Jackson and colleagues described evidence for two kinds of coronal precursors occurring before the onset of Skylab CMEs. The first were called “forerunners”, large, faint regions of enhanced brightness that were found to rim the CMEs themselves (Jackson and Hildner, 1978). The outer boundaries of the forerunners maintained a constant offset of from the CME. The reality of such features would be significant for two reasons:
Jackson (1981) noted that in some events forerunner material was actually in motion prior to the associated surface activity. However, using Solwind data, Karpen and Howard (1987) concluded that forerunners were either an artifact of the contouring process used or were structures not separate from the CME itself. Although this controversy was never fully resolved, with the larger dynamic range of LASCO rims of material detected ahead of fast LASCO CMEs are now considered evidence of shock waves, and emission can be detected ahead of slower speed CMEs as low-level brightness enhancements due to the expanding streamer (see Section 3.6).
A second type of CME precursor reported by Jackson et al. (1978) was the statistically significant temporal clustering of Culgoora type III radio bursts an average of 6 hours before Skylab CME onsets. Culgoora radioheliograph positional data revealed that these same type IIIs clustered spatially at the limb within 20° of the centroid of the CME. Recently, Jackson et al. (2010c) reported that imaging measurements from the French Nançay radio array showed similar activity before some solar disk events such as on 26 April 2008, but definitive studies are clearly necessary.
Data nearer the time of CME onset indicate the existence of precursor activity before some, but not all CMEs. During the SMM era it was found that the departure times of flare-associated CMEs often preceded flare onsets. Harrison (1991) concluded that CME onsets preceded any subsequent associated H or X-ray flares by an average of 17 minutes. CME onsets were associated with precursor X-ray arches having large scale sizes of 105 km and interconnecting two active regions. In addition, most X-ray flares observed by the SMM HXIS instrument were preceded by weak soft X-ray bursts, but more recent results using soft X-ray and EUV data do not show such a clear pattern (Harrison, 1991; Harrison et al., 1990; Yashiro et al., 2008a).
As discussed in Section 3.4, an S or reverse S-shaped structure called a sigmoid sometimes develops, and can be associated with a filament’s activation. Like the filaments themselves, sigmoids are indicative of sheared coronal magnetic fields. Since many CME onset models require a magnetic shear to be established for the field to erupt, these sigmoids may be a precursor of a CME. It is well known that various kinds of filament/prominence activity precede the eruption of the filament itself by tens of minutes. Since erupting prominences are the most common type of surface activity associated with CMEs and appear as bright cores within many CMEs (Webb and Hundhausen, 1987), pre-eruptive filament activity is a form of CME precursor. Tens of minutes before their eruption, some large filaments darken and get broader (e.g., Martin, 1980). The cancellation of magnetic flux near filament channels can also build energy prior to an eruption, a process already referred to as Flux Cancellation (Martin and Livi, 1992). Kahler et al. (1988) found that the eruption of H filaments began before the onset of associated flare impulsive phases, suggesting that these erupting filaments, and by analogy the CMEs associated with them, were driven before and independently of the flare and its impulsive phase.
Some of the most massive and energetic CMEs are the so-called streamer blowout events, which were first described in detail by Sheeley Jr et al. (1982) and Illing and Hundhausen (1986). The preliminary statistics of streamer-blowout CMEs observed by LASCO were presented by Vourlidas et al. (2002b). In such events, a pre-existing streamer typically increases in brightness for one to several days before erupting as a CME (Figure 2). Following the CME, the so-called helmet streamer disappears, and is often replaced by a thin ray and later a reforming helmet (Kahler and Hundhausen, 1992). These events appear on white light synoptic charts as “bugles”: portions of the streamer belt that brighten and widen with time until they disappear during a CME (Figure 28 – Hundhausen, 1993). Most streamer blowouts involve a pre-existing prominence sitting within a coronal void or cavity; this then erupts to form the classic “three-part” CME structure. Thus, the early filament/prominence activations discussed above are probably related to streamer swellings and blowouts.
Living Rev. Solar Phys. 9, (2012), 3
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