The Astrophysical Journal, 745:9 (7pp), 2012 January 20 C 2012.
doi:10.1088/0004-637X/745/1/9
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
SYMPATHETIC FILAMENT ERUPTIONS FROM A BIPOLAR HELMET STREAMER IN THE SUN 1
Jiayan Yang1,2 , Yunchun Jiang1 , Ruisheng Zheng1 , Yi Bi1 , Junchao Hong1 , and Bo Yang1 National Astronomical Observatory/Yunnan Astronomical Observatory, Chinese Academy of Sciences, P.O. Box 110, Kunming 650011, China;
[email protected] 2 Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650011, China Received 2011 September 14; accepted 2011 October 31; published 2011 December 27
ABSTRACT On 2005 August 5, two solar filaments erupted successively from different confined arcades underlying a common overarching multiple-arcade bipolar helmet streamer. We present detailed observations of these two events and identify them as sympathetic filament eruptions. The first (F1) is a small active-region filament located near the outskirts of the streamer arcade. It underwent a nonradial eruption, initially moving in the interior of the streamer arcade and resulting in an over-and-out coronal mass ejection. The second filament (F2), a larger quiescent one far away from F1, was clearly disturbed during the F1 eruption. It then underwent a very slow eruption and finally disappeared completely and permanently. Because two belt-shaped diffuse dimmings formed along the footprints of the streamer arcade in the first eruption and persisted throughout the complete disappearance of F2, the eruption series are interpreted as sympathetic: the simple expansion of the common streamer arcade forced by the F1 eruption weakened magnetic flux overlying F2 and thus led to its slow eruption, with the dimming formation indicating their physical connection. Our observations suggest that multiple-arcade bipolar helmet-streamer configurations are appropriate to producing sympathetic eruptions. Combined with the recent observations of unipolar-streamer sympathetic events, it appears that a multiple-arcade unipolar or bipolar helmet streamer can serve as a common magnetic configuration for sympathetic eruptions. Key words: Sun: activity – Sun: coronal mass ejections (CMEs) – Sun: filaments, prominences – Sun: flares – Sun: magnetic topology
in single-arcade streamers with a simple overlying bipolar field (Low 1996), a common magnetic configuration of the source fields for many CMEs should be large-scale complex magnetic systems with multiple arcades (Webb et al. 1997). Therefore, the multiple-arcade helmet-streamer configuration is crucial for the understanding of many CMEs. There are two kinds of helmet streamers, i.e., unipolar and bipolar streamers, which correspond to large-scale closed magnetic field regions containing an even or odd number of arcades sandwiched between like- or opposite-polarity open fields, respectively (Hundhausen 1972; Neugebauer et al. 2002; Wang et al. 2007). This means that both kinds of streamers can contain multiple filaments but have different ambient magnetic field structures. Based on observations and the MHD simulation model, T¨or¨ok et al. (2011) have more recently suggested that a multiplearcade unipolar streamer (or pseudo-streamer) appears to be prone to producing the 2010 August 1 sympathetic eruptions. Because CMEs can originate from either unipolar or bipolar streamers but show distinct dynamic characteristics due to their different background fields (Eselevich 1995; Zhao & Webb 2003; Liu 2007; Wang et al. 2009), it is naturally expected that a multiple-arcade bipolar streamer might also be favorable for the production of sympathetic eruptions and should involve a process different from that in a unipolar streamer. Coronal dimmings are a frequently occurring phenomenon in many eruption events and are usually observed by Yohkoh soft X-ray (SXR) and the Solar and Heliospheric Observatory (SOHO) extreme-ultraviolet (EUV) images (Hudson et al. 1996; Sterling & Hudson 1997; Thompson et al. 1998; Zarro et al. 1999). They can take complicated forms in a wide range of spatial scales (Attrill et al. 2007; Jiang et al. 2007; Mandrini et al. 2007; Zhukov & Veselovsky 2007; Innes et al. 2010; Hong et al. 2011; Yang et al. 2011; Zheng et al. 2011) and are usually regarded as key low-corona proxies of CMEs. As mentioned by many researchers, coronal dimmings are caused by the
