Extreme Ultraviolet Transitions of Ca xvi in Solar Flare and Laboratory Spectra

New R -matrix calculations of electron impact excitation rates for transitions among the 2 s 2 2 p , 2 s 2 p 2 , and 2 p 3 levels of Ca XVI are presented. These data are subsequently used, in conjunction with recent estimates for proton excitation rates, to derive theoretical electron density sensitive emission-line ratios involving 2 s 2 2 p -2 s 2 p 2 transitions in the ~155-225 Å wavelength range. A comparison of these with observational data for solar flares, obtained with the Naval Research Laboratory's S082A spectrograph on board Skylab , reveals excellent agreement between theory and observation. In addition, the theoretical ratios compare favorably with those measured from the TEXT tokamak plasma, for which the electron temperature and density have been independently determined. This provides experimental support for the accuracy of the atomic data, and hence line ratio calculations, employed in the present analysis.