Investigation of the Formation and Separation of An EUV Wave from the Expansion of A Coronal Mass Ejection
X. Cheng, J. Zhang, O. Olmedo, A. Vourlidas, M. D. Ding, Y. Liu
aa r X i v : . [ a s t r o - ph . S R ] D ec Investigation of the Formation and Separation of An EUV Wavefrom the Expansion of A Coronal Mass Ejection
X. Cheng , , , J. Zhang , O. Olmedo , A. Vourlidas , M. D. Ding , , Y. Liu Department of Astronomy, Nanjing University, Nanjing 210093, China [email protected] School of Physics, Astronomy and Computational Sciences, George Mason University,Fairfax, VA 22030, USA [email protected] Key Laboratory for Modern Astronomy and Astrophysics (Nanjing University), Ministryof Education, Nanjing 210093, China NRC, Naval Research Laboratory, Washington, DC 20375, USA Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA Space Science Laboratory, University of California, Berkeley, CA 94720, USA
ABSTRACT
We address the nature of EUV waves through direct observations of the for-mation of a diffuse wave driven by the expansion of a coronal mass ejection(CME) and its subsequent separation from the CME front. The wave and theCME on 2011 June 7 were well observed by Atmospheric Imaging Assembly on-board Solar Dynamic Observatory. Following the solar eruption onset, markedby the beginning of the rapid increasing of the CME velocity and the X-ray fluxof accompanying flare, the CME exhibits a strong lateral expansion. During thisimpulsive expansion phase, the expansion speed of the CME bubble increasesfrom 100 km s − to 450 km s − in only six minutes. An important finding is thata diffuse wave front starts to separate from the front of the expanding bubbleshortly after the lateral expansion slows down. Also a type-II burst is formed nearthe time of the separation. After the separation, two distinct fronts propagatewith different kinematic properties. The diffuse front travels across the entire so-lar disk; while the sharp front rises up, forming the CME ejecta with the diffusefront ahead of it. These observations suggest that the previously termed EUVwave is a composite phenomenon and driven by the CME expansion. While the 2 –CME expansion is accelerating, the wave front is cospatial with the CME front,thus the two fronts are indiscernible. Following the end of the acceleration phase,the wave moves away from the CME front with gradually an increasing distancebetween them. Subject headings:
Sun: corona — Sun: coronal mass ejections (CMEs) — wavesOnline-only material: animations, color figures
1. Introduction
Coronal mass ejections (CMEs) are large-scale eruptive phenomena from the Sun. Theycan carry large amounts of plasma and magnetic field energy into the interplanetary space,which may have severe effects on space environment and human technological systems aroundthe Earth (Gosling et al. 1993; Webb et al. 1994). A typical CME has a velocity of severalhundred km s − , while the fastest one recorded is over 3000 km s − (Yashiro et al. 2004). Themain acceleration of a CME usually occurs in the inner corona (e.g., ≤ R ⊙ ) (Zhang et al.2001). With the advent of the Atmospheric Imaging Assembly (AIA; Lemen et al. 2011)onboard Solar Dynamics Observatory ( SDO ), details on the initiation and structural for-mation of CME starts to emerge. Recently, it was found that a twisted hot channel ( ∼ SOHO spacecraft (Thompson et al. 1998, 1999). The physical nature of the moving front remainssomewhat unclear. The front has been interpreted as a fast-mode magnetohydrodynamic(MHD) wave (Thompson et al. 1999; Wang 2000; Warmuth et al. 2001; Veronig et al. 2008;Kienreich et al. 2009; Gopalswamy et al. 2009; Patsourakos & Vourlidas 2009a; Patsourakos et al.2009b; Liu et al. 2011; Olmedo et al. 2011), a soliton wave (Wills-Davey et al. 2007), or slow-mode wave (Wang et al. 2009). Whereas, others believe that it is not at all a true wave.Chen et al. (2002, 2005), Chen (2009), and Chen & Wu (2011) argued that the bright frontresults from the compression front driven by the successive stretching of the magnetic field 3 –overlying the erupting CME flux rope, although there should be a fast-mode wave aheadof the CME front. Attrill et al. (2007, 2009) and Dai et al. (2010) claimed that the brightfront is related to the magnetic reconnection between the outmost magnetic field of theCME and the magnetic loops from the quiet region. Delann´ee et al. (2008) suggested thatthe current shell in their numerical simulation can also form the bright front. In order tobring together these opposing views, some authors recently tend to appeal for the hybridmodel combining both wave and non-wave explanations (e.g., Cohen et al. 2009; Liu et al.2010; Downs et al. 2011). Details of various EUV wave models can be found in recent reviewpapers by Warmuth (2010), Gallagher & Long (2011), and Zhukov (2011).In this Letter, we investigate the physical nature of the EUV wave. From the observa-tions, we find that the wave front has two distinct evolution stages, i.e., a compression frontforming the CME front in the early stage and a stand-alone wave front separating from theCME front in the later stage. The data used are mainly from AIA onboard
SDO and SunEarth Connection Coronal and Heliospheric Investigation (SECCHI; Howard et al. 2008)onboard Solar TErrestrial RElations Observatory (
STEREO ). Observations and results arepresented in Section 2, followed by a summary and discussion in Section 3.
