Sun implies very likely that the corresponding network fields are not detectable through these diagnostics (see also Lin et al., 2004).

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Figure 3. The lower left part of region F6 (cf., Figure 8) shown in CaII H line wing. The bright points are proxies for magnetic fields. The corresponding flow cell map is superimposed.

4. Results 4.1. GENERAL

COMMENTS

In the following we will examine how the observed filaments relate to the photospheric flow cell pattern discussed in Section 3. Figures 4–6 from the 2000 observing season contain, respectively, Hα line center filtergrams showing filaments (upper left images), Dopplergrams obtained from ˚ images with contour of corresponding Hα line center and two wing (±0.3 A) filament inserted (upper right images), magnetograms obtained by SOHO MDI, with superimposed picture of the flow cell structure (lower left images), and finally a contour of the filament seen in Hα line center filtergram superposed on the corresponding flow cell image (lower right images). The Doppler Hα image in Figure 4b illustrates how the structures of the chromosphere immediately underneath the filament differ from the normal chromosphere. One assumes that this is an effect of the magnetic filament channel (corridor) above which filaments are embedded. It is noted that the observable barbs all end within this corridor. The MDI magnetograms are included here in order to illustrate the general correspondence between the magnetic network and the derived flow cells. The fact that the spatial resolution of these magnetograms is 4 arcsec, that they tend to show only the strongest magnetic elements, and that they are not exactly simultaneous make them less suitable for studying the detailed association between barbs and magnetic flux.


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Figure 4. Filament in region F2. Panel (a): Hα filtergram. Panel (b): Dopplergram with the contour of Hα filament. Dark and bright Doppler signals respectively, represent motions away from and towards the observer. Panel (c): magnetogram. The circles in (d) (with radius of 3 arcsec) indicate the regions of the apparent end points of the barbs. Image sizes are 182 × 182 arcsec.

Given the widths of the network cell boundaries discussed above, we introduce a search circle of radius 3 arcsec at the observed end points of the filament barbs (cf. Martin, 1998a). Barb end points are taken to be where their contrast vanishes as seen in high-resolution (≤0.5 arcsec) Hα line center images. In the case of the filament shown in Figure 4, the matching of barb ends and network was done using three Hα filtergrams recorded 15 min apart, but which gives only marginally different results. The distribution of end points of filament barbs relative to the network is listed in Table II. The three filament images from the 2003 series (F4 – F6) cover a smaller area. In these high resolution images the thin thread structure becomes more conspicuous and dominant. In the case of the filament shown in Figure 7 the location of barbs may be more easily noticed from lower right image of Figure 1. The

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TABLE II The observed end points of the filament barbs of the six filament sections (see Figures 4–8) falling in the network boundaries and inside network cells.

In network boundaries Inside network cells

Number of barb end points

Percentage

22 12

65 35

We regard the filament end points in the network boundaries if corks are inside the searching circles.

Figure 5. Same as Figure 4, but for region F1.

‘curtain’-like structure at the end of the F4 barb (upper panel of Figure 7), gives a visual impression of being rooted in the photosphere. The same can be said about the middle-right curved region of F5 (lower panel of Figure 7). We see, however, no other obvious barb structures in this part of the filament shown in Figure 7.


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Figure 6. Same as Figure 5, but for region F3.

4.2. ORIENTATION

OF FILAMENTS RELATIVE TO NETWORK

In Figure 5 the filament runs more or less parallel with network cell boundaries, whereas the filament in Figure 4 crosses over a large number of cells. Figure 6 shows a mixture of both types. For the case displayed in Figure 4, the supergranular flowing will lead to a transport of magnetic flux across the Polarity Inversion Line (PIL). This appears to be in agreement with Wang (2001) who concludes that the tendency of barbs to be rooted in minority polarity could be a result from the random walk exchange of magnetic flux elements caused by supergranular convection, in both directions across the filament channel. A notable flowing along the network cell boundaries is clearly illustrated in Figure 9. In this particular case the flows are directed solely from one side of the PIL to the other.

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Figure 7. Filaments in region F4 (upper panel) and F5 (lower panel). Photospheric flow cell maps showing the smaller mesogranular cells are superimposed on corresponding Hα filtergrams from observations with the SST (spatial resolution ≤ 13 arcsec) in August, 2003. The circles (with radius of 3 arcsec) indicate the regions of the apparent end points of the barbs. Image sizes are 75 × 75 arcsec.


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Figure 8. Filament in region F6. Photospheric flow cell map showing the smaller mesogranular cells is superimposed on corresponding Hα filtergram from observations with the SST (spatial resolution ≤ 13 arcsec) in August, 2003. The circles (with radius of 3 arcsec) indicate the regions of the apparent end points of the barbs. Image size is 75 arcsec × 75 arcsec.

Figure 9. Photospheric flows along cell boundaries in region F2 (cf., Figure 1), here represented by arrows indicating the direction of the flow. It is noticed in this particular case that the flows are directed predominantly from one polarity side to the other.

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5. Concluding Remarks The current picture of solar filaments (and prominences) is that they are located in the solar corona, upheld and controlled by magnetic fields, which inevitably must be anchored, within the magnetic filament channels, in the photosphere below. The present study has demonstrated that about 65% of the observed end points of barbs falls within the network boundaries, whereas the remaining 35% points into network cell areas (see Table II). It is noted, however, that these results apply to quiet solar regions where the network is defined from the derived flow cell pattern and that network field is too weak to be currently detected in magnetograms, or to appear as bright elements in CaII H line wing images. This conclusion is substantiated by the finding of Lin et al. (2004) that thin filaments threads appear to be rooted in weak magnetic fields in intergranular dark lanes. The issue of barbs being connected with parasitic or mixed polarities, as discussed by Martin and Echols (1994) and Aulanier, Srivastava and Martin (2000), remains observationally open for weak field regions until such high resolution field measurements are obtained. Plocienak and Rompolt (1973) defined the chromospheric network by means of spectroheliograms in CaII K, and found that 65% of the feet (barbs) connected with corners between three or more network cells, i.e., in so-called down drafts, 25% were between two cells and 10% were inside cells. We see no evidence from the current data of barbs being rooted in corners between three or more network cells. The fact that Plocienak and Rompolt were able to outline fairly completely the network cells from CaII K images suggests that their study refers basically to regions of strong magnetic fields, which may differ from the conditions in weak field regions.

Acknowledgements LRvdV’s research is funded by the European Commission’s Human Potential Programme through the European Solar Magnetism Network (contract HPRN-CT2002-00313). We thank the staff of the SST for their invaluable support with the observations. 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 also thank Lockheed Martin Solar and Astrophysics Laboratory and Dr. Tom Berger in particular for making their highly versatile narrow-band filter available for our program. We are most grateful to Sara Martin and Jack B. Zirker for numerous stimulating discussions in the course of the work, and for permission to use here part of data obtained in a joint observing program of May 2000. The help with these observations by the staff of the DST/NSO is gratefully


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acknowledged. We thank the anonymous referee for most helpful comments. Travel related to extensive discussions with US colleagues was in part supported by NASA grant NAG5-4180 and NAG5-10852 to Helio Research.

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