The Astrophysical Journal, 548:L99–L102, 2001 February 10 q 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.
MAGNETIC FLUX CANCELLATION ASSOCIATED WITH THE MAJOR SOLAR EVENT ON 2000 JULY 14 Jun Zhang,1 Jingxiu Wang,1 Yuanyong Deng,1 and Dejin Wu2 Received 2000 October 13; accepted 2000 November 29; published 2001 February 8
ABSTRACT The major solar event on 2000 July 14 is characterized by the simultaneous occurrence of a giant filament eruption, a great flare, and an extended Earth-directed coronal mass ejection. We examined in detail the magnetic evolution in its source active region, NOAA 9077, and found that the only obvious magnetic change in the course of the event is magnetic flux cancellation at many sites in the vicinity of the filament. Moreover, all the initial disturbance in the filament and the initial brightening around the filament took place at the cancellation sites. It is clearly indicated that the slow magnetic reconnection in the lower atmosphere, which is manifested as observed flux cancellation, is of overwhelming importance in leading to the global instability responsible for the major magnetic activity. Subject headings: Sun: coronal mass ejections (CMEs) — Sun: filaments — Sun: flares — Sun: magnetic fields
The optical data for this study include Ha filtergrams and photospheric vector magnetograms taken at the Huairou Solar Observing Station (HSOS) of the Beijing Astronomical Observatory. The UV and EUV observations were obtained from the Transition Region and Coronal Explorer (TRACE) satel˚ (Fe xii, ∼1.5 MK) and lite (Handy et al. 1999). The 195 A ˚ 1600 A images are used in this study. The full-disk EUV images and coronagraph observations were taken by the EUV Imaging Telescope (EIT) and the Large-Angle Spectrometric Coronagraph (LASCO), respectively, on board the Solar and Heliospheric Observatory (SOHO).
1. INTRODUCTION
On 2000 July 14, a great solar flare with X-ray importance of X5.7 launched near the disk center in active region NOAA 9077. It is the greatest solar event since 1989 (Title 2001). The flare was accompanied by a giant filament eruption and an extended Earth-directed coronal mass ejection (CME). The arrival at Earth of the massive electrified gas cloud from this CME caused vivid aurorae on July 16. The cause of a filament eruption is more likely in the rapid growth of filament instability. A filament will erupt when new magnetic flux emerges within or adjacent to the unipolar magnetic fields astride a filament in an orientation favorable for reconnection (Feynman & Martin 1995). Flux cancellation in the vicinity of a neutral line is suggested to be a necessary condition for filament formation and eventually for its eruption (Martin 1986). The association of flares and canceling magnetic fields was first noted by Martin et al. (1985) and was discussed later by Livi et al. (1989) and Wang & Shi (1993). However, the flux cancellations described by previous authors referred to either the quiet Sun or rather small active regions. Statistical studies have revealed that eruptive filaments are most closely correlated to CMEs among other manifestations of activity in the low layers of the solar atmosphere (Munro et al. 1979; Webb & Hundhausen 1987; St. Cyr & Webb 1991). An excellent observation of a CME initiation, related to a filament eruption, was presented by Dere et al. (1997). Feynman & Martin (1995) and Wang & Sheeley (1999) identified the association of emerging flux and CMEs. However, systematic efforts to identify the surface magnetic evolutions that lead to CME initiation have rarely been made so far. The major event on 2000 July 14 exhibits the almost simultaneous filament eruption, flare activity, and CME. It provides a good opportunity to study the surface magnetic activity that results in the rather global magnetic instability. In this Letter, we examine the detailed magnetic evolutions in the course of the major event with the emphasis on the evolutions that characterize this CME-producing active region.
2. TIMING OF THE EVENT
The analysis is based on the time sequences of observations from HSOS, TRACE, EIT, and LASCO on 2000 July 14. Figure 1 shows the general appearance of active region NOAA 9077, and the various manifestations of the event. The arrow in Figure 1a indicates a filament. Seen from the TRACE ˚ movie, we found that the brightening first appeared 1600 A from the region shown by a window in Figure 1b; then the bright material (the arrow in Fig. 1b) moved along the channel (dotted curve) of the filament to the right. Several hours before the filament eruption, some bright points (or patches) had already appeared on both sides of the filament. The arrow in Figure 1c indicates one main bright patch. It enlarged along the filament to form a bright ribbon. This bright patch was also ˚ images (see Figs. 1a and identified in TRACE 195 and 1600 A 1b). Comparing the magnetogram with the Ha filtergram in the figure, we noticed that the bright patch was located at the region (indicated by the arrow in Fig. 1d) where a pair of opposite polarity fields was closely contacting and canceling. The eruption of the filament and the onset of the flare were accompanied by an extended halo CME; see the running difference image of LASCO C2 in Figure 1f. To illustrate the process of the filament eruption and the flare onset, we present in Figure 2 the time sequence of TRACE ˚ images. The filament (indicated by the arrow at 195 A 09:30:15 UT) apparently consisted of two twisting threads (indicated by the upper two arrows at 09:46:06 UT); one was thicker and diffusive, and the other was thinner and compact. At an inflection point of the filament, the filament appeared to
1 National Astronomical Observatories, Chinese Academy of Sciences, 20 Datun Road, Chaoyang, Beijing 100012, China;
[email protected]. 2 Purple Mountain Observatory, Chinese Academy of Sciences, 2 Beijing Xi Lu, Nanjing, Jiangsu 210008, China.