2. Observations and Results2.1. Overview of the CME Eruption
On 2011 June 7, the active region (AR) NOAA 11226 produced an M2.5 class flare atthe location of S22 ◦ W53 ◦ , which started at 06:16 UT and peaked at 06:30 UT. Followingthe onset of the flare, the overlying magnetic field of the AR expanded rapidly and formeda plasma bubble with a sharp front. At ∼ ∼ The shape of the CME bubble and ejecta can be clearly seen and tracked, thanks tothe high cadence high quality AIA observations. So we are able to study the structuraland kinematic evolution of the CME with high precision. The CME bubble is fitted as acircle in the AIA FOV, from which the radius of the bubble is obtained. The top of the 4 –circle is regarded as the height of the bubble front. Figure 1(c) and (d) display the fittingresult, in which the bubble is represented by the blue dash-dotted lines. Once the CMEentered the FOV of COR1 (Figure 1(e) and (f)) and COR2 (not shown here), it appearedvery similar to a flux rope structure. Thus, we use the graduated cylindrical shell (GCS)model (Thernisien et al. 2006, 2009) to model the 3D structure of the CME. The height-timevariation of the CME front is shown in Figure 2(a), along with the radius-time plot of theCME bubble in the AIA FOV.Based on the height-time data, processed by the spline smoothing, we calculate the radialvelocity of the CME front using a piece-wise numerical derivative method. The temporalevolution of the velocity is plotted in Figure 2(b), in which we also plot the
GOES softX-ray 1–8 ˚A flux. One can see that the CME accelerated during the rise phase of the flare;the radial velocity of the CME increased from ∼
100 km s − at 06:19 UT to ∼ − at 06:35 UT. The average acceleration during this period is ∼ − . Moreover, wecalculate the expansion velocity of the CME bubble based on the radius-time data (blueline in Figure 2(b)). It is found that the expansion of the bubble experienced a differentkinematic evolution. In the first seven minutes, the expansion velocity of the bubble quicklyincreased from ∼
100 km s − to ∼
450 km s − with an average acceleration of ∼
830 m s − .Whereas, after ∼ One interesting finding from studying this event is that a diffuse wave is clearly seento separate from the sharp bubble front. The separating wave front is mostly visible at theflank of the bubble in the FOV of AIA (Figure 1(d)). The separation between the wave andthe bubble is also clearly seen in the SECCHI observations (Figure 1(e) and (f)). Inspectingthe evolution of the wave and the bubble, we find that the diffuse wave front is always closeto the bubble front at the top (or leading fronts along the radial direction). While, at theflanks of the bubble, the standoff distance between the two fronts gradually increases withtime. At 06:40 UT, the diffuse front had propagated further away from the bubble front asshown in Figure 1(e) and (f), from which we also find that the wave front above the limbcoincides very well with that on the disk.In order to investigate the detailed separation process of the wave front from the CMEbubble, we transform the AIA images from the observed cartesian coordinates (x,y distance 5 –from sun center) into helio-projective coordinates (polar angle along the X-axis and projectedheliocentric distance along the Y-axis). Figure 3(a) shows the AIA 211 ˚A difference imagesin the transformed system. Note that the 211 ˚A difference images show the diffuse wavefront best among all AIA passbands. One can see that the diffuse wave front can now bewell distinguished from the sharp front of the CME bubble (see also the online movie for abetter impression).Next, from each transformed image we extract three horizontal slices located at helio-centric heights of 1.15, 1.05, and 0.95 R ⊙ , respectively. We then stack each slice verticallyin a time sequence to make the slice-time plot. The results are shown in Figure 3(b)-(d).From the stacking slice-time plots, one can see that the diffuse wave front has a differentlateral evolution from the sharp bubble front. The wave front overlaps with the bubble frontfrom ∼ ∼ ∼ R ⊙ , the CME bubblefront reached an apparent lateral velocity of 960 km s − at ∼ − at ∼ ∼
600 km s − in the next nine minutes. Moreover, the wave traversed the nearby AR and 6 –continued to propagate at the velocity of ∼
600 km s − (squares in Figure 4(a)) (see also,Li et al. 2011). The apparent lateral propagation of the CME bubble and the wave at theheliocentric height of 1.05 R ⊙ is similar to that at 1.15 R ⊙ . At ∼ ∼
