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Title:
Active Region Loops: Hinode/Extreme-Ultraviolet Imaging Spectrometer Observations
Authors:
Tripathi, Durgesh; Mason, Helen E.; Dwivedi, Bhola N.; del Zanna, Giulio; Young, Peter R.
Affiliation:
AA(University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK ; Present address is Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK.; ), AB(Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK ), AC(Department of Applied Physics, Banaras Hindu University, Varanasi 221005, India ), AD(University College London, Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK ; Present address is Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Wilberforce Road, Cambridge CB3 0WA, UK.), AE(Naval Research Laboratory, Code 7673, Washington, DC 20375, USA ; George Mason University, 4400 University Dr. MS 6A2, Fairfax, VA 22030, USA )
Publication:
The Astrophysical Journal, Volume 694, Issue 2, pp. 1256-1265 (2009). (ApJ Homepage)
Publication Date:
04/2009
Origin:
IOP
ApJ Keywords:
Sun: activity, Sun: atmosphere, Sun: corona, Sun: fundamental parameters, Sun: transition region, Sun: UV radiation
DOI:
10.1088/0004-637X/694/2/1256
Bibliographic Code:
2009ApJ...694.1256T

Abstract

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.
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