2 Properties of CMEs

The measured properties of CMEs include their occurrence rates, locations relative to the solar disk, angular widths, speeds and accelerations, masses, and energies (e.g., Hundhausen, 1972; Kahler, 1992Jump To The Next Citation Point; St Cyr et al., 2000Jump To The Next Citation Point; Webb, 2002Jump To The Next Citation Point; Yashiro et al., 2004Jump To The Next Citation Point; Gopalswamy et al., 2005Jump To The Next Citation Point, 2006bJump To The Next Citation Point; Gopalswamy, 2010bJump To The Next Citation Point; Kahler, 2006Jump To The Next Citation Point; Vourlidas et al., 2010Jump To The Next Citation Point). There is a large range in the basic properties of CMEs, although some of this scatter is likely due to imaging projection effects (e.g., Burkepile et al., 2004Jump To The Next Citation Point; Cremades and Bothmer, 2004Jump To The Next Citation Point). Their speeds, accelerations, masses, and energies extend over 2 – 3 orders of magnitude (e.g., Vourlidas et al., 2002aJump To The Next Citation Point; Gopalswamy et al., 2006bJump To The Next Citation Point), and their angular widths exceed by factors of 3 – 10 the sizes of flaring active regions (e.g., Yashiro et al., 2004Jump To The Next Citation Point). Note that the measured values in the above cited publications make the assumption that all the CME material is in the “plane of the sky”, i.e., in the plane orthogonal to the Sun-Earth line. Thus, for example, unless a CME is exactly at the solar limb, its derived properties will be an underestimate and the width an overestimate. Recent developments using auxiliary data (Howard et al., 2007Jump To The Next Citation Point, 2008bJump To The Next Citation Point) and the multiple viewpoint capability of STEREO (e.g., Mierla et al., 2010Jump To The Next Citation Point, and references therein) have attempted to overcome this problem. These are discussed later. Table 1 summarizes the statistical properties from all of the near-Earth space borne coronagraph observations of CMEs (summaries of most CME parameters observed by the STEREO spacecraft are not yet available).


Table 1: Average statistical properties from near-Earth space borne coronagraph observations of CMEs. Updated from Gopalswamy (2004Jump To The Next Citation Point). a SMM values from Burkepile et al. (2004Jump To The Next Citation Point). b Updated by S. Yashiro (2011) priv. comm. c LASCO masses and energies from Vourlidas et al. (2010Jump To The Next Citation Point, 2011bJump To The Next Citation Point). The Solwind mass and energy values in Vourlidas et al. (2010Jump To The Next Citation Point, 2011bJump To The Next Citation Point) are in error (C.A. de Koning and A. Vourlidas (2012) priv. comm.). Although Solwind operated into 1985, we show only the values for 1979 – 81 from Howard et al. (1985Jump To The Next Citation Point).
Coronagraph OSO-7 Skylab Solwind SMMa LASCOb
Epoch 1971 1973 – 74 1979 – 81 1980, 84 – 89 1996 – present
FOV (R ⊙) 2.5 – 10 1.5 – 6 3 – 10 1.6 – 6 1.2 – 32
Total # CMEs 27 115 998 1351 > 10000
Speed (km s–1) 470 472 349 489
Acceleration (m s–2) –16 to +5
Width (°) 42 45 46 47
Mass (1015) gc 6.2 4.1 3.3 1.3
KE (1030) ergc 3.5 8.0 2.0
Mech. E (1030) ergc 4.2

UpdateJump To The Next Update Information

CMEs can exhibit a variety of forms, some having the classical “three-part” structure (Illing and Hundhausen, 1985), usually interpreted as compressed plasma ahead of a flux rope followed by a cavity surrounded by a bright filament/prominence (Figure 2View Image). Other CMEs display a more complex geometry. Some CMEs appear as narrow jets, some arise from pre-existing coronal streamers (the so-called streamer blowouts), while others appear as wide almost global eruptions. CMEs spanning very large angular ranges are probably not really global, but rather have a large component along the Sun-observer line and so appear large by perspective. These include the so-called halo CMEs (Howard et al., 1982) – see Section 2.3. The CDAW CME catalog (Yashiro et al., 2004Jump To The Next Citation Point) defines a “partial halo” as a CME with an apparent position angle range > 120°. Hence, again, the definition of a CME is restricted by its viewing perspective. Figure 4View Image illustrates several examples of partial and full halo CMEs observed by LASCO.

Figure 5View Image shows images of the same event (the Earth-directed CME from early April 2010) observed from three different viewpoints. Figure 5View Imageb shows the perspective from LASCO, which is along the Sun-Earth line, where the CME appears as a halo. Figures 5View Imagea and c show the same CME as observed by each STEREO spacecraft, which were separated in longitude by around 70° from LASCO at the time. The event appears in each COR-2 image as a limb CME directed towards the left (right) relative to STEREO-A (-B). The dramatic change in the appearance of this CME, with the only physical change being the viewing location, demonstrates the importance of perspective with respect to measuring CME properties.

View Image

Figure 4: Examples of a variety of halo CME observations, clockwise: a frontside full halo (arrow shows likely source near Sun center); a backside full halo; a partial halo; and an asymmetric full halo. Image reproduced with permission from Gopalswamy et al. (2003aJump To The Next Citation Point).
View Image

Figure 5: Images of the same Earth-directed CME obtained from three different viewing locations within an hour: a) from STEREO/COR2-B on 3 April 2010 at 11:39 UT, b) from LASCO/C2 at 10:55 UT, and c) from STEREO/COR2-A on the same day at 11:08 UT. At this time (April 2010) the STEREO spacecraft were approximately 70° in longitude from the Sun-Earth line and ∼ 140° from each other. The different appearances of this same CME observed at around the same time demonstrate the need to consider perspective in measuring CME properties.
 2.1 CME identification and measurement
 2.2 Frequency of occurrence
 2.3 Halo CMEs
 2.4 Locations, widths, geometry
 2.5 Kinematics
 2.6 Masses and energies

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