Study of an extended EUV filament using SoHO/SUMER observations of the hydrogen Lyman lines

Abstract

Context. In previous work we determined the 3D structure of an EUV extension of the filament observed by SoHO/CDS on 15 October 1999. The determination of the 3D structure depends on the optical thickness $\tau_{912}$ at the hydrogen Lyman-continuum edge at 912 A. The $\tau_{912}$ value was unknown, therefore it was taken as a free parameter for the 3D-structure determinations. Aims. The plasma properties (including $\tau_{912}$) of this EUV filament are estimated using non-LTE modelling of profiles of the hydrogen Lyman lines. Methods. The lines L β , L δ , L e , L6 and L7 were observed by SoHO/SUMER with the slit positioned across the EUV filament. In our modelling the EUV filament was approximated by a horizontal 1D slab with constant pressure and the temperature decreasing vertically from the edges to interior of the slab. The values of the geometrical thickness of the slab in the EUV extension were taken from its 3D structure determined for the $\tau_{912}$ values estimated iteratively. The synthetic profiles were fitted to observed ones using the $\chi^2$ minimization. Results. For the H α -filament part we obtained the expected results: a small geometrical thickness between several thousand and 15 000 km, temperatures around 5000 K in the interior and around 20 000 K in PCTRs, and pressure ~$0.1~{\rm dyn}~{\rm cm}^{-2}$. That leads to an optical thickness in the H α line center larger than 1. In the EUV extension, temperatures of 10 000–40 000 K in the interior and 35 000–50 000 K in the vertically extended PCTRs and a pressure of ~$0.01~{\rm dyn}~{\rm cm}^{-2}$ were determined. Such large variations of the temperature lead to large variations of $\tau_{912}$. Conclusions. The EUV extension is inhomogeneous – areas both optically thick and thin at 912 A are present close to each other. Parts optically thick at 912 A are visible also in H α as faint and tiny dark structures around the very dark and well pronounced H α filament. The volume blocking mechanism is an important mechanism throughout the whole EUV extension even in its optically thick parts. The absorption dominates only in the H α -filament part of the EUV filament.