Ratio of line intensities in helium-like ions as a density indicator

The ratio R of the forbidden- to intercombination-line intensities in helium4ike ions has been evaluated as a function of temperature. The effects of cascades to the n = 2 levels following recombinatina and collisional excitation of higher levels have been considered. Both the temperature variation and the effect of cascades are important for an accurate determination of R. The use of R as a density indicator in solar active regions and flares is discussed, and the theoretical results are compared with observations. We also discuss how in thermal cosmic X-ray sources this method may give information on the electron density in the source or on the ultraviolet flux in the vicinity of the hot plasma.