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Observations of an active region filament
Advances in Space Research 35 (2005) 1752–1755 www.elsevier.com/locate/asr
Observations of an active region filament W.G. Zong *, Y.H. Tang, C. Fang, A.A. Xu Department of Astronomy, Nanjing University, Nanjing 210093, PR China Received 27 October 2004; received in revised form 10 May 2005; accepted 10 May 2005
Abstract An active region filament was well observed on September 4, 2002 with THEMIS at the Teide observatory and SOHO/MDI. The ˚ lines. Using the data, we have studied the fine structure full Stokes parameters of the filament were obtained in Ha and FeI 6302 A of the filament and obtained the parameters at the barb endpoints, including intensity, velocity and longitudinal magnetic field. Our results indicate: (a) the Doppler velocities are quiet different at barb endpoints; (b) the longitudinal magnetic fields at the barb endpoints are very weak; (c) there is a strong magnetic field structure under the filament spine. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Filament; Barb; Endpoint
1. Introduction Filaments are commonly observed on the solar disk in Ha line. They appear as dark long structures called ‘‘spine’’ and lateral extensions named ‘‘barbs’’ or footpoints (Martin, 1998). Tandberg-Hanssen (1995) and Engvold (1998) distinguished filaments into three types: quiescent, active region and intermediate filaments. Quiescent filaments appear in quiescent regions or between weak remnants of active regions. They are long and sheet-like structures. While active region filaments form in or close to active regions. They are generally smaller and curved. Comparing to quiescent filaments, active region filaments are short-lived and more rapidly changing. In previous paper (Zong et al., 2003), we have studied the fine structure of a quiescent filament with the data from THEMIS in the multichannel subtractive double pass spectrograph (MSDP) mode. In this paper, we present the observations of an active region filament and give the parameters at the barb endpoints, and then *
Corresponding author. Tel.: +86 25 83686327. E-mail address: [email protected] (W.G. Zong).
the difference between the two type filaments is discussed.
2. Observations The observations were made on September 4, 2002 during 16:50–18:00 UT, with the multi-line spectroscopy (MTR) mode of the THEMIS telescope (Paletou et al., 2001a). By scanning the filament area of 8000 · 6000 , 2D ˚ lines spectral maps were obtained. Ha and FeI 6302 A were recorded simultaneously. The spatial sampling along the slit is 0.500 /pixel. The ˚ for Ha and 22 mA ˚ for FeI spectral sampling is 26 mA ˚ . The exposure time for all wavelengths is 6302 A 300 ms. Seeing was better than 1–200 during the observations, but the scanning procedure limited the spatial resolution of 2D maps to about 2–300 . The so-called 2 · 1 THEMIS spectro-polarimetric configuration was used. Two beams with orthogonal polarization exiting the analyzer are directed into a single camera. Beam inversion was performed for the polarization: the top part of the camera received sequentially I + Q, I Q, I + U, I U, I + V and I V, while
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.05.028
W.G. Zong et al. / Advances in Space Research 35 (2005) 1752–1755
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Fig. 1. Ha image from observatory of Paris at 11:04:00 UT (left) and the longitudinal magnetic field from MDI at 17:39:30 UT (right). The observed region is marked by black rectangle. It is around the active region NOAA10099. East is to the left and north is up.
the bottom part recorded I Q, I + Q, I U, I + U, I V and I + V. Dark current and flat-field correction were performed. The muller matrix was used to demodulate the Stokes parameters. To remove induced crosstalk and improve the seeing smearing, we sum the top and the bottom spectra for each parameter and then subtract each other. To do so, the two spectra are well aligned in Y-direction (slit direction). Ariste et al. (2000) and Paletou et al. (2001b) introduced the process of data reduction in detail. The full solar disk Ha image is shown in the left panel of Fig. 1. It was gotten from Observatory of Paris. It can be seen that the observed filament locates in the southern hemisphere near the center of the solar disk (W25 S15). It is just around the active region NOAA10099. The field of view (the black rectangles in Fig. 1) covers an area of 8000 · 6200 . The longitudinal magnetic field from the SOHO/MDI (Michelson Doppler Imager) is also presented in the right panel of Fig. 1.
