ACTIVE REGION LOOPS: HINODE/EXTREME-ULTRAVIOLET IMAGING SPECTROMETER OBSERVATIONS

We have carried out a study of active region loops using observations from the Extreme-ultraviolet Imaging Spectrometer (EIS) on board Hinode using 1'' raster data for an active region observed on 2007 May 19. We find that active region structures which are clearly discernible in cooler lines (∼1 MK) become 'fuzzy' at higher temperatures (∼2 MK). The active region was comprised of redshifted emissions (downflows) in the core and blueshifted emissions (upflows) at the boundary. The flow velocities estimated in the two regions located near the footpoints of coronal loop showed redshifted emission at transition region temperature and blueshifted emission at coronal temperature. The upflow speed in these regions increased with temperature. For more detailed study we selected one particular well-defined loop. Downward flows are detected along the coronal loop, being stronger in lower-temperature lines (rising up to 60 km s-1 near the footpoint). The downflow was localized toward the footpoint in transition region lines (Si VII) and toward the loop top in high-temperature line (Fe XV). By carefully accounting for the background emission we found that the loop structure was close to isothermal for each position along the loop, with the temperature rising from around 0.8 MK to 1.5 MK from the close to the base to higher up toward the apex (∼75 Mm). We derived electron density using well-established line ratio diagnostic techniques. Electron densities along the active region loop were found to vary from 1010 cm-3 close to the footpoint to 108.5 cm-3 higher up. A lower electron density, varying from 109 cm-3 close to the footpoint to 108.5 cm-3 higher up, was found for the lower temperature density diagnostic. Using these densities we derived filling factors in along the coronal loop which can be as low as 0.02 near the base of the loop. The filling factor increased with projected height of the loop. These results provide important constraints on coronal loop modeling.