880 km s − . However, after ∼ ∼ − . Similarly, the wave front at the heliocentric height of 0.95 R ⊙ accelerated from 150km s − at ∼ − ∼ ∼
600 km s − .Apparently, in the early evolution stage immediately following the eruption onset, thewave front can not be discerned from the CME bubble front, indicating that the wave frontis still undergoing compression from the expanding bubble. The standoff distance betweenthe two fronts is almost zero. Both of them obtain the maximum lateral velocity at thesame time. When the CME bubble starts to decrease the velocity, the wave front starts toseparate and propagate away from its driver. From Figure 4, one can notice that the wavefront has different peak lateral velocities at different heliocentric height. The wave’s lateralpeak velocities increase with the heliocentric heights; the corresponding peak times of thevelocities also delay with respect to the increasing height (Table 1). Such increase of thepeak lateral velocity and its time delay are most likely from the combination effect of theintrinsic expansion motion and the fast rising motion of the CME bubble.
3. Summary and Discussion
In this Letter, we focus on studying the separation process of two distinct fronts associ-ated with a solar eruption that occurred on 2011 June 7. Following the eruption onset, themagnetic field of the source region quickly expands and forms a circular bubble. In the earlystage of the eruption, the bubble expands strongly with an accelerating velocity. Afterwards,the apparent expansion velocity of the bubble close to the solar surface quickly deceleratesto almost zero. In the meantime, a diffuse wave front starts to separate from the sharpbubble front. We conclude that the wave originates from the compression of the surroundingplasma by the impulsively expanding CME bubble. Due to a small standoff distance betweenthe compression front and the driver front, the two fronts can not be distinguished duringthe early stage of the evolution when the driver is still undergoing acceleration. Throughexamining the radio data from CALLISTO radio spectrometer, we find that a type II radioburst started at ∼ STEREO -EUVI observations (5 minutes), the two distinct fronts were seen in only a fewframes. Here, the AIA observations in every 12 seconds not only reveal the existence of twofronts but also the detailed separation process of the diffuse front from the sharp bubble frontafter the expansion of the bubble slows down. These observations clearly demonstrate thatthe separated diffuse front is a true MHD wave, driven by the early accelerating expansionof the CME bubble.In conclusion, our observations help understand the physical nature of usually termedEUV waves. The EUV wave is actually a composite phenomenon, consisting of two distinctfronts, a wave front and a CME (compressed plasma) front. We further find that its evolutioncan be divided into two stages. The first stage takes place in the accelerating expansion phaseof the CME bubble, which acts as the piston-driver of the MHD wave. During this stage,the wave front is coupled together with the compression front of the CME bubble. In thesecond stage, when the expanding velocity of the CME bubble slows down, the MHD wavefront decouples from the compression bubble front, forms a distinct front, and propagatesacross the solar disk. The observational result for the coexistence of both wave and non-wave fronts are in general consistent with the models and numerical simulations for EUVwaves by Chen et al. (2002, 2005). We believe that the previous dispute about the natureof EUV waves resides in, at least partly, different parts of the composite phenomenon thatthe authors may have observed. In fact, the duration of the wave and CME compressionfront coupling is different for each event, depending on the dynamics of the CME and thesurrounding environment.We thank P. F. Chen for many valuable comments that helped to improve the manuscriptsignificantly. SDO is a mission of NASA’s Living With a Star Program. X.C., and M.D.D.are supported by NSFC under grants 10673004, 10828306, and 10933003 and NKBRSFunder grant 2011CB811402. X.C. is also supported by the scholarship granted by the ChinaScholarship Council (CSC) under file No. 2010619071. J.Z. is supported by NSF grant ATM-0748003 and NASA grant NNG05GG19G. A.V. is supported by NASA contract S-136361-Y.
