Spectroscopic diagnostics of an Hα and EUV filament observed with THEMIS and SOHO

A long filament has been observed with THEMIS/MSDP and SOHO/CDS - SUMER, during a coordinated campaign (JOPs 131/95) on May 5, 2000. The data were (a) 2-D Hα spectra, observed using THEMIS, (b) Lyman series spectra and Lyman continuum, observed using SOHO/SUMER, and (c) EUV spectra (in O V 629 Å, Mg X 624 Å, Si Xii 520 Å, Ca X 557 Å and He I 584 Å) observed using SOHO/CDS. A large depression of the line emissions in CDS images represents the EUV filament. A computed model shows that the EUV filament consists of an extended in height cloud of low gas pressure at an altitude lower than the top of the Hα filament, volume-blocking and absorbing coronal emission and absorbing transition region line emission. The optical thickness of the Lyman continuum is estimated by using the ratio of O V intensity inside and outside the EUV filament, while the optical thickness of Hα is computed from the Hα line profile by using an inversion technique. Using simultaneous Hα , Lyman lines and Lyman continuum spectroscopic data, we performed detailed, non-LTE radiative transfer diagnostics of the filament plasma conditions. The optical thickness of the Lyman continuum is larger than that of the Hα line by one to two orders of magnitude. This could be of a great importance for filament formation modeling, if we consider that more cool material exists in filament channels but is optically too thin to be visible in Hα images.