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Fig. 1.—Appearance of the active phenomena in the region NOAA 9077 on ˚ image. (b) A TRACE 1600 A ˚ image. (c) An 2000 July 14. (a) A TRACE 195 A Ha filtergram. (d) The corresponding line-of-sight magnetogram at 06:34 UT. (e) A running difference of the SOHO EIT image. There is a bright patch of the class X5.7 flare. (f) A running difference of the LASCO C2 image showing a halo CME (see the description in the text).
be bifurcated (see the lower arrow at 09:46:06 UT). At first, the thinner one of the two threads was cut off at the point indicated by an arrow in the frame of 09:48:17 UT; then the two threads broke off. At the broken point, a bright flare patch (shown by the arrow at 10:09:19 UT) appeared while the filament began to erupt. The filament tore into two pieces from the broken point. The upper piece seemed to be stable, while the lower one rose from one end close to the broken point with another end fixed in place. Several minutes later, another flare patch (see the arrow at 10:11:39 UT) appeared near the fixed end of the filament. This flare patch became larger and larger; meanwhile, the whole body of the lower piece of filament started to rise and erupt. The erupted piece appeared to rotate, and two threads (see arrows at 10:15:03 UT) were clearly seen to untwist during the eruption. Near the maximal phase of the flare, only a small segment of this piece of the filament remained. The whole time sequence of the solar event is summarized in Table 1.
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˚ images showing the evolution of Fig. 2.—Time sequence of TRACE 195 A the filament. The field of view is about 29000 # 29000 . The arrows in this figure are described in the text.
Moreover, the stressing or shearing of the magnetic fields in the atmosphere is created by the interaction of a topologyindependent flux system in the rather lower atmosphere (Wang 2000). With good measurements of vector magnetic fields in the photosphere for this highly active region, we examined in detail the fundamental magnetic evolutions that led to this major solar event. Figure 3 presents the time sequences of HSOS vector magnetograms on July 14 with a superposition of the filament in ˚ the lower right panel. From Ha, TRACE 195 and 1600 A
3. FLUX CANCELLATION
It is commonly accepted that the violent activities in the solar atmosphere, e.g., filament eruption and flare onset, are powered by the energy stored in the stressed magnetic field. TABLE 1 Time Sequence of the Solar Event Time (UT) 04:09 . . . . . . 08:22 . . . . . . 09:46 . . . . . . 09:48 10:00 10:03 10:09 10:24 10:54 11:18
...... ...... ...... ...... ...... ...... ......
Phenomena Ha brightenings on both sides of filament (from HSOS) Brightening of filament at region shown by window in ˚ images Fig. 1 in TRACE 1600 A Bifurcating of filament in the inflection point in TRACE ˚ images 195 A ˚ images Breaking of the thinner filament thread in 195 A Breaking of the filament seen in EIT images Appearance of the X5.7 flare in Ha images Appearance of flare patch at the broken point at EUV Maximum phase of the X5.7 flare (from EIT) First appearance of the halo CME in C2 field First appearance of the halo CME in C3 field
Fig. 3.—Time sequences of vector magnetograms observed at HSOS. The line-of-sight component of the magnetic field is presented by gray-scale patches and isogauss contours with levels of 5100, 200, 400, and 800 G. White patches represent positive polarity fields, and black patches represent negative fields. The transverse component is shown with short lines, with lengths proportional to the relative field strength. The dark ribbon at 08:12 UT is the Ha filament at 08:42 UT. The windows and letters in this figure are described in the text. On the x-axis, 1 unit p 00. 613; on the y-axis, 1 unit p 00. 425.