3. Data analysis After data reduction, the full-Stokes parameters of ˚ lines were obtained. Using the the Ha and FeI 6302 A Stokes parameters, we obtained 2D maps of the intensity and the velocity fields, which are given in Fig. 2. For comparison, both the longitudinal magnetic fields ˚ and from MDI are presented. derived from FeI 6302 A Magnetic contours on the strong magnetic structure are plotted in each magnetogram. It can be seen that the re˚ line and MDI are well sults derived from the FeI 6302 A consistent. The linear coherence coefficient is 0.91. For Ha data, the calibration has been done by using the area in the black rectangle (700 · 700 ) in Fig. 2(a) as the quiet background. The images at the Ha line wings have been used to ˚ from Ha line find barbs. In the image at Dk = 0.44 A center, four barbs are well identified and marked with
circles in Fig. 2. It can be seen that they are left-bearing, as expected for filaments in the southern hemisphere. This is consistent with the empirical chirality rule proposed by Martin (1998). Fig. 2(c) shows the velocity field in the observed area. The positions of the four barb endpoints are marked out. The results indicate that the mass was flowing up in some endpoints, but flowing down in the others. The values of the velocity cover a wide range, from tens to several hundred meters per second. The average values for five points at the endpoints marked 1–4 are 470 m s1 (up), 770 m s1 (down), 30 m s1 (up) and 680 m s1 (down), respectively. The accuracy of the velocity measurements is estimated to be ±60 m s1. It can be seen both in Fig. 2(b) and (d) that all the barbs terminate in weak magnetic field of only several Gauss. There are no strong polarities nearby. The magnetic field near the barb endpoints is less than 20 G. Just as mentioned above, the filament locates above a neutral line. But by comparing the intensity and the magnetic field in Fig. 2, it can be seen clearly that there is a strong magnetic polar under the filament spine. The contours have been drawn over the structure, and the levels are 80, 130 and 180 G. In Fig. 2(c), the strong magnetic structure is co-aligned with the velocity field. The direction of the velocity above the structure is down and the value is about 600 m s1. Near the strong magnetic structure (left upper to it), there is an area with high velocity (mass flows up), and the value reaches 2 km s1.
4. Discussion and summary A filament in the active region NOAA 10099 was well observed on September 4, 2002 with THEMIS/MTR. By use of the images at the Ha line wings, four barbs of the filament have been identified, and the parameters at the endpoints of the barbs are derived. The intensity, velocity and longitudinal magnetic fields are all presented.
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W.G. Zong et al. / Advances in Space Research 35 (2005) 1752–1755
˚ from Ha line center. Four barbs are well identified and their endpoints are Fig. 2. Main results of the observations. (a) The image at Dk = 0.44 A ˚ marked with circles. The area in black rectangle was selected as the quiet background for calibration. (b) The magnetic field derived from FeI 6302 A line observation. White is up and black is down. (c) The velocity field derived from Ha line with the center of gravity (COG) method. Black is up and white is down. (d) The photospheric longitudinal magnetic field from MDI. The black line is the neutral line of large scale magnetic field. The contour levels are 80, 130 and 180 G. All four images correspond to the same area on the solar disk.