1. INTRODUCTION A sympathetic solar event is defined as consecutive eruptions occurring in different locations but having a certain physical connection, the existence of which is evidenced by previous statistical studies (Pearce & Harrison 1990; Moon et al. 2002; Wheatland & Craig 2006) and observations of individual sympathetic flares (Shi et al. 1997; Bagala et al. 2000; Gopalswamy et al. 1999). The key point of the sympathy of successive eruptions is to determine their causal linkage. Some studies have suggested that a direct magnetic interaction between different magnetic structures or a traveling disturbance indicated by surges/jets can link two eruptions as a sympathetic pair (Wang et al. 2001; Jiang et al. 2008, 2009a). Schrijver & Title (2011) showed that all eruptions in the 2010 August 1–2 global sympathetic event were connected by a system of separatrices, separators, and quasi-separatrix layers at the topological divisions of coronal magnetic force lines. However, an alternative driving mechanism for the sympathetic event is that an eruptive magnetic structure can easily weaken or remove the magnetic fields overlying other magnetic structures and thus can trigger another interrelated eruption (Gary & Moore 2004; Ding et al. 2006; Wang et al. 2006; Peng & Hu 2007; Zuccarello et al. 2009). Therefore, the creditable identification of the causal linkage between sympathetic eruptions touchs upon two major questions: do there exist some peculiar types of magnetic field configuration exclusively in favor of sympathetic eruptions? If so, are there any clear observational signatures of their physical connections? A multiple-arcade helmet streamer might be a potential candidate for such a configuration. It is well known that coronal helmet streamers are quite common structures in the Sun and are generally associated with coronal mass ejections (CMEs; Illing & Hundhausen 1986; Sime 1989). While some CMEs are due to the symmetrical eruptions of concentric filaments 1
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expansion of coronal magnetic fields, which is forced by CME eruptions and thus marks the footprints of the expanded fields. It has become increasingly clear that the dimming configuration may reflect the structure of the large-scale magnetic field involved in the associated CMEs (Cliver & Hudson 2002; van Driel-Gesztelyi et al. 2008). Therefore, the formation process of dimmings might represent the relaxation and restructuring of the closed field configuration before CME, and thus should provide clues to identifying the physical connection between some sympathetic eruptions. Consistent with previous speculations and observations (Watanabe et al. 1992; Srivastava et al. 2000; Liu et al. 2009), Jiang et al. (2011) have recently shown that diffuse coronal dimmings extending to surround adjacent filaments can serve as a clear indicator of causal connection between sequential filament eruptions from different arcades. It is very likely that a similar situation could also appear under the CME-associated, multiple-arcade streamer configuration. On 2005 August 5, two filament eruptions were observed along different photospheric neutral lines on the solar disk. During the first eruption followed by a flare and a CME, diffuse dimmings formed and extended to surround the second filament, and then persisted throughout the second eruption. The computed coronal magnetic-field configuration, using the potential-field source-surface (PFSS) model of the Schrijver & DeRosa (2003) version, shows that the two filaments were located in the interior of a bipolar helmet streamer. In this paper, we will show that the two eruptions were sympathetic and causally linked by the dimming process. Our observations bridge the gap between unipolar and bipolar streamers and thus expand the magnetic configurations favoring sympathetic eruptions to general multiple-arcade helmet streamers.