REFERENCES
Attrill, G. D. R., Harra, L. K., van Driel-Gesztelyi, L., & D´emoulin, P. 2007, ApJ, 656, L101Attrill, G. D. R., Engell, A. J., Wills-Davey, M. J., Grigis, P., & Testa, P. 2009, ApJ, 704,1296 8 –Chen, P. F. 2009, ApJ, 698, L112Chen, P. F., Fang, C., & Shibata, K. 2005, ApJ, 622, 1202Chen, P. F., Wu, S. T., Shibata, K., & Fang, C. 2002, ApJ, 572, L99Chen, P. F., & Wu, Y. 2011, ApJ, 732, L20Cheng, X., Zhang, J., Liu, Y., & Ding, M. D. 2011, ApJ, 732, L25Cohen, O., Attrill, G. D. R., Manchester, W. B., IV, & Wills-Davey, M. J. 2009, ApJ, 705,587Dai, Y., Auch`ere, F., Vial, J.-C., Tang, Y. H., & Zong, W. G. 2010, ApJ, 708, 913Delann´ee, C., T¨or¨ok, T., Aulanier, G., & Hochedez, J.-F. 2008, Sol. Phys., 247, 123Downs, C., Roussev, I. I., van der Holst, B., et al. 2011, ApJ, 728, 2Gallagher, P. T., & Long, D. M. 2011, Space Sci. Rev., 158, 365Gopalswamy, N., Yashiro, S., Temmer, M., et al. 2009, ApJ, 691, L123Gosling, J. T. 1993, J. Geophys. Res., 98, 18937Howard, R. A., et al. 2008, Space Sci. Rev., 136, 67Kienreich, I. W., Temmer, M., & Veronig, A. M. 2009, ApJ, 703, L118Li, T., Zhang, J., Yang, S. H., & Liu, W. 2011, ApJ, in pressLiu, W., Nitta, N. V., Schrijver, C. J., Title, A. M., & Tarbell, T. D. 2010, ApJ, 723, L53Liu, Y., Luhmann, J. G., Bale, S. D., & Lin, R. P. 2009, ApJ, 691, L151Liu, Y., Luhmann, J. G., Bale, S. D., & Lin, R. P. 2011, ApJ, 734, 84Lemen, J. R., et al. 2011, Sol. Phys., 106Ma, S., Raymond, J. C., Golub, L., et al. 2011, ApJ, 738, 160Olmedo, O., Vourlidas, A., Zhang J., & Cheng, X. 2011, ApJ, under reviewPatsourakos, S., & Vourlidas, A. 2009a, ApJ, 700, L182Patsourakos, S., Vourlidas, A., Wang, Y. M., Stenborg, G., & Thernisien, A. 2009b,Sol. Phys., 259, 49 9 –Patsourakos, S., Vourlidas, A., & Kliem, B. 2010, A&A, 522, A100Thernisien, A. F. R., Howard, R. A., & Vourlidas, A. 2006, ApJ, 652, 763Thernisien, A., Vourlidas, A., & Howard, R. A. 2009, Sol. Phys., 256, 111Thompson, B. J., Gurman, J. B., Neupert, W. M., et al. 1999, ApJ, 517, L151Thompson, B. J., Plunkett, S. P., Gurman, J. B., et al. 1998, Geophys. Res. Lett., 25, 2465Veronig, A. M., Temmer, M., & Vrˇsnak, B. 2008, ApJ, 681, L113Wang, Y.-M. 2000, ApJ, 543, L89Wang, H., Shen, C., & Lin, J. 2009, ApJ, 700, 1716Warmuth, A. 2010, Advances in Space Research, 45, 527Warmuth, A., Vrˇsnak, B., Aurass, H., & Hanslmeier, A. 2001, ApJ, 560, L105Webb, D. F., Forbes, T. G., & Aurass, H. et al. 1994, Sol. Phys., 153, 73Wills-Davey, M. J., DeForest, C. E., & Stenflo, J. O. 2007, ApJ, 664, 556Yashiro, S., Gopalswamy, N., Michalek, G., et al. 2004, Journal of Geophysical Research(Space Physics), 109, 7105Zhang, J., Cheng, X., & Ding, M. D. 2011, Nature Communications, under reviewZhang, J., Dere, K. P., Howard, R. A., Kundu, M. R., & White, S. M. 2001, ApJ, 559, 452Zhukov, A. N. 2011, Journal of Atmospheric and Solar-Terrestrial Physics, 73, 1096
