are the faint structures ahead of solar coronal mass ... - IOPscience

The Astrophysical Journal Letters, 796:L16 (5pp), 2014 November 20 C 2014.

doi:10.1088/2041-8205/796/1/L16

The American Astronomical Society. All rights reserved. Printed in the U.S.A.

ARE THE FAINT STRUCTURES AHEAD OF SOLAR CORONAL MASS EJECTIONS REAL SIGNATURES OF DRIVEN SHOCKS? Jae-Ok Lee1 , Y.-J. Moon1,2 , Jin-Yi Lee2 , Kyoung-Sun Lee3 , Sujin Kim4 , and Kangjin Lee1 1

School of Space Research, Kyung Hee University, Yongin 446-701, Korea; [email protected], [email protected] 2 Astronomy and Space Science, Kyung Hee University, Yongin 446-701, Korea 3 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-0231, Japan 4 Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea Received 2014 March 25; accepted 2014 October 20; published 2014 November 7

ABSTRACT Recently, several studies have assumed that the faint structures ahead of coronal mass ejections (CMEs) are caused by CME-driven shocks. In this study, we have conducted a statistical investigation to determine whether or not the appearance of such faint structures depends on CME speeds. For this purpose, we use 127 Solar and Heliospheric Observatory/Large Angle Spectroscopic COronagraph (LASCO) front-side halo (partial and full) CMEs near the limb from 1997 to 2011. We classify these CMEs into two groups by visual inspection of CMEs in the LASCO–C2 field of view: Group 1 has the faint structure ahead of a CME and Group 2 does not have such a structure. We find the following results. (1) Eighty-seven CMEs belong to Group 1 and 40 CMEs belong to Group 2. (2) Group 1 events have much higher speeds (average = 1230 km s−1 and median = 1199 km s−1 ) than Group 2 events (average = 598 km s−1 and median = 518 km s−1 ). (3) The fraction of CMEs with faint structures strongly depends on CME speeds (V): 0.93 (50/54) for fast CMEs with V 1000 km s−1 , 0.65 (34/52) for intermediate CMEs with 500 km s−1 V < 1000 km s−1 , and 0.14 (3/21) for slow CMEs with V < 500 km s−1 . We also find that the fraction of CMEs with deca–hecto metric type II radio bursts is consistent with the above tendency. Our results indicate that the observed faint structures ahead of fast CMEs are most likely an enhanced density manifestation of CME-driven shocks. Key words: shock waves – Sun: coronal mass ejections (CMEs)

physical properties of the faint structures, such as density compression ratio (Vourlidas et al. 2003; Ontiveros & Vourlidas 2009). Ontiveros & Vourlidas (2009) used 15 CMEs from 1997 to 1999 with the following criteria: (1) their speeds are greater than 1500 km s−1 ; (2) they should be associated, spatially and temporally, with streamer deflections; and (3) their associated faint structures must outline the outermost envelope of the CMEs. They identified shock signatures in the LASCO images for 13 out of the 15 CMEs by measuring their density compression ratios. Through the examination of thousands of CMEs, Vourlidas et al. (2013) found that a “faint front followed by a bright loop” is a common occurrence. In addition, they showed that the synthetic images of fast CMEs from the MHD simulation of a magnetic breakout CME eruption model are similar to LASCO coronagraph observations. The above studies support the idea that the faint structures ahead of fast CMEs are the signatures of CME-driven shocks. However, there has been no comprehensive statistical study concerning the dependence of the structures on CME speed. In this study, we make conduct a statistical investigation on whether or not the appearance of such faint structures depends on CME speeds. For this purpose, we use 127 SOHO/LASCO front-side halo (partial and full) CMEs near the limb from 1997 to 2011. We classify these CMEs into two groups with and without faint structures. These structures mimic the shape of CMEs (Figure 1 in Ontiveros & Vourlidas 2009), which are different from the streamer deflections noted by Sheeley et al. (2000). Then, we estimate the average and median speeds of these events. In addition, we examine the fraction of CMEs with faint structures depending on CME speeds. This Letter is organized as follows. In Section 2, we describe the data and analysis. We provide our results and discussion in Section 3. We present a brief summary and our conclusion in Section 4.