Japan (UeNo et al. 2004), and the Polarimeter for Inner Coronal Studies at Mauna Loa Solar Observatory (MLSO; MacQueen et al. 1998) were also used. These images have varied cadences and samplings from about 0. 6 to 2. 9 pixel−1 . We also examined full-disk He i 10830 Å velocity images from the Chromospheric Helium Imaging Photometer at MLSO, acquired at seven filter positions covering the spectral region from 10826 Å to 10834 Å, and provided a measure of the line-of-sight velocity component over the range of ±100 km s−1 (Gilbert et al. 2001). Finally, all of the above images on August 5 were differentially rotated to a reference time close to the flare in the first eruption (August 5 08:05 UT) and cropped to follow the two filament eruptions. To study the long-term disk-passage evolution of the two eruptive filaments, however, Hα images on other days were not subjected to such differential rotation correction. 3. OBSERVATIONAL RESULTS The long-duration C2.6 X-ray flare occurred on NOAA active region (AR) 10792 (N14◦ W19◦ ). It started at around 06:59 UT on 2005 August 5, reached the maximal flux at 08:05 UT, and ended at about 08:47 UT. Figure 1 shows the flare process and associated phenomena in Hα and EIT 195 Å observations. The flare originated from the eruption of a small active-region filament F1 and was associated with a partial halo CME. The CME’s central position angle (P.A.) is 23◦ and its width is larger than 233◦ . Before the flare, F1 was situated along the northeastern (NE) boundary of the AR and was centered at a P.A. of about 298◦ . There was a larger quiescent filament F2 far away from the AR. It was located at the northeast of F1, along another neutral line (see the first Hα image). F2 was about 0.54 R from the F1’s centroid, with a P.A. of 16◦ and an angular extent of about 13◦ relative to the solar disk center. After the flare, however, both F1 and F2 became completely invisible (see the second Hα image). In EIT observations, the F1 eruption showed two distinct characteristics. One is that the eruption was nonradial. At 07:11 UT, the erupting F1 appeared as a loop-like protrusion (indicated by the arrow in panel (d1)), and then the loop-like structure transformed into a nearly straight streak pointing toward the NE direction by 07:45 UT (indicated by the arrow in panel (d2)). Taking the projection effect into account, therefore, F1 did not erupt vertically but had an inclined path along the NE direction. The other is that two diffuse, beltshaped dimming regions, “D1” and “D2,” formed during the F1 eruption. Nearly along the NE eruption direction of F1, they developed to occupy areas larger than that of the relative compact flare brightening. D1 showed up as a dark region, but D2 clearly consisted of three patches, “D2a,” “D2b,” and “D2c,” that darkened one by one: first D2a, then D2b, and finally D2c. By comparison with MDI magnetograms, it is clear that D1 and D2 are located on opposite-polarity sides of the erupting F1 (see panel (b)): D1 corresponded to a positive-polarity region and D2 to a negative-polarity region. In Figure 1, the final CME direction, determined from its central P.A., is plotted by white dashed lines, and the radial direction of F1, defined as a straight line connecting its centroid to the center of the solar disk, is plotted by solid lines. It is clear that the directions of the F1 eruption and the dimming expansion were consistent with that of the CME but offset from the radial direction of F1. Interestingly, F2 was weakly disturbed in the course of the F1 eruption and the dimming formation (indicated by the black arrow in panel (d2)). Because D1 and D2 expanded so much that they eventually surrounded F2, and because they can be clearly discerned in the SXI SXR observations by the next day
