4.8 Erupting filaments and prominences

The best observed erupting filament during the STEREO era occurred on 2007 May 19 (Liewer et al., 2008Jump To The Next Citation Point, 2009Jump To The Next Citation Point; Culhane et al., 2008Jump To The Next Citation Point; Li et al., 2008Jump To The Next Citation Point; Gissot et al., 2008Jump To The Next Citation Point; Bone et al., 2009; Xu et al., 2010), early in the STEREO mission when the two spacecraft were separated by 8.5°, at an angle that is most suitable for stereoscopy. The filament could be followed in STEREO/EUVI 304 Å from ≈ 12 hours before to about 2 hours after the eruption, allowing to determine the 3D trajectory of the erupting filament (Figure 40View Image), (Liewer et al., 2008, 2009Jump To The Next Citation Point). The trajectory of the erupting filament was also determined with an optical-flow (Velociraptor) algorithm (Gissot et al., 2008). The magnetic field topology was found to be highly non-potential, with a multipolar configuration, hosting frequent flares, multiple filament eruptions, and CMEs (Li et al., 2008). The 3D reconstruction of the filament and the chromospheric ribbons in the early stage of the eruption suggest that simultaneous heating occurred in the rising filament plasma and in the chromosphere below, as expected from a flare-like magnetic reconnection process (Liewer et al., 2009Jump To The Next Citation Point; Culhane et al., 2008). Simultaneous analysis of Hinode/SOT, TRACE, and EUVI data lead to conclusion that a pre-eruption sheared-core magnetic field is gradually destabilized by evolutionary tether-cutting flux cancellation, which was driven by converging photospheric flows, where the main filament ejection is triggered by flux cancellation between the positive flux elements and the surrounding negative field (Sterling et al., 2010). Comparisons of He ii and Hα images show that emission in He ii occurs together with disappearence in Hα and, thus, the disappearance results from heating and motion, rather than from draining and loss of filamentary material (Liewer et al., 2009Jump To The Next Citation Point).
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Figure 40: Two views of 3D reconstruction of the 2007 May 19 erupting filament during 11 time intervals, each one rendered with a different color. The filament erupts with the south-west end rising fastest, performing some counter-clockwise rotation (from Liewer et al., 2009).

STEREO observations of other eruptive filaments revealed also oscillating threads and filament disappearance (Gosain et al., 2009), a rotating ribbon-like 2D geometry rather than a twisted 3D flux tube (Bemporad, 2009; Bemporad et al., 2009), the eruption of a twisted filament (2008 Mar 25) that is consistent with the kink-unstable flux rope model or the sheared and opposite-polarity emerging twisted magnetic flux rope model (Aschwanden et al., 2009bJump To The Next Citation Point), a slow gradual filament eruption with a weak but persistent acceleration of 3 m s–2 over 17 hours (Figure 41View Image), followed by a gradual CME (Li et al., 2010aJump To The Next Citation Point), and an initial mass off-loading phase that triggered the rise and catastrophic loss of equilibrium of a flux rope (Seaton et al., 2011).

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Figure 41: 3D reconstruction of the gradual filament eruption observed on 2009 Sept 26, shown from STEREO/A (left) and reconstructed in 3D (with the z-coordinate in the range of 1.0 –1.9 R⊙ indicated with colors) (from Li et al., 2010a).

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