A Technique for Removing Background Features in SECCHI - EUVI He II 304 Å Filtergrams: Application to the Filament Eruption of 22 May 2008

The STEREO mission has been providing a stereoscopic view of filament eruptions in the EUV. The clearest view during a filament eruption is seen in He II 304 Å observations. One of the main problems in visualizing filament dynamics in He II 304 Å is the strong background contrast due to surface features. We present a technique that removes background features and leaves behind only the filamentary structure, as seen by STEREO-A and -B. The technique uses a pair of STEREO He II 304 Å images observed simultaneously. The STEREO-B image is geometrically transformed to a STEREO-A view so that the background images appear similar. Filaments, being elevated structures, i.e., not lying on the same spherical surface as background features, do not appear similar in the transformed view. Thus, subtracting the two images cancels the background but leaves behind the filament structure. We apply this technique to study the dynamics of the filament-eruption event of 22 May 2008, which was observed by STEREO and followed by several ground-based observatories participating in the Joint Observing Programme (JOP 178).