4.7 Quiescent filaments and prominences

EUV filaments (when seen in absorption on the solar disk) or prominences (when seen in emission above the limb) reveal the location of the magnetic neutral line in active regions and often represent progenitors of CMEs. They may exist in a quiescent state for a long time, but some become magnetically unstable and initiate the eruptive state of a CME. Analysis of spectra and images of prominence plasma is not trivial, since non-LTE (departure from Local Thermodynamic Equilibrium) radiative transfer models are required (see recent review by Labrosse et al., 2010b).

Some attempts of 3D reconstruction of quiescent EUV filaments have been made in the pre-STEREO era, using the solar rotation and SOHO/EIT data (Foullon, 2003). The 3D geometry of filaments can often be approximated by a vertical slab or sheet-like plasma structure. A method to estimate the width and inclination of such filament sheets has been applied to STEREO He ii 304 Å data (Gosain and Schmieder, 2010). On the other side, helical twisted flux ropes have also been observed as precursors of filaments (Raouafi, 2009). The automated detection of filaments in He 304 Å images is often difficult due to the varying brightness level of the background, but useful methods have been developed to remove this background (Artzner et al., 2010). Automated detection of limb prominences in He ii 304 Å data has been developed, which can discriminate limb prominences from active regions or the quiet corona in 93% of the cases (Labrosse et al., 2010a). Another solar prominence catcher and tracker (SLIPCAT) algorithm produced a catalog of 9477 well-tracked prominences during the 2007 April – 2009 October period of STEREO 304 Å observations (Wang et al., 2010Jump To The Next Citation Point). The statistical results showed that most prominences occur below a latitude of 60°, have a length of ≈ 50 Mm, a height of ≈ 26 Mm, and 80% show no obvious motion (Wang et al., 2010). Another statistical study of 68 quiescent filament channels with STEREO/EUVI and Hinode/XRT revealed an asymmetry in the morphology due to the variation in axial flux of the flux rope along the channel (Su et al., 2010).

A particular topology is the “prominence cavity”, which is the limb manifestation of a longitudinally extended polar-crown filament channel (or tunnel), which has a lower density than the surrounding corona. The 3D geometry and morphology of such a prominence cavity has been reconstructed from STEREO data (Gibson et al., 2010).

A complex evolution of a quiescent and active filament, from the formation, interaction, to merging, accompanied by repeated heating via magnetic reconnection preceding the eruption on 2007 May 19, has been observed with STEREO (Bone et al., 2009Jump To The Next Citation Point). The same filament was stereoscopically triangulated and the height of the EUV emission was determined to ≈ 48 ± 10 Mm prior to eruption (Gissot et al., 2008Jump To The Next Citation Point; Liewer et al., 2009Jump To The Next Citation Point; Xu et al., 2010Jump To The Next Citation Point), which was found be higher than the location of Hα emission (Xu et al., 2010Jump To The Next Citation Point).

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