Since Carrington’s accidental flare observation in 1859, it was established that geomagnetic storms are linked to solar activity and come in a very irregular fashion. More than one hundred years later, a different category of highly energetic transient events on the Sun was discovered (Brueckner, 1974; Gosling et al., 1974), in the course of which huge amounts of gas are ejected from the Sun into interplanetary space. After some years of unclear terminology, the use of the name coronal mass ejections (CMEs) became common. Their pronounced significance for the Earth was soon revealed. Although the discovery of CMEs was made only some 30 years ago, a very vast literature has been piled up, and the flood of new publications is still growing. A great number of review articles and books is available to the interested reader (see, e.g., Hildner, 1977; MacQueen, 1980; Wagner, 1984; Schwenn, 1986; Hundhausen, 1987; Kahler, 1992; Crooker et al., 1997; Hundhausen, 1997; Low, 2001; Webb et al., 2001; Gopalswamy, 2004; a comprehensive book on CMEs was issued by ISSI in 2006).
Before going into details, I consider it worthwhile to repeat the definition of a CME (Hundhausen et al., 1984, see also Schwenn, 1996):
We define a coronal mass ejection (CME) to be an observable change in coronal structure that 1) occurs on a time scale of a few minutes and several hours and 2) involves the appearance (and outward motion) of a new, discrete, bright, white light feature in the coronagraph field of view.
Note that this definition does not specify the origin of the “feature”, nor its nature, be it the ejecta themselves or the effects driven by them.
CMEs cause gigantic plasma clouds to leave the Sun, which then drive large-scale density waves out into space. In the animation of Figure 25 a typical CME as observed on February 27, 2000, by the coronagraph LASCO C3 onboard the SOHO spacecraft is shown. This dramatic CME (often termed the “light bulb CME”, for obvious reasons) originated from an erupting mid latitude filament in the north-east quadrant of the Earth-facing Sun disk, and it probably was pointed well-above the Earth. The apparent position angle of the centroid of about 30∘ (measured anticlockwise from the north pole) is probably due to a projection effect. Note how well the 60∘ cone angle and the general shape are maintained during the whole 12 hour passage through the LASCO field of view. This maintained “self-similarity” is characteristic for most CMEs (Low, 2001).
In Figure 26, and in the animated Figures 27, 28, and 29, I show a collection of CME images that may indicate the huge variety of CME shapes. However, much of that variety is due to projection effects. CMEs occurring close to the disk center often appear to surround the occulting disk of the coronagraph and are thus known as halo CMEs (Howard et al., 1982), CMEs with apparent widths between 120∘ and 360∘ are known as partial halos. Halos can be front-sided or back-sided, and for differentiation simultaneous disk observations are required. A “normal” CME, seen above the limb with an angular width of, say, 60∘, will appear as a halo CME or partial halo CME when oriented along the Sun–Earth-line (both: towards to or away from Earth) or some 40∘ off that line, respectively.
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