3D Evolution of a Filament Disappearance Event Observed by STEREO

A filament disappearance event was observed on 22 May 2008 during our recent campaign JOP 178. The filament, situated in the Southern Hemisphere, showed sinistral chirality consistent with the hemispheric rule. The event was well observed by several observatories, in particular by THEMIS. One day, before the disappearance, H alpha observations showed up- and down-flows in adjacent locations along the filament, which suggest plasma motions along twisted flux rope. THEMIS and GONG observations show shearing photospheric motions leading to magnetic flux canceling around barbs. STEREO A, B spacecraft with separation angle 52.4A degrees, showed quite different views of this untwisting flux rope in He ii 304 images. Here, we reconstruct the three-dimensional geometry of the filament during its eruption phase using STEREO EUV He ii 304 images and find that the filament was highly inclined to the solar normal. The He ii 304 movies show individual threads, which oscillate and rise to an altitude of about 120 Mm with apparent velocities of about 100 km s(-1) during the rapid evolution phase. Finally, as the flux rope expands into the corona, the filament disappears by becoming optically thin to undetectable levels. No CME was detected by STEREO, only a faint CME was recorded by LASCO at the beginning of the disappearance phase at 02:00 UT, which could be due to partial filament eruption. Further, STEREO Fe xii 195 images showed bright loops beneath the filament prior to the disappearance phase, suggesting magnetic reconnection below the flux rope.