2. DATA In the present study, we used full-disk EUV images with a pixel resolution of 2. 6 from the Extreme Ultraviolet Imaging Telescope (EIT; Delaboudini´ere et al. 1995), and line-of-sight magnetograms with a pixel size of 2 and a cadence of 96 minutes from the Michelson Doppler Imager (MDI; Scherrer et al. 1995). EIT and MDI are both on board SOHO. EIT images are taken in four spectral bands centered on Fe ix/x 171 Å, Fe xii 195 Å, Fe xv 284 Å, and He ii 304 Å. For this event, EIT 195 Å images were obtained continuously with a cadence of 12 minutes, while 171, 284, and 304 Å images were taken only once every 6 hours. To identify the CME associated with the event, we examined the C2 and C3 white-light coronagraph movies from the Large Angle and Spectrometric Coronagraphs (LASCO; Brueckner et al. 1995) on board SOHO, and the CME height–time data that are available at the LASCO Web site. We used the SXR light curves observed by the Geostationary Operational Environmental Satellite (GOES) to track the flare time. To trace the dimming evolution, we also examined the Corona-Hole type full-disk SXR images at level-1 from the Solar X-Ray Imager (SXI; Hill et al. 2005) on board the GOES-12 satellite. These images have a resolution of 5 pixel−1 and a cadence of 6 hr. Observations from the Transition Region and Coronal Explorer (TRACE; Handy et al. 1999) with a varied cadence and a pixel size of 0. 5 provided 171 Å images for the event. Its limited field of view (FOV) centered on the flare core and partially covered the dimming regions. Besides the data from the space telescope, ground-based fulldisk Hα line-center images from Kanzelh¨ohe Solar Observatory (KSO), the Solar Magnetic Activity Research Telescope (SMART) at Hida Observatory (HO) of Kyoto University in 2
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Figure 1. KSO and SMART (labeled as “smt”) Hα images (a1–a2), MDI magnetogram (b), EIT 195 Å direct image (c), and 195 Å (d1–d5), 284 Å (e), and SXI SXR (f1–f2) fixed-base difference images. Note that two Hα filaments, “F1” and “F2,” can be seen at 06:33 UT but have disappeared by 21:50 UT. Two dimmings, “D1” and “D2,” were formed during the F1 eruption, and D2 consisted of three components: “D2a,” “D2b,” and “D2c.” The asterisks in (d1) and (f1) mark F1’s original centroid determined from the 06:33 UT Hα image. The solid lines indicate F1’s radial direction, and the dashed lines the final CME direction. F2’s outlines determined from the 06:33 UT Hα image are superimposed as white contours, and the outlines of the erupting F1 determined from the 07:45 UT EIT 195 Å image are superimposed as white curves. The FOV is 1220 × 1140 . The dotted black/white boxes indicate the FOVs in Figure 2/4, and the solid boxes mark the areas, in which 195 Å light curves are measured and displayed in Figure 3.
after the F2 disappearance (see panel (f2)), it is probable that the F2 disappearance was associated with the dimming formation process during the F1 eruption. The zoomed-in view of F1’s eruption in KSO Hα and TRACE 171 Å images is presented in Figure 2. The eruptive F1 was the northernmost section of a longer filament surrounded by AR 10792. It was clearly visible in both Hα and 171 Å images (see panels (c1) and (d)) before the flare but disappeared near the flare peak time (see panel (c2)). Our Hα observations did not cover its eruption, but TRACE 171 Å images showed that the erupting F1 first took a form of bright loop-like structure (indicated by the arrow in panel (e1)) and then post-flare loops were formed to span the original F1 after the eruption (see panel (e3)). Note that the southern end of the longer filament was disturbed in the eruption (see panels (e1–e2)). Although the following evolution of the erupting F1 could not be traced by the TRACE observations due to the limited FOV, some coronal loops to the north of F1 were seen to disappear during the eruption and thus led to the formation of dimming patch D2a (indicated by the white thick arrows). As shown by Khan & Hudson (2000) and Jiang et al. (2008), therefore, it is probable that the dimmings in this event were due to expanding or even opening of pre-eruptive coronal loops caused by F1’s nonradial eruption. Figure 3 shows the GOES-10 1–8 Å SXR flux; the light curves of EIT 195 Å intensities in the areas D1, D2a, D2b, and D2c; and the CME height–time profile. The average speed and acceleration of the CME fronts are also indicated in Figure 3(c). It is clear that the 195 Å intensities in D1 decreased almost coincidentally with the flare start (06:59 UT). Consistent with the observations shown in Figure 1, however, the steep intensity drops in D2a, D2b, and D2c lagged slightly and successively occurred at