1. INTRODUCTION Large coronal mass ejections (CMEs) are transient, giant, and eruptive phenomena that expel a massive amount of coronal material into the interplanetary space. These CMEs can be detected as intensity enhancements of the ejected material by electron Thomson scattering of photospheric light in sequences of white light coronagraph images. The Large Angle Spectroscopic COronagraph (LASCO; Brueckner et al. 1995) on board the Solar and Heliospheric Observatory (SOHO) have observed more than 20,000 CMEs during the period from 1996 to 2013. Information and data concerning the CMEs observed by LASCO are available in the SOHO/LASCO CME catalog.5 Statistical studies of CME properties based on LASCO observations have found that the range of CME speeds is broad, ranging from a few tens of km s−1 to a few thousands of km s−1 (Yashiro et al. 2004; Gopalswamy 2006; Gopalswamy et al. 2009b). Several researchers expect that fast CMEs, whose speeds exceed the local characteristic speeds (428 km s−1 ∼ 740 km s−1 at 1.45 R ∼ 6 R ; Mann et al. 2003), can propel CME-driven shocks in the corona (Hundhausen et al. 1987; Gopalswamy et al. 2001, 2008). There have been many studies on the signatures of CMEdriven shocks using LASCO observations (Sheeley et al. 2000; Vourlidas et al. 2003, 2013; Gopalswamy et al. 2009a; Ontiveros & Vourlidas 2009; Gopalswamy & Yashiro 2011; Kim et al. 2012). Sheeley et al. (2000) observed streamer deflections as the signatures of CME-driven shocks by using fast CMEs from 1998 to 1999 with initial speeds of 800 km s−1 . In order to demonstrate that the faint structures ahead of CMEs are caused by CME-driven shocks, several researchers investigated 5

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The Astrophysical Journal Letters, 796:L16 (5pp), 2014 November 20

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Figure 1. Examples of the SOHO/LASCO-C2 running-difference images for Group 1 CME (left) and Group 2 CME (right). The white circle indicates the location and size of the solar disk. The black arrow indicates a coronal mass ejection (CME). The white arrow indicates the faint structure ahead of the CME front.

Figure 2. LASCO–C2 base difference image (left) of the Group 1 CME presented in the left panel of Figure 1 and its normalized solar brightness profile (right). In the left panel, the solid line indicates the location where solar brightness profile is shown. In both figures, the black and white arrows represent the CME front and the front of a faint structure, respectively. The box shows a background region of 100 by 100 pixels. In the right panel, the solid and dashed lines represent the heliocentric distances indicated by the arrows. The horizontal dotted lines indicate the noise levels of the LASCO–C2 image beyond the CME.

2. DATA AND ANALYSIS

We classify the CMEs into two groups through visual inspection of CMEs in the LASCO–C2 field of view: Group 1 CMEs have a faint structure ahead of them and Group 2 CMEs do not have such a structure. Here, we regard several ambiguous events as Group 2. Figure 1 shows an example of the LASCO–C2 running-difference images for Group 1 and Group 2 CMEs. The faint structure appears ahead of the CME observed at 6:30 UT on 2000 March 3 (left panel of Figure 1). However, in the right panel of Figure 1, we do not find such a faint structure ahead of the CME observed at 19:48 UT on 2011 March 16. Figures 2 and 3 show normalized solar brightness profiles of two CMEs in Figure 1 with their base difference images. For this purpose, we used LASCO–C2 level 1 data that have been corrected for the flat field response of the detector, radiometric sensitivity, stray light, geometric distortion, and vignetting. The normalized solar brightness is defined as the mean solar brightness (B ) divided by B at a specific heliocentric distance (R ): 2.7 × 10−10 B at 3.5 R for Figure 2 and 2.0 × 10−10 B

