A study of hydrogen density in emerging flux loops from a coordinated Transition Region and Coronal Explorer and Canary Islands observation campaign

During an international ground-based campaign in the Canary Islands coordinated with space instruments (i.e., Transition Region and Coronal Explorer [TRACE]), we observed an active region on 1998 September 10 with high spatial and temporal resolution. New emerging flux in the central part of the active region was observed in magnetograms of the Swedish Vacuum Solar Telescope, La Palma. Emerging loops (arch-filament systems [AFSs]) are well developed in H alpha and Ca II according to the observations made at the Vacuum Tower Telescope (VIT) and THEMIS telescope in Tenerife with the Multichannel Subtractive Double Pass (MSDP) spectrographs. The TRACE images obtained at 171 and 195 Angstrom show low-emission regions that are easily identified as the individual AFS. They are due to absorption by hydrogen and helium continua in the cool filament plasma. We compare two techniques of measuring the hydrogen density in the cool dense fibrils of AFSs. The first method based on TRACE observations derived the neutral hydrogen column density of the plasma absorbing coronal lines. The second one using H alpha line profiles provided by the MSDP spectrographs is based on the cloud model. The results are consistent. We derive also electron density values using H alpha lines that are in good agreement with those derived from the 8542 Angstrom Ca II line observed with THEMIS (Mein et al.). The three types of observations (TRACE, VTT, THEMIS) are well complementary: absorption of coronal lines giving a good approximation for the maximum value of the neutral hydrogen column density, the H alpha line giving a good determination of n(e), and the 8542 Angstrom Ca II line a good determination of the electronic temperature.