07:11, 07:23, and 07:45 UT, respectively. Finally, by the use of second-order polynomial fitting, the extrapolated CME onset time was also inferred. The CME began at about 07:41 UT in the course of the flare and the dimming formation, indicating that it was tightly correlated with the eruption of F1. As mentioned above, F2 was disturbed by F1’s eruption and eventually disappeared. Figure 4 presents the close-up view of F2’s evolution in Hα, EIT 195 Å, and He i 10830 Å velocity observations. First, the F2’s disturbance can be identified as the appearance of a bright streak in its body that is easily seen in both Hα and EIT 195 Å fixed-base difference images during the flare (panels (b1–b3) and (c1–c3)). Note that the extending D2b was close to F2 (panel (c3)) by 08:09 UT. Second, when we superimpose F2’s outlines at 06:33 UT (see panel (a1)) on the following Hα images (panels (a2–a6)), it is found that F2 continuously shifted westward after the disturbance. From 06:33 to 20:29 UT, its centroid migrated a distance of about 58,000 km, giving a mean projected velocity of about 1.16 km s−1 . Because F2 nearly disappeared by 21:50 UT (panel (a6)), it probably underwent a very slow eruption that lasted about 15 hr. This was confirmed by MLSO He i 10830 Å velocity observations started at 16:30 UT (panels (d1–d4)), showing that blueshifts were dominant in F2 and gradually faded away with time. LASCO did not detect any CME associated with this very slow eruption, and it is noteworthy that its eruption direction was clearly different from those of the F1 eruption and the dimming expansion. Jiang et al. (2009b) showed that, during three eruptions from AR 10792 in the preceding days (two eruptions on July 30 and the other one on August 1), F2 had obvious oscillation but remained in existence after each eruption (see their Table 1), and they further suggested that the interplay between different magnetic 3
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located far away from F1, D1 and D2 exhibited large expansion approaching and surrounding F2 and then persisted throughout the whole F2 eruption. To more clearly understand the relationship between the two successive eruptions, the formation processes of the dimmings and the magnetic connectivity between them are crucial. The PFSS model (Schrijver & DeRosa 2003) is used to construct the coronal fields based on synoptic magnetic maps from MDI, and representative coronal magnetic field lines associated with the dimmings and the two eruptive filaments are shown in Figure 6. As the most remarkable characteristic of the event, F1 and F2 are located below the bipolar helmet-streamer belt indicated by the PFSS model. The cusp of the streamer belt is identified with the computed source surface neural line (SSNL) at 2.5 R where the radial field is set to zero, i.e., the base of the heliospheric current sheet (Smith 2001). In Figure 6, the SSNL’s projection onto the photospheric magnetogram shows that it separates opposite-polarity field regions. There are three lower confined arcades, “a1,” “a2,” and “a3,” underneath the overarching streamer arcade, “SA.” The identification of the three arcades below the streamer is consistent with the coronagraph white-light edge-on observations at the southeast limb on July 29 when these arcades were perpendicular to the limb (Jiang et al. 2009b, see their Figure 6). D1 and D2 are obviously linked by SA. To better visualize their magnetic connectivity, the SA’s outermost field lines are highlighted in white and its other field lines are highlighted in green. It is clear that F1 is held by a1 and F2 is held by a2 in the SA’s interior, but F1 and a1 are located closer to the SA’s inner edge. Similar to the near-limb July 29 event from the same AR (Jiang et al. 2009b), therefore, the CME in the present on-disk event was also an over-and-out one (Harrison 1986; Moore & Sterling 2007; Yang et al. 2011). However, a conspicuous difference is that the erupting F1 ran a relatively long distance in a path nearly parallel instead of perpendicular to the SSNL, implying that the eruption was not laterally guided by the adjacent, overlying field lines but was mainly channeled by the streamer