Although the barb endpoints of the active region filament still locate in weak magnetic fields, they are different from that of the quiescent filament discussed in our previous paper (Zong et al., 2003). According to our result, the barb endpoints of the quiescent filament are located in weak magnetic fields near neutral lines of the majority polarities and minority polarities. The magnetic field of the majority polarities is more than 70 G and those of minority polarities is greater than 30 G. However, all the barb endpoints discussed here are far away from the strong polarities, and the magnetic field of the polarities nearby is less than 20 G. The velocities at the four barb endpoints have been given. The range is from tens to several hundred meters per second. However, as Mein et al. (1996) and Molowny-Horas et al. (1999) indicated, a Ha filament is observed against a strong chromospheric line, the measured line shift will be notably less than the real Doppler shift produced by the filament. In addition, the spatial resolution of our observations is 2–300 , the Doppler velocities deduced from our observations are the results from spatial smearing of sub-arc sec structures. All these effects lead to an underestimate of the velocities in barbs. It implies that there is significant
mass flowing through the barb endpoints, either into or out from the barbs. There are mainly two kinds of model involving filament barbs. Martin (1998) proposed that the filament barbs are anchored in parasitic polarity elements. Aulanier and De´moulin (1998) proposed that barbs are located at ‘‘bald patches’’. In our observation, all the four barbs end in very weak longitudinal magnetic fields. But we can not rule out the possibility that the magnetic fields in barbs are horizontal, as suggested by the model of Aulanier and De´moulin (1998). However, their model cannot be reconciled with the flowing of plasma in our observations. As generally believed, the filaments locate near the neutral lines of magnetic field and thus it seems there is only weak magnetic field under the filament spine. However, in our observation, a strong magnetic structure (about 200 G) near the filament was found. This can be clearly seen both in MDI map and our derived magnetogram. A similar phenomenon had also been found in our previous observations (Zong et al., 2003), in which a structure with 300 G under the quiescent filament spine was detected. In the case of the active region filament, the distance between the strong
W.G. Zong et al. / Advances in Space Research 35 (2005) 1752–1755
magnetic structure and the neutral line shown in Fig. 2(d) is about 2000 . Considering that the filament locates at a heliocentral angle of about 26 on the solar disk, if the height of the filament is 50,000 km, than the distance would have been about 4000 . So only if the height of the filament is less than, say, 25,000 km, than the coverage of the strong magnetic structure on the filament can be probably explained by the projection effect. If not, then we are still not understand the physical meaning about the strong magnetic structure. To resolve the puzzle, more work has to be done. Acknowledgments We give our sincere thanks to Dr. G. Ceppatelli, Dr. C. Briand and other staffs at the Spanish Observatorio del Teide of the Institute de Astrofisica de Canarias for their enthusiastic help during C.F. and Y.H.TÕs stay at the observatory. We also thank the referees for their valuable comments and suggestions to improve both the science and the language. This work has been supported by NKBRSF of China G20000784 and by NSFC key Project of China No. 10333040, as well as NSFC under Project of Nos. 10073005 and 10403003.
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References Ariste, A. Lo´pez, Rayrole, J., Semel, M. First results from THEMIS spectropolarimetric mode. A&AS 142, 137–148, 2000. Aulanier, G., De´moulin, P. 3-D magnetic configurations supporting prominences. A&A 329, 1125–1137, 1998. Engvold, O. Observations of fialment structure and dynamics, in: Webb, D., Rust, D.M., Schmieder, B. (Eds.), New Perspectives on Solar Prominences, ASP Conf. Ser., vol. 150, pp. 23–31, 1998. Martin, S.F. Conditions for the formation and maintenance of filaments. Solar Phys. 182, 107–137, 1998. Mein, N., Mein, P., Heinzel, P., Vial, J.-C., Malherbe, J.M., Staiger, J. Cloud model with variable source function for solar Ha structures. A&A 309, 275–283, 1996. Molowny-Horas, R., Heinzel, P., Mein, P., Mein, N. A non-LTE inversion procedure for chromospheric cloud-like features. A&A 345, 618–628, 1999. Paletou, F., Molodij, G. Multi-line spectropolarimetry at THEMIS, in: Sigwarth, Michael (Ed.), Advanced Solar Polarimetry – Theory, Observation, and Instrumentation, Asp Conf. Ser., vol. 236, pp. 1– 7, 2001a. Paletou, F., Ariste, A. Lo´pez, Bommier, V., Semel, M. Full-Stokes spectropolarimetry of solar prominences. A&A 375, L39–L42, 2001b. Tandberg-Hanssen, E.. The Nature of Solar Prominences. Kluwer Academic Publishers, Dordrecht, 1995, pp. 3–11. Zong, W.G., Tang, Y.H., Fang, C., Mein, P., Mein, N., Xu, A.A. Spectro-polarimetric observations of the fine structure of a quiescent filament. A&A 412, 267–270, 2003.
