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

A long filament has been observed with THEMIS/MSDP and SOHOICDS - SUMER, during a coordinated campaign (JOPs 131/95) on May 5, 2000. The data were (a) 2-D Ha spectra, observed using THEMIS, (b) Lyman series spectra and Lyman continuum, observed using SOHO/SUMER, and (c) EUV spectra (in 0 v 629 Angstrom, Mg x 624 Angstrom, Si xii 520 Angstrom, Ca x 557 Angstrom and He I 594 Angstrom) 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 Halpha 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 Halpha is computed from the Halpha line profile by using an inversion technique. Using simultaneous Halpha, 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 Halpha 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 Halpha images.