arcade along its axis. Such erupting behavior of F1, which has not been reported before, was clearly consistent with the northeastward-traveling dimming formation and the TRACE 171 Å loop disappearance. It is noted that the photospheric magnetic field settings at the footprints of a1–a3 are not favorable for reconnection among them when a1’s field was forced to expand by the erupting F1. Furthermore, no signature of interaction between the erupting F1 with a3, a2, or F2 was observed. Therefore, it is very likely that F1’s eruption flew over a3 and a2 but pushed SA outward to form D1 and D2. After F1’s eruption, F2 was still held by a2 while the upper SA field was removed. As a result, the overall closed magnetic field overlying F2 was weakened, which might be the cause of the F2’s slow eruption. Because F1 was located southwest of F2, its NE eruption as well as the CME weaken the SA field at F2’s western side more intensively than that at its eastern side. The dimming configuration also supported this conjecture. So, consistent with the recent results of Bi et al. (2011), F2 moved westward, suggesting that filament tends to erupt toward a region with a weakened overlying magnetic field. Under this scenario, the nonradially erupting F1 not only opened a1 but also resulted in the SA’s expansion, thus the same overarching SA also shared by F2 represents the most important connection between the two filaments. Although their successive eruptions seemingly had no close continuity due to F2’s very slow eruption, we still call them “sympathetic filament eruptions” because they originated from different compact
Figure 2. MDI magnetogram (a), TRACE white light (b), KSO Hα (c1–c2), and TRACE 171 Å direct (d) and fixed-base difference (e1–e3) images. F1’s outlines determined from the 06:33 UT Hα image are superimposed as black or white contours. The thin white arrow indicates F1’s original centroid, the black arrow indicates the erupting F1, and the thick white arrows indicate the disappearing loops forming a part of D2. The FOV is 250 × 270 .
arcades overlying F1 and F2 might have partially removed but did not completely destroy the restraining condition of overlying fields to F2. The entire disappearance of F2 in the event studied here was clearly different from the cases in these previous eruptions. To examine whether F2 resumed or not, Figure 5 presents the long-term evolution of AR 10792 and F2 at their disk passage in BBSO and KSO Hα images. It is clear that F2 kept its manifestation from July 29 to August 4. After the August 5 event, however, besides a tiny dark feature indicated by the arrows, its major part did not recover at least by August 9 when AR 10792 was close to the west limb. Therefore, it is very likely that the F2 disappearance that resulted from its slow eruption on August 5 was permanent, implying that the overall field configuration supporting F2 might have been largely altered or even destroyed completely. 4. MAGNETIC CONFIGURATION ANALYSIS We can reasonably surmise that F2’s slow eruption was related to the eruption of F1 due to F2 showing a clear disturbance signature in the course of F1’s eruption. Although F2 was 4
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Figure 3. Time profiles of GOES-10 1–8 Å SXR flux (the dashed curve in (a)) and the light curves of EIT 195 Å intensities in areas centered on D1 (the solid curve in (a)) and the three components of D2 (b). The light curves are computed from the intensity integrated and normalized over the regions indicated by the solid boxes in Figure 1. (c) Heights of the CME fronts as a function of time, and the back extrapolations by the use of second-order polynomial fitting. The vertical solid lines indicate the flare’s start time, the dashed lines indicate the extrapolated onset time of the CME, and the horizontal bars indicate the flare duration.
Figure 4. KSO, MLSO, and SMART (labeled as “smt”) direct Hα images (a1–a6) and MLSO He i 10830 Å velocity images (d1–d4) showing the slow F2 eruption. KSO Hα (b1–b3) and EIT 195 Å (c1–c3) fixed-base difference images (labeled as “Diff”) showing the F2 disturbance in the F1 eruption. F2’s outlines as in Figure 1 are superimposed as white contours. The FOV is 500 × 300 .