Observations of an active region filament W.G. Zong *, Y.H. Tang, C. Fang, A.A. Xu Department of Astronomy, Nanjing University, Nanjing 210093, PR China Received 27 October 2004; received in revised form 10 May 2005; accepted 10 May 2005
Abstract An active region filament was well observed on September 4, 2002 with THEMIS at the Teide observatory and SOHO/MDI. The ˚ lines. Using the data, we have studied the fine structure full Stokes parameters of the filament were obtained in Ha and FeI 6302 A of the filament and obtained the parameters at the barb endpoints, including intensity, velocity and longitudinal magnetic field. Our results indicate: (a) the Doppler velocities are quiet different at barb endpoints; (b) the longitudinal magnetic fields at the barb endpoints are very weak; (c) there is a strong magnetic field structure under the filament spine. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Filament; Barb; Endpoint
1. Introduction Filaments are commonly observed on the solar disk in Ha line. They appear as dark long structures called ‘‘spine’’ and lateral extensions named ‘‘barbs’’ or footpoints (Martin, 1998). Tandberg-Hanssen (1995) and Engvold (1998) distinguished filaments into three types: quiescent, active region and intermediate filaments. Quiescent filaments appear in quiescent regions or between weak remnants of active regions. They are long and sheet-like structures. While active region filaments form in or close to active regions. They are generally smaller and curved. Comparing to quiescent filaments, active region filaments are short-lived and more rapidly changing. In previous paper (Zong et al., 2003), we have studied the fine structure of a quiescent filament with the data from THEMIS in the multichannel subtractive double pass spectrograph (MSDP) mode. In this paper, we present the observations of an active region filament and give the parameters at the barb endpoints, and then *
Corresponding author. Tel.: +86 25 83686327. E-mail address: [email protected] (W.G. Zong).
the difference between the two type filaments is discussed.
2. Observations The observations were made on September 4, 2002 during 16:50–18:00 UT, with the multi-line spectroscopy (MTR) mode of the THEMIS telescope (Paletou et al., 2001a). By scanning the filament area of 8000 · 6000 , 2D ˚ lines spectral maps were obtained. Ha and FeI 6302 A were recorded simultaneously. The spatial sampling along the slit is 0.500 /pixel. The ˚ for Ha and 22 mA ˚ for FeI spectral sampling is 26 mA ˚ . The exposure time for all wavelengths is 6302 A 300 ms. Seeing was better than 1–200 during the observations, but the scanning procedure limited the spatial resolution of 2D maps to about 2–300 . The so-called 2 · 1 THEMIS spectro-polarimetric configuration was used. Two beams with orthogonal polarization exiting the analyzer are directed into a single camera. Beam inversion was performed for the polarization: the top part of the camera received sequentially I + Q, I Q, I + U, I U, I + V and I V, while
0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.05.028
W.G. Zong et al. / Advances in Space Research 35 (2005) 1752–1755
1753
Fig. 1. Ha image from observatory of Paris at 11:04:00 UT (left) and the longitudinal magnetic field from MDI at 17:39:30 UT (right). The observed region is marked by black rectangle. It is around the active region NOAA10099. East is to the left and north is up.
the bottom part recorded I Q, I + Q, I U, I + U, I V and I + V. Dark current and flat-field correction were performed. The muller matrix was used to demodulate the Stokes parameters. To remove induced crosstalk and improve the seeing smearing, we sum the top and the bottom spectra for each parameter and then subtract each other. To do so, the two spectra are well aligned in Y-direction (slit direction). Ariste et al. (2000) and Paletou et al. (2001b) introduced the process of data reduction in detail. The full solar disk Ha image is shown in the left panel of Fig. 1. It was gotten from Observatory of Paris. It can be seen that the observed filament locates in the southern hemisphere near the center of the solar disk (W25 S15). It is just around the active region NOAA10099. The field of view (the black rectangles in Fig. 1) covers an area of 8000 · 6200 . The longitudinal magnetic field from the SOHO/MDI (Michelson Doppler Imager) is also presented in the right panel of Fig. 1.
