2.1 Morphology and mass

CMEs present many different shapes, and much of the variety is believed simply due to the projection effects (Schwenn, 2006Jump To The Next Citation Point). However, fundamental difference can be found between narrow CMEs and others (sometimes called normal CMEs). The narrow CMEs show jet-like motions probably along open magnetic field, whereas normal CMEs are characterized by a closed frontal loop, as shown in Figure 1View Image. The typical morphology for normal CMEs is the so-called three-part structure, i.e., a bright frontal loop, which is immediately followed by a dark cavity with an embedded bright core (Illing and Hundhausen, 1985Jump To The Next Citation Point). The bright core corresponds to the erupting filament (House et al., 1981Jump To The Next Citation Point).
View Image

Figure 1: White-light images of two types of typical CMEs (from SOHO/LASCO database). (a) A narrow CME on 1997 March 11, where the 195 Å disk image is overplotted on the occulting disk; (b) a normal CME on 2000 February 27 with a three-part structure, i.e., a frontal loop, a cavity, and a bright core, where the white circle marks the solar limb.

The three-part structure is considered to be the standard morphology for CMEs, although observations indicate that only ∼ 30% of CME events possess all the three parts (Webb and Hundhausen, 1987). Among the events without a bright core, some are due to that the filament materials drained down to the solar surface along the stretched magnetic field, some are due to that thermal instability had not started to form a filament in the pre-eruption structure, and others might not be related to filament or filament-supporting structure at all.

Based on the Thomson-scattering formulae (see Billings, 1966), the mass of a CME can be estimated (Hundhausen, 1993Jump To The Next Citation Point). Without the knowledge of the exact position of the density-enhanced structure, it is often assumed that the CME is close to the plane of the sky, which would underestimate the mass of the CME, especially for halo events.

Typically, the mass of a CME falls in the range of 1 × 1011 – 4 × 1013 kg, averaged at 3 × 1012 kg (Jackson, 1985; Gopalswamy and Kundu, 1992; Hudson et al., 1996). About 15% of the CMEs have a mass less than 1011 kg (Vourlidas et al., 2002Jump To The Next Citation Point).

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