SYMPATHETIC CORONAL MASS EJECTIONS Y.-J. Moon, 1,2 G. S. Choe, 3 Haimin Wang, 1 and Y. D. Park 2

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1 The Astrophysical Journal, 588: , 2003 May 10 # The American Astronomical Society. All rights reserved. Printed in U.S.A. SYMPATHETIC CORONAL MASS EJECTIONS Y.-J. Moon, 1,2 G. S. Choe, 3 Haimin Wang, 1 and Y. D. Park 2 Received 2002 November 27; accepted 2003 January 23 Q1 ABSTRACT We address the question whether there exist sympathetic coronal mass ejections (CMEs), which take place almost simultaneously in different locations with a certain physical connection. For this study, the following three investigations are performed. First, we have examined the waiting-time distribution of the CMEs that were observed by the SOHO Large Angle and Spectrometric Coronagraph (LASCO) from 1999 February to 2001 December. The observed waiting-time distribution is found to be well approximated by a timedependent Poisson distribution without any noticeable overabundance at short waiting times. Second, we have investigated the angular difference distribution of successive CME pairs to examine their spatial correlations. A remarkable overabundance relative to background levels is found within 10 of the position angle difference, which supports the existence of quasi-homologous CMEs that occur sequentially in the same active region. Both of the above results indicate that sympathetic (interdependent) CMEs are far less frequent than independent CMEs. Third, we have examined the EUV Imaging Telescope running difference images and the LASCO images of quasi-simultaneous CME pairs and found a candidate sympathetic CME pair, of which the second CME may be initiated by the eruption of the first CME. Possible mechanisms of the sympathetic CME triggering are discussed. Subject headings: Sun: corona Sun: coronal mass ejections (CMEs) 1. INTRODUCTION Sympathetic solar eruptive events, which include coronal mass ejections (CMEs) and flares, are defined as a pair of Q2 successive events that occur almost simultaneously in different positions with a certain physical connection. As expounded in Moon et al. (2002a), sympathetic events can be contrasted to quasi-simultaneous events, which occur almost simultaneously in different locations irrespective of physical connections. Thus, the set of sympathetic events is a subset of quasi-simultaneous events, and quasi-simultaneous events are either sympathetic (with a certain physical connection) or random-coincident. In this paper, a pair of CMEs is considered as quasi-simultaneous if the activities of both CMEs are observed in the same field of view (the Large Angle and Spectrometric Coronagraph [LASCO] C2 field of view in our case) within the observing LASCO time cadence of a few tens of minutes. Sympathetic CMEs, which would take place almost simultaneously in different regions, would be distinguished from homologous (recurrent) CMEs, which take place in the same active region within a certain time interval. Sympathetic flares have been studied for quite a long time (Fritzova-Svestkova, Chase, & Svestka 1976; Pearce & Harrison 1990; Bumba & Klvana 1993; Biesecker & Thompson 2000; Wang et al. 2001; Moon et al. 2001, 2002a), but there has been a controversy about their existence (Pearce & Harrison 1990; Bumba & Klvana 1993; Biesecker & Thompson 2000). With an analysis of the GOES X-ray flare data, Moon et al. (2002a) recently presented strong statistical evidence for the existence of 1 Big Bear Solar Observatory, New Jersey Institute of Technology, North Shore Lane, Big Bear City, CA 92314; yjmoon@bbso.njit.edu. 2 Korea Astronomy Observatory, San 36-1 Whaamdong, Yuseong-gu, Daejeon, , Korea. 3 Plasma Physics Laboratory, Princeton University, Princeton, NJ sympathetic flares. As for coronal mass ejections, which have much larger spatial scales than flares, their sympathetic occurrence has rarely been reported. Simnett & Hudson (1997) studied a pair of plausible sympathetic CME events observed on 1997 February 23 and argued that an initial ejection of mass to the northeast led to a further destabilization of a much larger magnetic loop system and an ejection of mass over a latitude range of 60. On the other hand, homologous CMEs, which are also rare, are more frequently reported than sympathetic CMEs. Lyons & Simnett (1999) studied two sets of multiple (recurrent) CMEs, which respectively occurred over a period of 16 hr. They suggested a rapid destabilization of structures within the global coronal field as a