3. Data analysis After data reduction, the full-Stokes parameters of ˚ lines were obtained. Using the the Ha and FeI 6302 A Stokes parameters, we obtained 2D maps of the intensity and the velocity fields, which are given in Fig. 2. For comparison, both the longitudinal magnetic fields ˚ and from MDI are presented. derived from FeI 6302 A Magnetic contours on the strong magnetic structure are plotted in each magnetogram. It can be seen that the re˚ line and MDI are well sults derived from the FeI 6302 A consistent. The linear coherence coefficient is 0.91. For Ha data, the calibration has been done by using the area in the black rectangle (700 · 700 ) in Fig. 2(a) as the quiet background. The images at the Ha line wings have been used to ˚ from Ha line find barbs. In the image at Dk = 0.44 A center, four barbs are well identified and marked with
circles in Fig. 2. It can be seen that they are left-bearing, as expected for filaments in the southern hemisphere. This is consistent with the empirical chirality rule proposed by Martin (1998). Fig. 2(c) shows the velocity field in the observed area. The positions of the four barb endpoints are marked out. The results indicate that the mass was flowing up in some endpoints, but flowing down in the others. The values of the velocity cover a wide range, from tens to several hundred meters per second. The average values for five points at the endpoints marked 1–4 are 470 m s1 (up), 770 m s1 (down), 30 m s1 (up) and 680 m s1 (down), respectively. The accuracy of the velocity measurements is estimated to be ±60 m s1. It can be seen both in Fig. 2(b) and (d) that all the barbs terminate in weak magnetic field of only several Gauss. There are no strong polarities nearby. The magnetic field near the barb endpoints is less than 20 G. Just as mentioned above, the filament locates above a neutral line. But by comparing the intensity and the magnetic field in Fig. 2, it can be seen clearly that there is a strong magnetic polar under the filament spine. The contours have been drawn over the structure, and the levels are 80, 130 and 180 G. In Fig. 2(c), the strong magnetic structure is co-aligned with the velocity field. The direction of the velocity above the structure is down and the value is about 600 m s1. Near the strong magnetic structure (left upper to it), there is an area with high velocity (mass flows up), and the value reaches 2 km s1.
4. Discussion and summary A filament in the active region NOAA 10099 was well observed on September 4, 2002 with THEMIS/MTR. By use of the images at the Ha line wings, four barbs of the filament have been identified, and the parameters at the endpoints of the barbs are derived. The intensity, velocity and longitudinal magnetic fields are all presented.
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W.G. Zong et al. / Advances in Space Research 35 (2005) 1752–1755
˚ from Ha line center. Four barbs are well identified and their endpoints are Fig. 2. Main results of the observations. (a) The image at Dk = 0.44 A ˚ marked with circles. The area in black rectangle was selected as the quiet background for calibration. (b) The magnetic field derived from FeI 6302 A line observation. White is up and black is down. (c) The velocity field derived from Ha line with the center of gravity (COG) method. Black is up and white is down. (d) The photospheric longitudinal magnetic field from MDI. The black line is the neutral line of large scale magnetic field. The contour levels are 80, 130 and 180 G. All four images correspond to the same area on the solar disk.