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Figure 5. KSO and BBSO Hα images showing the disk-passage evolution of AR 10792 and F2.
arcades, showed clear physical linkage, and F1’s eruption was immediately followed by that of F2.
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5. CONCLUSIONS AND DISCUSSION The derived PFSS magnetic configuration shows that the twin eruptive filaments were located inside different confined arcades underlying the common overarching arcade of the bipolar helmet-streamer belt. Originated from AR 10792 adjacent to the inner boundary of the streamer arcade, F1’s eruption initially had a nonradial direction nearly parallel to the axis of the streamer arcade, accordingly leading to the diffuse dimmings forming along both of its footprints and the over-and-out partial halo CME. It appears that the dimmings resulted from the simple expansion of the streamer arcade without involvement of reconnection between different arcades. F2 was obviously disturbed during F1’s eruption, then underwent a very slow eruption and eventually disappeared permanently. The dimmings extended to surround F2 during F1’s eruption and remained in existence after F2’s disappearance, and the computed magnetic connectivity between them suggests that the common streamer-arcade magnetic field overlying both F1 and F2 was removed by the F1’s eruption. In support of the previous suggestion (Jiang et al. 2011), the two consecutive filament eruptions are thus considered to be a sympathetic pair causally connected by the dimming formation process. As a key signature indicative of the removal of the overlying magnetic field and thus the sympathy of the two eruptions, the dimming process revealed that the F1 eruption makes a notable impact on the stability of F2 (Jiang et al. 2011). Similar sympathetic eruptions occurring in a relatively long time period were also shown by Schrijver & Title (2011) and T¨or¨ok et al. (2011). Because F1 and F2 were initially located within SA, however, our sympathetic eruptions were different from the global sympathetic events studied by Schrijver & Title (2011), in which there was little activity within the deep interiors of domains of magnetic connectivity. In the unipolar-streamer sympathetic eruptions studied by T¨or¨ok et al. (2011), two
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Figure 6. Overlay of MDI magnetogram (a) and EIT 195 Å base difference image (b) with the extrapolated field lines, emphasizing the magnetic connectivity between D1 and D2. The outlines of the pre-eruptive F1 and F2 (the red solid contours) and the erupting F1 as in Figure 1 (the red dashed curves) are overlaid, and F1’s radial direction and the final CME direction are also indicated (the white straight lines). White and green field lines correspond to the overarching streamer arcade, “SA,” forming the bipolar helmet-streamer belt, with a source surface neutral line, “SSNL” (the pink–black curves). The plus/minus signs mark the corresponding positive/negative magnetic polarities in the photosphere. Blue, orange, and pink field lines underneath SA correspond to three confined arcades, “a1,” “a2,” and “a3.” Note that a1/a2 hold F1/F2 and SA is anchored around the two dimmings. The FOV is 1300 × 1040 .
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coronal flux ropes were located within the streamer and one was located next to it. A sequence of eruptions is initiated by triggering the eruption of the flux rope next to the streamer, with the involvement of two consecutive reconnection events and removal of overlying flux. However, it seems that in our event a similar decrease resulted from the SA’s simple expansion pushed out by the F1 eruption and did not involve interaction or reconnection between different arcades. Further observations are necessary to clarify such a possibility. Besides a unipolar streamer, our example further showed that a multiple-arcade bipolar streamer represents another type of large-scale magnetic configuration that favors the occurrence of sympathetic eruptions. It is not uncommon that both unipolar and bipolar helmet streamers can contain multiple arcades and filaments. In combination with the result of unipolar-streamer sympathetic eruptions (T¨or¨ok et al. 2011), therefore, our bipolarstreamer sympathetic event implies that the favorable magnetic configuration for producing sympathetic eruptions might be broadened to general multiple-arcade streamers, regardless of its unipolar or bipolar nature. Clearly, robust observational and theoretical studies will be greatly helpful in pinning down this mechanism.
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