For data selection, we use the following procedure: (1) we consider 1493 wide CMEs from 1997 to 2011 using the SOHO/LASCO CME catalog whose angular widths (AWs) are greater than or equal to 120◦ ; (2) we select 145 CMEs associated with X-ray flares whose source longitudes are greater than or equal to 60◦ from the Sun center in order to minimize projection effects using the flare list complied by the National Geophysical Data Center6 ; and (3) finally, we use 127 CMEs that are well identified in the LASCO–C2 field of view. The CME data, speeds and running-difference images, are taken from the SOHO/LASCO CME catalog. The LASCO–C2 runningdifference images are made using the raw daily “QuickLook” level 0.5 data and displayed with the same intensity range (K. Battams & S. Yashiro 2014, private communication). 6

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Figure 3. LASCO–C2 base difference image (left) of the Group 2 CME presented in the right panel of Figure 1 and its normalized solar brightness profile (right). In the left panel, the solid line indicates the location where solar brightness profile is shown. The box shows a background region of 100 by 100 pixels. In both figures, the black arrow represents the CME front. In the right panel, the solid line represent the heliocentric distance indicated by the arrow. The horizontal dotted lines indicate the noise levels of the LASCO–C2 image beyond the CME.

diate CMEs (500 km s−1 V < 1000 km s−1 ), and fast CMEs (V 1000 km s−1 ).

at 3.0 R for Figure 3. To determine the noise levels of the LASCO–C2 images beyond the CMEs, we use the following procedure: (1) we select two different images, a pre-CME image and a CME image; (2) we select a background region of 100 by 100 pixels that is represented by the dashed line boxes in the left panels of Figures 2 and 3; (3) we make a histogram of the difference between two background brightness; (4) we perform a Gaussian fitting of the histogram; and (5) finally, we use the 1σ level of the Gaussian distribution as the noise level. The noise levels for the images in Group 1 CME (Figure 2) and Group 2 CME (Figure 3) are about 4.4 × 10−12 B and 2.9 × 10−12 B , respectively. As seen in the right panel of Figure 2, there is an enhanced region ahead of the CME front that corresponds to the region between two vertical lines. The normalized solar brightness of the enhanced region slowly decreases to the noise level (from 0.12 to 0.016) between 3.91 R and 4.56 R . On the other hand, there is no enhancement in the right panel of Figure 3. The normalized solar brightness ahead of the CME front steeply decreases to the noise level (from 0.14 to 0.014) between 3.65 R and 3.66 R . The characteristic of the brightness enhancement profile ahead of the CME front is consistent with Ontiveros & Vourlidas (2009), who estimated the compression ratios of CME-associated shocks. As shown in the left panel of Figure 3, there is a faint haze near the top portion of the image. We can see that the haze is more visible in the running-difference image using level 1 data (left panel of Figure 3) than that using level 0.5 data (right panel of Figure 1). This may be a noise signature caused by calibration effects such as vignetting and stray light. In our analysis, we classify CMEs into Group 1 in the case that their faint structures mimic the shape of CMEs like Figure 1 in Ontiveros & Vourlidas (2009), which are different from the streamer deflections noted by Sheeley et al. (2000). It seems that the haze does not belong to either case. We compare the speed distributions between Group 1 and Group 2. In addition, we examine the fraction of CMEs with faint structures depending on three groups divided according to CME speeds: slow CMEs (V < 500 km s−1 ), interme-