possible triggering mechanism. The homologous CME-flare events, which occurred sequentially in NOAA Active Region 9236 on 2000 November 24 25, were rather well observed and have been studied in several aspects: multi-wavelength characteristics (Nitta & Hudson 2001), their homology associated with moving magnetic features (Zhang & Wang 2002), the CME-flare relationship (Moon et al. 2002b), and the temporal evolution of magnetic helicity change rate (Moon et al. 2003). One of the most useful tools for investigating the interdependence of successive events is the analysis of the socalled waiting-time distribution, which is defined as the time interval between two successive events. The waitingtime distribution can provide us with statistical information on the probability of the next event occurring within a certain time interval after one event takes place. If the number of sympathetic events is not negligible, an overabundance will appear in short intervals of the waiting-time distribution relative to a Poisson distribution, because sympathetic events literally have an interdependency, while a Poisson distribution implies a totally random process. Recently, the waiting-time distribution has been used to examine the existence of sympathetic flares (e.g., Biesecker & Thompson 2000; Moon et al. 2001, 2002a). Biesecker & Thompson (2000) found no evidence of sympathetic flares from the

2 2 MOON ET AL. Vol. 588 distribution of solar X-ray flares. Examining the waitingtime distribution of GOES X-ray flares with short waiting times and the angular correlation function of simultaneous flares, Moon et al. (2001) also confirmed that the number of sympathetic flares is not significant in a statistical sense. However, Moon et al. (2002a) found strong statistical evidence of sympathetic flares, analyzing the waiting-time distribution in carefully selected active region pairs, which have an actual flaring overlap time larger than the random coincidence overlap time. Unfortunately, the approach taken by Moon et al. (2002a) cannot be applied to LASCO CMEs because the locations of their origin on the solar surface are not fully known and because their time durations cannot be well defined. In this paper we examine sympathetic properties of CMEs and investigate the underlying physical connections. To do this, we take the following approaches. First, we examine the waiting-time distribution of the LASCO CMEs that occurred from 1999 February to 2001 December looking for statistical evidence for sympathetic CMEs. Second, we investigate the angular difference distribution of successive CME pairs and examine their spatial correlations. Third, we inspect the EUV Imaging Telescope (EIT) images, their running difference images, and the LASCO images of quasi-simultaneous LASCO CMEs searching for a set of nearly sympathetic CMEs/flares. In x 2 a brief description is given of our data and statistical procedure. In x 3 we report the results of our statistical analysis and present a plausible example of sympathetic CMEs identified by EIT and LASCO images. Finally, a brief summary and conclusion is delivered in x DATA ANALYSIS AND STATISTICAL PROCEDURES 2.1. Waiting-Time and Angular Difference Distribution For the waiting-time distribution, we consider 3817 SOHO/LASCO CMEs, which occurred from 1999 February to 2001 December, as a continuous CME data sample. For the event time of these CMEs, we take their first appearance times in the LASCO C2 field of view, which are listed in the LASCO CME online catalog (Yashiro et al. 2002). 4 According to the primary CME list created by the LASCO operators, 5 the LASCO CME data during the entire observing period have about 20 intermittent data gaps ranging Q3 from a few hours to five days. The missing days are in total 57 days whose fraction in the total observing period is about 5%. The statistical effect of the data gaps can be regarded as Q4 insignificant because of their randomness and the small fraction. We can make two assumptions for the calculation of waiting times for the sample CMEs. First, observed LASCO CMEs are treated as discrete events in time. Second, the waiting time is defined as the interval between the first C2 appearance times of one event and that of the next. If individual CME events are independent of each other and take place with a constant mean occurrence rate m, the waitingtime distribution should be represented by a Poisson interval distribution in which the probability of an event 4 Available at 5 Available at Q5 occurring between time t and