Comparison between observed and theoretical O IV line ratios in the UV/EUV solar spectrum as derived by SUMER, CDS and EIS

Abstract

Aims. The joint use of SoHO Solar Ultraviolet Measurement of Emitted Radiation (SUMER), Coronal Diagnostic Spectrometer (CDS) and Hinode Extreme-ultraviolet Imaging Spectrometer (EIS) allow observation of several O iv line ratios, useful for temperature plasma diagnostics. Accurate atomic data are required to avoid interpretation errors in deriving the electron temperature from these ratios. Muglach et al. (2010) found that the measured ratio I(787.72 Å)/I(279.93 Å) is lower than the predicted value by a factor 2-5. Here the predicted value for this ratio is revised using updated atomic data. A comparison with other observed O iv line ratios is shown and the electron temperature is derived. Methods. The analysis is based on new observations made during the observational campaign of April 2009 and including three O iv multiplets. The theoretical ratios have been derived using the Atomic Data and Analysis Structure (ADAS) database and include comparison with the most recent calculations available in the literature. Results. The discrepancy for the O iv I(787.72 Å)/I(279.93 Å) ratio has been solved by adding transitions involving higher excited levels, which have been omitted in previous atomic models. This results in a decrease of the theoretical line ratio, providing electron temperatures in the range of log T = 5.17-5.39, close to the temperature expected from a plasma in ionisation equilibrium.

On Extreme-Ultraviolet Helium Line Intensity Enhancement Factors on the Sun

“…In addition, this work is focused on the review and update of atomic data for light elements, concentrating on the precision and accuracy of the previous calculations and performing new computations to extend and top-up preferred data from the literature. A recommendation of atomic data with appropriate accuracy and including finite density effects is very important to avoid interpretation errors (Giunta et al 2012). New calculations for silicon ionization, for both ground to ground state ionization and metastable resolved, have been performed and used for this analysis.…”

“…The pointing accuracy of SUMER and CDS is within 5″, while typical values of EIS pointing uncertainty are 5″-10″. However, the fields of view covered by the three spectrometers are wide enough to allow a good overlap and to perform an appropriate co-alignment (Giunta et al 2012).…”

“…This doublet is complicated by a blend with several coronal lines, mainly Si X 256.37, Fe X 256.41 Å, and Fe XIII 256.42 Å. However, for disk observations, He II has been estimated to contribute 70%-80% to the blend (Young et al 2009;Giunta et al 2012).…”

“…The effective collision strengths have been taken from the review of Sampson et al (1994). For the O 3+ ion, the data assessed by Giunta et al (2012) have been used. For Si 9+ , only the line that arises from the transition between the ground level 2s 2p P 2 2 1 2 and the excited level…”

Diagnosing transient plasma status: from solar atmosphere to tokamak divertor

“…Fortunately, theoretical and experimental results are becoming available and more reliable for many ions and the precision of atomic physics calculations has improved considerably [1]. Nonetheless, misinterpretation of a density sensitivity, a non-Maxwellian plasma or a transient is still common because of poor data or too low a truncation point [10]. This is the finite density problem and it affects also ionisation state through equivalent issues for effective recombination and ionisation processes.…”