This preprint was prepared with the AAS L A TEX macros v5.2.
10 –Fig. 1.— (a) and (b) AIA 193 ˚A and 211 ˚A images of the 2011 June 07 CME. The arrowsindicate the CME bubble at 06:26 UT. (c) and (d) Sequence of 193 ˚A running differenceimages showing the expanding and rising motion of the CME bubble. The blue dash-dottedlines denoted the circle fitting of the CME bubble; the red dash-dotted lines depict the diffusewave front. (e) and (f) Difference images of the composite image of COR1 white light EUVI195 ˚Apassband. The red and blue dash-dotted lines depict the diffuse wave front and theCME ejecta front, respectively. The blue solid lines indicate the heliographic latitude spacedby 15 ◦ .(An animation of this figure is available in the online journal) 11 – H e i gh t ( R s un ) V e l o c i t y ( k m / s ) -6 -5 GO ES ( W a tt s m - ) (b)(a) Fig. 2.— Kinematic evolution of the CME bubble. (a) Height-time plot of the CME bubbleleading front (black line) and the radius-time plot of the bubble intrinsic radii from circularfitting (blue line). (b) The velocity evolution of the CME bubble leading front, and theintrinsic expansion velocity of the CME bubble (blue line). GOES Soft X-ray 1–8 ˚A flux ofthe associated flare is also plotted (red line).Table 1: Properties of the apparent lateral expansion velocity of the CME bubble front andthe diffuse wave front of 2011 June 07 solar eruption.Height Peak velocity Time a ( R ⊙ ) (km s − ) (UT)1.15 960 ±
48 06:27:031.05 880 ±
27 06:26:240.95 830 ±
43 06:25:03 a Time of the peak velocity. 12 –Fig. 3.— (a) AIA 211 ˚A difference image in the transformed helio-projective coordinatesystem. The red and blue arrow lines indicate the diffuse wave front and the CME bubblefront, respectively. Two vertical dash lines indicate the boundaries of an AR in the northand the coronal hole in the south. The straight horizontal lines (black) show slices at theprojected heliocentric distance of 1.15, 1.05 and 0.95 R ⊙ , respectively. (b–d) Slice-time plotsshowing the time evolutions of the bright fronts along the lateral direction at three differentheights. The lateral displacement of the CME bubble front is indicated by the blue plussymbols, while the diffuse wave front is indicated by the red symbols. Two white horizontaldashed lines indicate the location of the same feature as indicated by white vertical dashedlines in (a).(An animation of this figure is available in the online journal) 13 –Fig. 4.— Kinematic evolution of the apparent lateral expansion of the CME bubble front(blue) and the diffuse wave front (red) along the line of the projected heliocentric distance(height) of 1.15 (a), 1.05 (b) and 0.95 R ⊙⊙