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 coordin ated with space instruments (i.e., Transition Region and Coronal Explorer [ TRACE]), we observed an active region on 1998 September 10 with high spatia l and temporal resolution. New emerging flux in the central part of the act ive region was observed in magnetograms of the Swedish Vacuum Solar Telesco pe, La Palma. Emerging loops (arch-filament systems [AFSs]) are well develo ped in H alpha and Ca II according to the observations made at the Vacuum T ower 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 co ntinua in the cool filament plasma. We compare two techniques of measuring the hydrogen density in the cool dense fibrils of AFSs. The first method ba sed on TRACE observations derived the neutral hydrogen column density of th e plasma absorbing coronal lines. The second one using H alpha line profile s provided by the MSDP spectrographs is based on the cloud model. The resul ts are consistent. We derive also electron density values using H alpha lin es 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 g iving a good approximation for the maximum value of the neutral hydrogen co lumn 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 temperatur e.