Determination of the 3D structure of an EUV-filament observed by SoHO/CDS, SoHO/SUMER and VTT/MSDP

Heinzel et al. (2001) found that filaments seem to be much more extended in EUV spectral lines than in Halpha. In this paper we reconstruct the 3D structure of the extensions of the EUV-filament observed on 15 October 1999 by SoHO/CDS and SoHO/SUMER. Small Halpha structures were observed by VTT/MSDP. We computed the bottom and top heights of the EUV-filament extensions for several values of the hydrogen Lyman-continuum optical thickness tau(912) using the spectroscopic model of Heinzel et al. (2003a). For this we used CDS observations of the two coronal lines Mg x 624.94 Angstrom and Si XII 520.60 Angstrom. Further, we develop a new method for estimating tau(912) using the O V line measured by CDS and the O VI measured by SUMER onboard SoHO. The latter line is used as a proxy to deduce the intensity variations of 0 v underneath the filament. This method also takes into account the presence of prominence-corona transition regions. Resulting heights of the EUV-filament extensions are around 15 000 km for the bottom and around 40 000 km for the top boundaries. However, there are significant variations around these representative values and we show that in about half of observed pixels the Lyman-continuum absorption dominates in the coronal-line depression, while in the other half it is the volume blocking effect. The latter situation corresponds to a large (larger than 30 000 km) geometrical thickness of the EUV-filament extension. From the heights of the boundaries of the EUV-filament the geometrical thickness can be computed and used to estimate the total mass loading which was found to be comparable to that of typical CMEs.