On extreme-ultraviolet helium line intensity enhancement factors on the sun

Journal Article · Mon Apr 20 04:00:00 EDT 2015 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/803/2/66· OSTI ID:22882568

Giunta, A. S.; Fludra, A. ^[1]; Lanzafame, A. C. ^[2]; O’Mullane, M. G.; Summers, H. P. ^[3]

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1. STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon. OX11 0QX (United Kingdom)
2. Dipartimento di Fisica e Astronomia, Università di Catania, via S. Sofia 78, I-95123 Catania (Italy)
3. Department of Physics, University of Strathclyde, 107 Rottenrow, Glasgow G4 0NG (United Kingdom)

Helium lines in the solar EUV spectrum provide useful diagnostics of the solar atmosphere plasma. Helium is one of the few elements that exhibits strong emission lines formed in the lower transition region, and it is the second most abundant element in the Sun. However, the analysis of helium lines is complicated by their optical thickness and the unusual behavior of their intensities, with enhancements by a factor of up to 15 reported in the literature. Detailed study requires spatially and spectrally resolved observations in the EUV range, as well as sophisticated atomic modeling. The present work focuses on the application of the differential emission measure distribution to reproduce the observed fluxes of the He i and He ii lines observed by the Solar and Heliospheric Observatory (SOHO)/Coronal Diagnostic Spectrometer and Hinode/EIS spectrometers, using the latest atomic data. It is found that the comparison between observed and reconstructed intensities for He i resonance lines λλ537.03, 522.21, and 515.62 and the intercombination line λ591.41 does not show a real enhancement. By contrast, He i λ584.33, the first line of the 1s{sup 2} {sup 1}S−1s np {sup 1}P resonance series, shows a depletion of a factor ∼2, due to the opacity effect, as supported by non-LTE radiative transfer calculations. For single ionized helium lines 303.78 and 256.32 Å, the enhancement factors obtained are higher and agree with those of previous work. The different behavior of He i and He ii lines suggests a mechanism that affects ionized helium only.

• OSTI ID: 22882568
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 803; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United Kingdom
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CHROMOSPHERE
COMPARATIVE EVALUATIONS
COMPUTERIZED SIMULATION
DISTRIBUTION
EMISSION
EXTREME ULTRAVIOLET RADIATION
HELIUM
LTE
PLASMA
RADIANT HEAT TRANSFER
RESONANCE
SUN