In pre-SOHO coronagraph observations the angular size distribution of CMEs seemed to vary little over the cycle, maintaining an average width of about 45° (SMM – Hundhausen, 1993; Solwind – Howard et al., 1985). However, the CME size distribution observed by LASCO and the CORs is affected by their increased detection of very wide CMEs, especially halos. Including halo CMEs from January 1996 – June 1998, St Cyr et al. (2000) found the average (median) width of LASCO CMEs was 72° (50°). Including all measured LASCO CMEs of 20 – 120° in width through 2002, Yashiro et al. (2004) found the average widths to vary, from 47° at minimum to 61° at maximum (1999), then declining again. Figure 12 from Gopalswamy et al. (2010a) gives the updated distributions of LASCO CME speeds and widths. The average width of 41° corresponds to non-halo (width 120°) CMEs, whereas inclusion of all CMEs yields an average width of 60°. On the bottom are the speed and width distributions of all LASCO CMEs with widths > 30°. That the CACTus automatic catalog contains many more narrow CMEs is illustrated in Figure 13 from Robbrecht et al. (2009b). Shown on a log-log scale are the CACTus and CDAW width distributions for each year from 1997 – 2006; CACTus does not measure structures with widths below 10°.
Along with their white light imaging capabilities, the benefits of polarized images have also been demonstrated with some instruments. A polarizing strip across a fixed radial was part of the C/P instrument on board SMM and polarizing capabilities were part of the Skylab and Solwind coronagraphs as well (Sheeley Jr et al., 1980; Crifo et al., 1983). Polaroid filters can help determine distances of CME material along the line of sight and, therefore, give an idea of its three-dimensional structure. This is because the Thomson scattered light that enables us to observe CMEs has a polarization degree that is dependent on the direction of observation (Billings, 1966; Howard and Tappin, 2009). In what has become two of only a few studies making use of the SOHO/LASCO polarizing capabilities, Moran and Davila (2004) and Dere et al. (2005) presented analyses of LASCO C2 polarized CME observations and showed loop arcades and filamentary structure in six CMEs. The STEREO coronagraphs provide a constant stream of polarized images enabling for the first time their regular utility for 3-D property extraction. Publications making use of this ability include Mierla et al. (2009), Moran et al. (2010), and de Koning and Pizzo (2011).
The STEREO instruments allow us to attempt to remove the projection effects using geometry, that is to use geometric triangulation on features commonly observed between observers. An early attempt to do this using LASCO and COR2 data was performed by Howard and Tappin (2008). They measured two events observed as southwest limb CMEs in LASCO observed in November 2007 when the STEREO spacecraft were each 20° from the Sun-Earth line (and 40° from each other). Figure 14 shows the results from a geometric localization technique, also using LASCO and COR2 data, devised by de Koning et al. (2009). Rather than attempt to perform 3-D triangulation on a series of points comprising the CME, they confine the CME to within a polygon bound by the limits of the CME’s extent. While this does not provide as much information as one may assume can be obtained with 3-D triangulation, it is actually a powerful technique, as the optical thinness of CMEs makes it nearly impossible to identify the same point in 3-D space when observing from different perspectives.
Many workers have now devised geometrical techniques for determining 3-D information on CMEs, including forward modeling (e.g., Thernisien et al., 2006; Wood et al., 2009), tie-pointing (e.g., Mierla et al., 2009), and inverse reconstruction (Antunes et al., 2009). Other triangulation efforts have also been made by (for example) de Koning et al. (2009), Liewer et al. (2009), and Temmer et al. (2009). The review by Mierla et al. (2010) discusses many of these new and emerging techniques. Attempts to identify the 3-D structure using triangulation has proven to be difficult, and techniques that place the CME within a volume bound by a polygon (e.g., de Koning et al., 2009; Byrne et al., 2010; Feng et al., 2012) may have greater success.
Living Rev. Solar Phys. 9, (2012), 3
This work is licensed under a Creative Commons License.