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, 1992; St Cyr et al., 2000; Webb, 2002; Yashiro et al., 2004; Gopalswamy et al., 2005, 2006b; Gopalswamy, 2010b; Kahler, 2006; Vourlidas et al., 2010). 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., 2004; Cremades and Bothmer, 2004). Their speeds, accelerations, masses, and energies extend over 2 – 3 orders of magnitude (e.g., Vourlidas et al., 2002a; Gopalswamy et al., 2006b), and their angular widths exceed by factors of 3 – 10 the sizes of flaring active regions (e.g., Yashiro et al., 2004). 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., 2007, 2008b) and the multiple viewpoint capability of STEREO (e.g., Mierla et al., 2010, 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).
|Epoch||1971||1973 – 74||1979 – 81||1980, 84 – 89||1996 – present|
|FOV ()||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|
|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|
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 2). 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., 2004) 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 4 illustrates several examples of partial and full halo CMEs observed by LASCO.
Figure 5 shows images of the same event (the Earth-directed CME from early April 2010) observed from three different viewpoints. Figure 5b shows the perspective from LASCO, which is along the Sun-Earth line, where the CME appears as a halo. Figures 5a 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.
Living Rev. Solar Phys. 9, (2012), 3
This work is licensed under a Creative Commons License.