3. RESULTS AND DISCUSSION To examine the distribution of CME speeds depending on the existence of a faint structure ahead of a CME, we divide 127 CMEs into two groups through visual inspection of the CMEs in the LASCO–C2 running-difference images. Group 1 CMEs have a faint structure ahead of them and Group 2 CMEs do not have such a structure. We find that 87 CMEs belong to Group 1 and 40 CMEs belong to Group 2. Figures 4(a) and (b) show the speed distributions for Group 1 and Group 2, respectively. The average and median speeds of Group 1 CMEs are 1230 km s−1 and 1199 km s−1 , respectively. The average and median speeds of Group 2 CMEs are 598 km s−1 and 518 km s−1 , respectively. We find that Group 1 events have much higher speeds than Group 2 events. We note that there are no Group 1 CMEs with V < 468 km s−1 , which corresponds to the approximate value of the coronal Alfven ´ speed (Cho et al. 2005; Mann et al. 2003). Thus, within Group 1 with CMEs that have a faint structure ahead of them, the condition that the CME be faster than the local Alfv´en speed for a shock to be produced is more likely to be met. It is well known that a CME-driven shock can generate a type II radio burst in the solar corona. Concerning this, Gopalswamy et al. (2008) studied the properties of two fast and wide CME groups (V 900 km s−1 and AW 60◦ ) with and without type II radio bursts, such as metric or deca–hecto metric (DH) type II radio bursts. They found that the average speed of the CMEs with type II radio bursts (1438 km s−1 ) is higher than the average speed of the CMEs without type II radio bursts (1117 km s−1 ). Our results together with theirs support the idea that CME-driven shocks, which may be manifested by type II bursts, are caused by fast CMEs. To find out the fraction of CMEs with faint structures depending on speed, we classify the CMEs into three groups according to their speeds: slow CMEs (V < 500 km s−1 ), 3


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(a)

(b)

Figure 4. Speed distributions for two groups: (a) Group 1 events, which have the faint structures ahead of CMEs, and (b) Group 2 events, which do not have such structures.

400 km s−1 ) in the LASCO–C2 field of view. The increase in speed of the CME indicates that the CME has accelerated over the coronagraph field of view (St. Cyr et al. 2000). However, it is hard to explain why the other three CMEs cannot generate the faint structures ahead of them as signatures of CME-driven shocks. Therefore, it is necessary to further investigate the criteria for the generation of CME-driven shocks and their associated signatures. We examine whether or not our visual inspection of the enhanced leading structure is similar to their identification from brightness profiles. There are five CMEs with 600 km s−1 < V 650 km s−1 in Group 1, and four CMEs with 600 km s−1 < V 650 km s−1 in Group 2: a total of nine CMEs. By investigating the brightness profiles of these CMEs as well as their appearance in the LASCO level 1.0 images, we find that all Group 1 events have sloping profiles ahead of the CME fronts; that is, the normalized solar brightness of the enhanced region slowly decreases to the noise level. On the other hand, three out of four Group 2 events do not have profile enhancements, as shown in the right panel of Figure 3; that is, the brightness profile for three of the four Group 2 events rapidly decreases to the noise level near the vicinity of the CME front. In summary, all of the events except for one are consistent with our identifications by visual inspection. Therefore, we think that our identifications would not be quite different from those using brightness profiles. As for the presence or absence of the faint structure indicated in visual inspections of the LASCO–C2 running-difference image, we cannot rule out the possibility of an artifact of the slow CMEs. In particular, faint structures below the noise levels could not be detected by SOHO/LASCO instruments. Therefore, our identifications are limited by the capability of the LASCO instrument.

Fraction of faint structures

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Intermediate CME Groups

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Figure 5. Fraction of CMEs with faint structures for three CME speed ranges: slow CMEs with V < 500 km s−1 , intermediate CMEs with 500 km s−1 V < 1000 km s−1 , and fast CMEs with V 1000 km s−1 .

intermediate CMEs (500 km s−1 V < 1000 km s−1 ), and fast CMEs (V 1000 km s−1 ). Figure 5 shows the fraction depending on CME speeds. We find that the fraction remarkably increases with CME speed: 0.14 (3/21) for slow CMEs, 0.65 (34/ 52) for intermediate CMEs, and 0.93 (50/54) for fast CMEs. These results statistically demonstrate that the faint structures ahead of fast CMEs are a common feature. We examine the fraction of CMEs with DH type II radio bursts, which are taken from the Wind/WAVES Type II burst and CMEs catalog.7 We find that the fraction depends on three groups: 0.0 (0/21) for slow CMEs, 0.13 (7/52) for intermediate CMEs, and 0.67 (36/54) for fast CMEs. This tendency is consistent with that of the fraction of CMEs that have faint structures. We check 4 CMEs that do not have faint structures from among 54 fast ones. Three CMEs do not have faint structures along their leading edges but have faint structures near their flank regions, which are all associated with nearby helmet streamers. One CME, whose linear fit speed in the LASCO–C2 and C3 field of view is 1180 km s−1 , has a quite slow speed (about 7