t þ dt is PðtÞdt, where PðtÞ ¼m expð mtþ ; with a mean frequency m. However, the mean occurrence rate of CMEs is not constant on a long timescale since it depends on the solar cycle, the spatial distribution of active regions, etc. Thus, we compare the observed waiting-time distribution with a nonstationary (time-dependent) Poisson distribution in which the mean frequency varies in time. For a slowly varying CME occurrence rate, the waiting-time distribution of a nonstationary Poisson process with rates m i and intervals t i is given by where PðDtÞ X i ð1þ ðm i Þm i expð m i DtÞ ; ð2þ ðm i Þ¼ m it i Pj m jt j ð3þ is the fraction of events associated with a given rate m i (Wheatland 2000; Moon et al. 2001, 2002a). The value of t j should be a time unit that has the same potential to generate CMEs in a statistical sense (Moon et al. 2001). If one selects too small a value of t j, the number of CMEs during the interval would not be sufficient for statistics. Conversely, if one considers too large a value of t j, the approximation as a nonstationary process loses its validity. To determine appropriate values of t j, Wheatland (2000) used the method of Scargle (1998) based on Bayesian statistics and decomposed the GOES flares over 25 yr into 390 Bayesian blocks. On the other hand, Moon et al. (2001) considered t j as a constant free parameter and determined it by comparing the observed waiting-time distributions with the corresponding nonstationary Poisson distributions. They found that 3 days is an appropriate value for all GOES flares stronger than C1 class during the last solar maximum from 1989 to They also showed that the waiting time distribution in individual active regions can be approximated by a stationary Poisson distribution, which implies that the value of t j should be about 14 days, the crossing time of an active region on the solar disk. In another study, Moon et al. (2002a) assumed that the value of t j is the total crossing time of each active region pair, ranging from 15 to 25 days. In the present study, we take two different values of t j : 3 and 13.5 days; the latter is half the solar synoptic rotational period. To examine the spatial correlation between the successive CMEs, we consider the angular difference distribution for all successive CME pairs in the data set. Since the positional information of the CMEs on the solar disk is only partially available, we adopt their position angles seen in the LASCO C2 field of view. In order to find any enhancement of the spatial correlation between successive CMEs, the number of CME pairs within a certain angular difference range is compared with the background-level frequency in the same range. We have determined the background level as a function of angular difference as follows: (1) The series of CME position angles are divided into 38 segments so that each segment contains 100 position angles. (2) One hundred random numbers are generated for each segment and then sorted in ascending order. (3) A surrogate set of position angles are created by redistributing original positional

3 No. 2, 2003 SYMPATHETIC CORONAL MASS EJECTIONS 3 angles according to the sorted order of the random numbers. (4) The background-level distribution is defined as the mean angular difference distribution of 200 surrogate sets. This procedure is proposed to take into account the occurrence and migration of active latitudes in a solar cycle. This method is conceptually similar to the one used for the angular correlation function by Moon et al. (2001) SOHO EIT and LASCO Observations To directly search for sympathetic CMEs and examine their characteristics, we inspected the 195 Å EIT full-disk images and the LASCO C2 images of quasi-simultaneous CMEs, whose positional angle difference is less than 50. EIT provides spectro-heliogams of the corona and the transition region on the solar disk and up to 1.5 solar radii above the solar limb. It allows diagnostics of solar plasma at certain temperatures in the range of to K (Delaboudiniere et al. 1995). The time cadence of the EIT images used in this study ranges from 10 to 30 minutes, and their pixel resolution is about 2>6. Their running difference images are used to examine how the perturbation originated from the first CME propagates to the second CME. We have also inspected LASCO C2 images and movies of the quasi-simultaneous CMEs to look for sympathetic CMEs. The C2 instrument is an externally occulted whitelight coronagraph that observes Thompson scattered visible light through a broadband