According to the original definition, CMEs are an observable change in the coronal structure that involves the appearance (Hundhausen et al., 1984) and outward motion (Schwenn, 1996) of a new, discrete, bright, white-light feature in the coronagraph field of view. Further observations indicate that CMEs can also be observed in other wavelengths, such as soft X-rays (SXR, e.g., Rust, 1983; Klimchuk et al., 1994; Gopalswamy et al., 1996), extreme ultra-violet (EUV, Dere et al., 1997; Chen, 2009a), radio (Gopalswamy and Kundu, 1992; Maia et al., 1999), and so on (see Hudson and Cliver, 2001, for more details). The multiwavelength observations are thought to be crucial to a complete understanding of CMEs (Hudson and Cliver, 2001). Recently, it was proposed that Ly line at the ultra-violet (UV) wavelength might be very suitable for the detection of CMEs (Vial et al., 2007).
The white-light emission of the corona comes from the photospheric radiation Thomson-scattered by free electrons in the corona, and any enhanced brightness means that the coronal density somewhere along the line of sight is increased. In addition to the density enhancement, the Thomson-scattered radiation depends on the photospheric radiation incident to the electrons and the angle between the incidence and the line of sight, which makes CMEs favorably observed near the plane of the sky. With the continual observations from various ground-based and space-born coronagraphs, more than ten thousand CME events have been recorded, which enables the statistical investigation of their properties.
Living Rev. Solar Phys. 8, (2011), 1
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