4. SUMMARY AND CONCLUSION In this study, we have performed a statistical investigation of whether or not the appearance of faint structures ahead of CMEs depends on CME speeds. For this purpose, we used 127 front-side halo (partial and full) CMEs near the limb observed from 1997 to 2011. We have compared the speed distributions between Group 1, which has the faint structure ahead of a CME, and Group 2, which does not have such a structure. We have examined the fraction of CMEs with faint structures depending on

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CME catalog is generated and maintained at the CDAW Data Center by NASA and The Catholic University of America in cooperation with the Naval Research Laboratory. SOHO is a project of international cooperation between the ESA and NASA.

CME speed ranges: slow CMEs with V < 500 km s , intermediate CMEs with 500 km s−1 V < 1000 km s−1 , and fast CMEs with V 1000 km s−1 . We have also investigated the fraction of CMEs with DH type II radio bursts for these groups. The main results of this study can be summarized as follows. First, 87 CMEs belong to Group 1 and 40 CMEs belong to Group 2. Second, Group 1 events have much higher speeds than Group 2 events. Third, the fraction of CMEs having faint structures remarkably increases with CME speed. This tendency is consistent with that of the fraction of CMEs having DH type II radio bursts. Noting that fast CMEs, whose speeds exceed the local characteristic solar wind speed, can drive piston-driven shocks (Hundhausen et al. 1987; Gopalswamy et al. 2001, 2008), these results indicate that the observed faint structures ahead of fast CMEs are most likely an enhanced density manifestation of CME-driven shocks.

REFERENCES Brueckner, G. E., Howard, R. A., Koomen, M. J., et al. 1995, SoPh, 162, 357 Cho, K.-S., Moon, Y.-J., Dryer, M., et al. 2005, JGR, 110, A12101 Gopalswamy, N. 2006, JA&A, 27, 243 Gopalswamy, N., Lara, A., Kaiser, M. L., & Bougeret, J.-L. 2001, JGR, 106, 25261 Gopalswamy, N., Thompson, W. T., Davila, J. M., et al. 2009a, SoPh, 259, 227 Gopalswamy, N., & Yashiro, S. 2011, ApJL, 736, L17 Gopalswamy, N., Yashiro, S., Michalek, G., et al. 2009b, EM&P, 104, 295 Gopalswamy, N., Yashiro, S., Xie, H., et al. 2008, ApJ, 674, 560 Hundhausen, A. J., Holzer, T. E., & Low, B. C. 1987, JGR, 92, 11173 Kim, R.-S., Gopalswamy, N., Moon, Y.-J., Cho, K.-S., & Yahiro, S. 2012, ApJ, 746, 118 Mann, G., Klassen, A., Aurass, H., & Classen, H. T. 2003, A&A, 400, 329 Ontiveros, V., & Vourlidas, A. 2009, ApJ, 693, 267 Sheeley, N. R., Hakala, W. N., & Wang, Y.-M. 2000, JGR, 105, 5081 St. Cyr, O. C., Howard, R. A., Sheeley, N. R., et al. 2000, JGR, 105, 18169 Vourlidas, A., Lynch, B. J., Howard, R. A., & Li, Y. 2013, SoPh, 284, 179 Vourlidas, A., Wu, S. T., Wang, A. H., Subramanian, P., & Howard, R. A. 2003, ApJ, 534, 456 Yashiro, S., Gopalswamy, N., Michalek, G., et al. 2004, JGR, 109, A07105

This work was supported by the BK21 plus program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea, Basic Science Research Program through the NRF funded by the Ministry of Education(NRF-2013R1A1A2012763, 2013R1A1A2058409), NRF of Korea grant funded by the Korean Government(NRF2013M1A3A3A02042232), and the Korea Meteorological Administration/National Meteorological Satellite Center. The

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