filter. It covers 2 6 solar radii with a pixel resolution of 12>1 (Brueckner et al. 1995). The C2 images are used to see the configuration of a pair of sympathetic CMEs at a later time. 3. RESULTS 3.1. Waiting-Time and Angular Difference Distribution Figure 1 shows the waiting-time distribution of 3817 LASCO CMEs from 1999 February to 2001 December. It is well represented by two time-dependent Poisson distributions with t j ¼ 3 and 13.5 days, except at the shortest waiting time. The time-dependent Poisson distribution does not strongly depend on t j as long as t j is not too small. As seen in the figure, we cannot find any significant overabundance at short waiting times in the waiting-time distribution, even when it is compared with a stationary Poisson distribution (Fig. 1, dotted line). Actually, it is found that there is some underabundance at the shortest waiting time of less than 1 hr. This may be attributed to the fact that an eruptive event is detected as a CME only if the eruption scale is sizable and/or there is some identification (reporting) bias. Our finding that there is no significant statistical sign of sympathetic CMEs is very similar to the previous results for solar flares (Biesecker & Thompson 2000; Moon et al. 2001). Biesecker & Thompson (2000) compared the distribution of solar X-ray flares in time with an expected time-varying Poisson distribution and then argued that there is no evidence for sympathetic flares. Moon et al. (2001) also reached a similar conclusion that sympathetic flares are not significant in the statistical sense after examining the waiting-time distribution of GOES X-ray flares with short waiting times as well as their angular correlation function. Nevertheless, Moon et al. (2002a) presented very convincing statistical evidence for sympathetic flares by considering the waiting-time distributions of carefully selected flare samples. Thus, our results do not absolutely deny the existence Fig. 1. Waiting-time distribution of CMEs that occurred from 1998 October to 2001 December. Two nonstationary Poisson distributions with t j ¼ 3(dashed line) and 13.5 days (solid line) are presented for comparison. Dotted line: Stationary Poisson distribution with a mean event rate of m ¼ 3:6 per day. Error bars: Square root of the number of events in each bin. of sympathetic CMEs, but rather imply that the population of sympathetic (or interdependent) CMEs is much smaller than that of independent CMEs. Figure 2 represents the position angle distribution of the CMEs under consideration. It is evident that there are two major peaks around 90 and 270, which is consistent with Fig. 2. Position angle distribution of the CMEs. Error bars: Square root of the number of events in each bin.

4 4 MOON ET AL. Vol. 588 bution. The standard deviation of the background-level frequency for 200 surrogate sets ranges from 11 to 15, which is close to the square root of the number of events in each bin. When compared to the background level, a remarkable excess (larger than 3 ) is found only within 10 of position angle difference. This excess may be interpreted as the exis- of quasi-homologous CMEs that occurred sequen- Q7tence tially in the same active regions (Nitta & Hudson 2001; Zhang & Wang 2002; Moon et al. 2002b). The overabundant data within 10 are about 3% of the whole data set. There is no noticeable overabundance compared to the background level at large angular differences. This implies that the number of sympathetic CMEs is small and even smaller than the number of possible homologous CMEs. This result is consistent with the result from Figure 1. Fig. 3. Angular difference distribution of successive CME pairs. The background level (dashed line) is the mean angular difference distribution of 200 surrogate sets (for details, see x 2.1). Error bars: Square root of the number of events in each bin. the well-known fact that CMEs occur more frequently in Q6 the equatorial region during the solar maximum period (e.g., Hundhausen 1999). This tendency seems to be related to the existence of helmet streamers near the equator. The ratio of the CMEs that originated from the equatorial region to those from the polar regions is about 3.4. Figure 3 shows the angular difference distribution of all the successive CME pairs compared with the background-level distri A Candidate Pair of Sympathetic CMEs Searching for sympathetic CMEs, we have inspected several tens of quasi-simultaneous CMEs in a series of EIT images, their running difference images, and LASCO C2 images. For