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Fig. 4.—Time sequence of vector magnetograms in the area labeled window A in Fig. 3 showing the magnetic field evolution. On the x-axis, 1 unit p 00. 613; on the y-axis, 1 unit p 00. 425.
movies, it is identified that there are a few key sites in the filament where the activities initiated. We have marked these sites by windows A, B, and D and square brackets in the magnetograms. It is further identified that all these sites are places where magnetic flux cancellation took place. To show the magnetic field evolution on some key sites, we present two time sequences of vector magnetograms in windows A and B in Figures 4 and 5, respectively. In Figure 4, the straight line at 00:08 UT indicates a piece of a magnetic neutral line. The transverse field alignments show that the positive magnetic field (above the line) and the negative field (below the line and on the left, shown by arrow 2 in Fig. 3) were a single couple of magnetic features. It is identified from the history of flux evolution from Michelson Doppler Imager and HSOS magnetograms that they represented an emerging flux region in this superactive region. The negative magnetic field squeezed upward to the left of the positive magnetic flux. During the 10 hr observations from 23:32 UT July 13, the negative polarity field moved about 7.1 # 10 3 km (related to the negative polarity field shown by arrow 1 in Fig. 3). The mean speed was 0.2 km s21. The positive polarity field moved downward and canceled with a nearby negative field (indicated by an open square bracket at 01:01 UT) until the disappearance of negative flux. We have also noticed the flux cancellation when some positive magnetic patches (the two arrows at 01:01 and 04:14 UT) slid and intruded into the negative magnetic fields to its south. As a result of the shearing motion of opposite polarities in the emerging flux region and related flux
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Fig. 5.—Time sequence of vector magnetograms in the area labeled window B in Fig. 3 showing the magnetic field evolution. On the x-axis, 1 unit p 00. 613; on the y-axis, 1 unit p 00. 425.
cancellation, the orientation of the magnetic neutral line altered obviously. In the period from 00:08 to 08:12 UT, its alignment changed 707 (see the solid and dotted lines at 08:12 UT, which represent the neutral lines at 08:12 and 00:08 UT, respectively). The magnetic field evolution in the area of the inflection point of the filament is shown in Figure 5. Again, it is found that the only obvious change in this window is flux cancellation. The square brackets at 00:08 UT indicate a canceling magnetic feature in the vicinity of the inflection point. Another canceling magnetic feature is indicated by an open bracket at 01:01 UT. A negative flux patch (indicated by circle g at 04:14 UT in Fig. 3) increased in flux from 00:08 to 04:14 UT and then decreased by the cancellation with the positive flux to its north. At 08:12 UT, it almost completely disappeared. The transverse field alignment indicated that the negative flux and the positive flux to its right were topology-connected and thus a magnetic bipole. In order to show the mutual flux disappearance in the flux cancellation, we present in Figure 6 the temporal evolution of the magnetic flux in several canceling magnetic features. It is found that, in the observation interval, approximately 1.4 # 10 20 Mx total flux disappeared in the vicinity of the filament. 4. DISCUSSION
Canceling magnetic fields are defined by the mutual flux disappearance at the boundaries between two closely spaced magnetic features of opposite polarity. Our observations have confirmed the earlier results that the two components of can-
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Fig. 6.—Upper panel: Magnetic flux in region C (shown in Fig. 3) as functions of time. The dotted, solid, and dashed curves indicate the negative, positive, and total (the negative plus the positive) flux, respectively. Middle panel: Same as the upper panel, but for region D in Fig. 3. Lower panel: Magnetic flux as functions of time. The solid, dotted, dot-dashed, and dashed curves indicate magnetic patches e, f, g, and the total (e 1 f 1 g) flux, respectively. On the y-axis, 1 unit p 1019 Mx.
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celing magnetic features are not connected by transverse fields (Wang & Shi 1993). Moreover, the Ha brightenings always appear at the sites of canceling magnetic features. This indicates that some energy release takes place at the site of flux cancellation. Thus, magnetic reconnection must have taken place in canceling magnetic features. Martin & Livi (1992) suggested a physical link between canceling magnetic fields and eruptive flares. They proposed that canceling magnetic fields serve as the direct transfer of magnetic flux, and thus magnetic energy, from the photosphere into the filament. Wang & Shi (1993) presented a two-step reconnection scenario for flare process. The first step of reconnection takes place in the photosphere, or lower atmosphere, and is seen as flux cancellation observed in the photosphere. It is slow but continuous. This slow reconnection might convert the magnetic energy into heat and kinetic energy; but, more importantly, it transports the magnetic energy and complexity into the rather large-scale magnetic structure higher in the corona. The second step of reconnection, which is explosive in nature and directly responsible for transient solar activities, can take place only when some critical status is achieved in the corona. The key idea of these authors is the overwhelming importance of magnetic reconnection in the lower atmosphere in the energy process of explosive solar activities. The major solar event manifests itself as a giant filament eruption, a great flare, and an extended Earth-directed CME. For such a major event, the only obvious magnetic change in its source region was magnetic flux cancellation at many sites in the vicinity of the filament. More importantly, all the initial disturbance in the filament and initial brightening around the filament took place at the sites of canceling magnetic features. This shows the first evidence of flux cancellation in a superactive region and the intrinsic association of flux cancellation with a major solar event. It is indicative that the slow magnetic reconnection beneath the filament plays a decisive role in producing the global instability responsible for this major solar event. This work is supported by the National Natural Science Foundation of China (grants 19791090 and 19973009). The authors are indebted to the HSOS, TRACE, and SOHO EIT and LASCO teams for providing the wonderful data.
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