3.3 Erupting prominences

Prominences are observed in coronagraphs often as the bright, central core of the CME structure (the filament component of the classic three-part CME). They are also observed by instruments that observe the solar disk, so through erupting prominences a direct comparison between coronagraphs and solar data can be made. Prominences are believed to be caused by the formation of a flux rope low in the magnetic structure that eventually erupts to form the CME. Many CME onset models (e.g., flux cancellation, mass loading) require the presence or formation of a prominence in order for the CME to erupt.

The latitude distribution of LASCO CMEs peaks at the equator (Section 2.4), but the distribution of EIT EUV activity including prominence eruptions associated with these CMEs is bimodal with peaks 30° north and south of the equator (Plunkett et al., 2002). This offset is confirmed for the distribution of disk source regions associated with halo CMEs (Figure 22View Image). This pattern indicates that many CMEs involve more complex, multiple-polarity systems (Webb et al., 1997Jump To The Next Citation Point) such as those modeled by Antiochos et al. (1999Jump To The Next Citation Point). Prominences themselves tend to be offset to one side of the CME axis and, occasionally, two prominences can erupt under the same CME canopy (Webb et al., 1997; Simnett, 2000). A particularly good example of the former is shown in Figures 7 – 11 of Hundhausen (1988) which combine Mauna Loa and SMM Hα and white light data of a CME (see also Webb, 1992Jump To The Next Citation Point).

View Image

Figure 22: Histograms of “source” longitudes of halo CMEs. Image reproduced with permission from Webb (2002Jump To The Next Citation Point).

Using SOHO LASCO, EIT, and MDI and ground-based Hα data, Cremades and Bothmer (2004) concluded that a simple scheme can be used to relate CME white light topology to the heliographic position and orientation of the underlying magnetic neutral line. When the neutral line is approximately parallel to the solar limb, the CME appears as a linear feature parallel to the limb having a broad, diffuse inner core. When the neutral line is approximately perpendicular to the solar limb, the CME is observed along its symmetry axis, and the core material lies along the line of sight. Joy’s law implies that the frontside neutral line will typically lie perpendicular to the east limb and parallel to the west limb. The neutral line and CME orientations are reversed for the solar backside, so backside CMEs are viewed predominately orthogonally to frontside CMEs at each limb. These CME orientations are generally valid only for CMEs with source regions in the active region belts, < 50° heliolatitude. The CME orientations will be different for polar crown filaments (McAllister et al., 2002; Gopalswamy et al., 2003a) or for CME source regions outside the active regions, where the neutral lines do not obey Joy’s law. However, in an older, related study using SMM data, Webb (1988) found no clear pattern between the orientation of filaments, i.e., neutral lines, and the morphology or widths of associated CMEs.

There have been several recent studies of the kinematics and rotations of prominences using STEREO EUVI data. Joshi and Srivastava (2011) used a stereoscopic reconstruction technique to study the motions of two polar crown prominences. They found evidence of two different motions, a helical twist in the prominence spine and overall non-radial equatorward motion of the entire prominence structure, and two phases of acceleration during the eruptions. Bemporad et al. (2011) used the tie-pointing technique with COR1 and EUVI data to reconstruct the 3-D shape and trajectory of an erupting prominence. They found evidence for a progressive clockwise rotation of the prominence by ∼ 90°, and helical motion providing evidence for the conversion of twist into writhe. Finally, (Vourlidas et al., 2011a) used SECCHI and LASCO data with a forward-fitting model to determine the 3-D orientation of a 3-part CME with embedded prominence. The found that the CME had a fast rotation rate, and suggested it was possibly due to disconnection of one of the CME footpoints.

The physical (as opposed to observational, defined earlier) definition of a CME involves material in a magnetic field that is expelled from the corona (Hundhausen, 1999), so we assume that all the material observed moving away from the Sun in coronagraphs escapes the corona. However, in a few CMEs with relatively slow speeds material in bright cores has been observed to collapse back to the Sun with speeds of ∼ 50 to 200 km s–1 (Wang and Sheeley Jr, 2002). These collapses have been interpreted in terms of gravitational and magnetic tension forces as well as the drag forces of the ambient solar wind. It is not clear whether these collapses are only a minor part of some CMEs or more generally important for the CME dynamics.

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