most events, it is very difficult to identify any interaction between coronal mass ejections because they are not very visible against the solar disk in the EIT images and in their running difference images. Fortunately, we have found one candidate pair of sympathetic CMEs that originated from near the solar limb. Figures 4 and 5 are respectively EIT 195 Å images and their running difference images for the example observed on 1998 June 11. The initial eruption (before 09:35 UT) is quite earlier than the GOES starting time (09:57 UT) and the Yoh- hard X-ray telescope starting time (10:02 UT) of the Q8koh associated M1.4 flare that occurred in NOAA AR A detailed description of the relationship between the CME and the flare in this event was given by Zhang et al. (2001). As seen in Figure 4, two distinct bundles of coronal loops were ejected. The north loops originated from NOAA AR 8243 and the south loops from NOAA AR At the time Fig. 4. Series of EIT 195 Å images for a candidate pair of sympathetic CMEs that occurred on 1998 June 11. The field of view is about

5 No. 2, 2003 SYMPATHETIC CORONAL MASS EJECTIONS 5 Fig. 5. Series of EIT running difference images for the event shown in Fig. 4 of the event, the positions of two active regions are roughly N18 E92 and S20 E78, respectively, when extrapolated from the NOAA coordinate information of June 12 and 13. Since NOAA AR 8243 was marginally behind the limb at that time, its position may obscure the information about the actual geometry associated with the eruptive event. From the coordinate information, the separation between the two sources is estimated at approximately 500 Mm, which gives the interconnecting loop a length of about 780 Q9Mm, assuming semicircular-shaped loops. After the north loops started to expand the south loops also began to expand. This time evolution is more evidently shown in the running difference images in Figure 5. As seen in the first running difference image (09:35 09:20 UT), the eruption of the north loops started before 09:35 UT. The following three running difference images (10:00 09:35 UT, 10:09 10:00 UT, and 10:20 10:09 UT) show that the loops in the north erupted first and then expand eastward and southward, which seems to have eventually brought about the eruption of the south loops. Figure 6 shows two CME loops in a LASCO C2 image taken at 10:28 UT on 1998 June 11. This structure is regarded as resulting from two ejecting coronal loops (north and south loops) identified in Figures 4 and 5. According to the on-line CME catalog (S. Yashiro 2002, private communication), # the north CME has a position angle of 96 Q10 and the south one has 123. The speeds estimated by linear fits in the C2 and C3 field of view are 1078 km s 1 for the north one and 1223 km s 1 for the south one. Even though the north loop was first ejected, both loops had the nearly the same heights (3.8 solar radii) at this time because the south loop was faster. It is interesting that there is broad brightening around the intersection area of two LASCO CME loops. This may be a mere overlap of two separate structures along the line of sight. However, a density 6 Available at Fig. 6. LASCO C2 image taken at 10:28 UT on 1998 June 11. The field of view is about

6 6 MOON ET AL. Vol. 588 enhancement may be caused by compression of plasma where two expanding structures come into contact. But none of these arguments are conclusive until additional information (such as Ultraviolet Coronograph Spectrometer data) is available. As for the apparently successive eruptions, we can make several speculations. First, one may think that the two eruptions are totally independent and just coincidently took place in nearby active regions with a very short time interval. For example, there may be simultaneous flux emergence by chance. Second, we may reason that the first eruption eventually acts as a triggering process for the second eruption. In this case, the disturbance generated by the first eruption propagates to the other active region in the form of fast-mode waves, fast shocks, or Alfvén waves. In order for Alfvén waves to be disturbance-carrying agents, there must be connecting field lines between two active regions. In addition to this condition, the first eruption must involve either magnetic reconnection or a sudden increase of currents in order to exert a destabilizing impact to the other active region. The fast-mode waves or fast shocks do not require connecting field lines between two active regions. Regarding this, Khan & Hudson (2000) showed a possibility that shock waves associated with flaring activity can destabilize largescale structures that produced CMEs. Considering the Q11 phase speed of the fast-mode and Alfén waves of the order of 1000 km s 1 in the solar corona (Lyons & Simnett 1999), the waves take about 13 minutes to traverse the interconnecting loop of 780 Mm between the two ejection sources shown in Figure 4. This estimate is comparable to the time interval (10 20 minutes) conjectured from the third and fourth panels of Figure 4. Third, we can consider the two eruptions as sequential manifestations of one global eruption. However, it is uncertain whether the time lag in the two eruptions is merely an apparent effect or a real one. If the latter is the case, we have to raise another question, why a part of the global system is more vulnerable to eruption than the rest part. For the present, we reserve judgement among these speculations until further information is obtained. 4. SUMMARY AND CONCLUSION In this paper we have investigated sympathetic coronal mass ejections. For this study we have examined the waiting-time distribution and the angular-difference distribution Q12 of SOHO/LASCO CMEs that occurred from 1999 February to 2001 December. We have also inspected the EIT running difference images and the LASCO images of quasi-simultaneous CMEs. The major results of the study can be summarized as follows. First, the waiting-time distribution is quite well approximated by a time-dependent Poisson distribution, and there is no noticeable overabundance at short waiting times. Second, the angular difference distribution shows a noticeable excess only within 10 difference, which supports the noticeable existence of quasihomologous CMEs. Although these statistical results do not provide any evidence for sympathetic coronal mass ejections, they may rather imply that the number of sympathetic (or interdependent) CMEs is much smaller than that of independent CMEs. Finally, in a direct survey of EIT and LASCO images we have found a possible candidate pair of sympathetic CMEs originated from two distinct active regions near the solar limb. The EIT running difference images show that the first CME loop expanded toward the other active region, and the second active region eventually produced another CME. The LASCO image shows two distinct LASCO CME structures. As for the triggering mechanism of the second CME, we may consider the destabilization of the magnetic structure induced by the impact of the first CME expansion. Q13 that Q14 It is also possible that the two CMEs are a sequential development of global destabilization in a large-scale magnetic field system encompassing two active regions. However, we cannot exclude the possibility that the two CMEs did not have any physical connections and just took place coincidently with a short time interval. The candidate for sympathetic CMEs that we have presented in this paper is a very rare case. Another example may be the event on 1997 February 23 studied by Simnett & Hudson (1997). Recently, Gopalswamy et al. (2001, 2002) studied the interactions between slow CMEs and fast CMEs through the heliosphere. Our study is different from theirs in that we have paid attention to the CME initiation induced by another CME. We very much thank the referee for his/her constructive comments, which helped us to improve the present paper. We are also very thankful to H. S. Hudson for his valuable comments and discussions. We thank S. Yashiro for his comments on LASCO CME data gaps and P. T. Gallagher for his useful comments on data analysis. This work has been supported by NASA grants NAG and NAG , by MURI grant of AFOSR, by the US-Korea Cooperative Science Program (NSF INT ), and by NRL grant M J of the Korean government. G. S. C. has been supported by DoE contract DE-AC02-76-CH03073 and NSF grant ATM The CME catalog we used is generated and maintained by the Center for Solar Physics and Space Weather, the Catholic University of America, in cooperation with the Naval Research Laboratory and NASA. SOHO is a project of international cooperation between ESA and NASA. Biesecker, D. A., & Thompson, B. J. 2000, J. Atmos. Sol.-Terr. Phys., 62, 1449 Brueckner, G. E., et al. 1995, Sol. Phys., 162, 357 Bumba, V., & Klvana, M. 1993, Ap&SS, 199, 45 Delaboudiniere, J.-P., et al. 1995, Sol. Phys., 162, 291 Fritzova-Svestkova, L., Chase, R. C., & Svestka, Z. 1976, Sol. Phys., 48, 275 Gopalswamy, N., Yashiro, S., Kaiser, M. 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