Although the barb endpoints of the active region filament still locate in weak magnetic fields, they are different from that of the quiescent filament discussed in our previous paper (Zong et al., 2003). According to our result, the barb endpoints of the quiescent filament are located in weak magnetic fields near neutral lines of the majority polarities and minority polarities. The magnetic field of the majority polarities is more than 70 G and those of minority polarities is greater than 30 G. However, all the barb endpoints discussed here are far away from the strong polarities, and the magnetic field of the polarities nearby is less than 20 G. The velocities at the four barb endpoints have been given. The range is from tens to several hundred meters per second. However, as Mein et al. (1996) and Molowny-Horas et al. (1999) indicated, a Ha filament is observed against a strong chromospheric line, the measured line shift will be notably less than the real Doppler shift produced by the filament. In addition, the spatial resolution of our observations is 2–300 , the Doppler velocities deduced from our observations are the results from spatial smearing of sub-arc sec structures. All these effects lead to an underestimate of the velocities in barbs. It implies that there is significant
mass flowing through the barb endpoints, either into or out from the barbs. There are mainly two kinds of model involving filament barbs. Martin (1998) proposed that the filament barbs are anchored in parasitic polarity elements. Aulanier and De´moulin (1998) proposed that barbs are located at ‘‘bald patches’’. In our observation, all the four barbs end in very weak longitudinal magnetic fields. But we can not rule out the possibility that the magnetic fields in barbs are horizontal, as suggested by the model of Aulanier and De´moulin (1998). However, their model cannot be reconciled with the flowing of plasma in our observations. As generally believed, the filaments locate near the neutral lines of magnetic field and thus it seems there is only weak magnetic field under the filament spine. However, in our observation, a strong magnetic structure (about 200 G) near the filament was found. This can be clearly seen both in MDI map and our derived magnetogram. A similar phenomenon had also been found in our previous observations (Zong et al., 2003), in which a structure with 300 G under the quiescent filament spine was detected. In the case of the active region filament, the distance between the strong
W.G. Zong et al. / Advances in Space Research 35 (2005) 1752–1755
magnetic structure and the neutral line shown in Fig. 2(d) is about 2000 . Considering that the filament locates at a heliocentral angle of about 26 on the solar disk, if the height of the filament is 50,000 km, than the distance would have been about 4000 . So only if the height of the filament is less than, say, 25,000 km, than the coverage of the strong magnetic structure on the filament can be probably explained by the projection effect. If not, then we are still not understand the physical meaning about the strong magnetic structure. To resolve the puzzle, more work has to be done. Acknowledgments We give our sincere thanks to Dr. G. Ceppatelli, Dr. C. Briand and other staffs at the Spanish Observatorio del Teide of the Institute de Astrofisica de Canarias for their enthusiastic help during C.F. and Y.H.TÕs stay at the observatory. We also thank the referees for their valuable comments and suggestions to improve both the science and the language. This work has been supported by NKBRSF of China G20000784 and by NSFC key Project of China No. 10333040, as well as NSFC under Project of Nos. 10073005 and 10403003.
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References Ariste, A. Lo´pez, Rayrole, J., Semel, M. First results from THEMIS spectropolarimetric mode. A&AS 142, 137–148, 2000. Aulanier, G., De´moulin, P. 3-D magnetic configurations supporting prominences. A&A 329, 1125–1137, 1998. Engvold, O. Observations of fialment structure and dynamics, in: Webb, D., Rust, D.M., Schmieder, B. (Eds.), New Perspectives on Solar Prominences, ASP Conf. Ser., vol. 150, pp. 23–31, 1998. Martin, S.F. Conditions for the formation and maintenance of filaments. Solar Phys. 182, 107–137, 1998. Mein, N., Mein, P., Heinzel, P., Vial, J.-C., Malherbe, J.M., Staiger, J. Cloud model with variable source function for solar Ha structures. A&A 309, 275–283, 1996. Molowny-Horas, R., Heinzel, P., Mein, P., Mein, N. A non-LTE inversion procedure for chromospheric cloud-like features. A&A 345, 618–628, 1999. Paletou, F., Molodij, G. Multi-line spectropolarimetry at THEMIS, in: Sigwarth, Michael (Ed.), Advanced Solar Polarimetry – Theory, Observation, and Instrumentation, Asp Conf. Ser., vol. 236, pp. 1– 7, 2001a. Paletou, F., Ariste, A. Lo´pez, Bommier, V., Semel, M. Full-Stokes spectropolarimetry of solar prominences. A&A 375, L39–L42, 2001b. Tandberg-Hanssen, E.. The Nature of Solar Prominences. Kluwer Academic Publishers, Dordrecht, 1995, pp. 3–11. Zong, W.G., Tang, Y.H., Fang, C., Mein, P., Mein, N., Xu, A.A. Spectro-polarimetric observations of the fine structure of a quiescent filament. A&A 412, 267–270, 2003.