High spatial resolution VAULT H-Ly alpha observations and multiwavelength analysis of an active region filament

Context. The search for the fine structure of prominences has received considerable new attention thanks to the Swedish Solar Telescope (SST) H alpha pictures that provide an unsurpassed spatial resolution. Recently, it has been shown that the filaments' coronal environment, at least for quiescent filaments, is perturbed by either cool absorbing material (in the EUV) or an "emissivity blocking" (actually a lack of transition region and coronal material). Aims. The aim is to assess the fine structure in an active region filament and to determine the nature of the EUV absorption or lack of emission phenomena, using the very optically thick line H-Ly alpha, formed at a temperature higher than H alpha. Methods. We performed a multiwavelength study where high-resolution imaging in the H-Ly alpha line (VAULT) was analysed and compared with observations of an active region filament in H alpha (BBSO) and EUV lines (EIT and TRACE). Results. As for the SST data, small-scale structures were detected at a typical scale of about one to two arcseconds with, for some cuts, an indication of fine scales down to 0.4 arcsec in the optically thick H-Ly alpha line. The filament intensity relative to the intensity of the (active) region it is embedded in is about 0.2 in H-Ly alpha. This ratio (Lymana ratio intensity or "LRI") is the lowest value compared to other lines, e. g. H alpha. The filament environment was also investigated and evidence of an UV extension was found. The comparison of spatial cuts in different lines across the filament shows evidence of strong absorption, and consequently of cool plasma on one side of the filament, but not on the other (that side is obscured by the filament itself). Conclusions. The absence of very fine structure in H-Ly alpha compared to H alpha is explained by the formation temperature of the H-Ly alpha line (similar to 20 000 K), where the transition regions of the thin threads begin to merge. From the detection of H-Ly alpha absorption on the observable side of the filament side, we derive the presence of absorbing (cool) material and possibly also of emissivity blocking (or coronal void). This poses the question whether these absorption effects are typical of active region filaments.