Extreme Ultraviolet Emission Lines of ca xv in Solar and Laboratory Spectra

Abstract

New R-matrix calculations of electron impact excitation rates in Ca xv are used to derive theoretical electron density diagnostic emission line intensity ratios involving 2s22p2–2s2p3 transitions, specifically R1=I(208.70 A)/I(200.98 A), R2=I(181.91 A)/I(200.98 A), and R3=I(215.38 A)/I(200.98 A), for a range of electron temperatures (Te=106.4–106.8 K) and densities (Ne=109–1013 cm−3) appropriate to solar coronal plasmas. Electron densities deduced from the observed values of R1, R2, and R3 for several solar flares, measured from spectra obtained with the Naval Research Laboratory's S082A spectrograph on board Skylab, are found to be consistent. In addition, the derived electron densities are in excellent agreement with those determined from line ratios in Ca xvi, which is formed at a similar electron temperature to Ca xv. These results provide some experimental verification for the accuracy of the line ratio calculations, and hence the atomic data on which they are based. A set of eight theoretical Ca xv line ratios involving 2s22p2–2s2p3 transitions in the wavelength range ∼ 140–216 A are also found to be in good agreement with those measured from spectra of the TEXT tokamak plasma, for which the electron temperature and density have been independently determined. This provides additional support for the accuracy of the theoretical line ratios and atomic data.