On the Rate of Abundance Modifications versus Time in Active Region Plasmas

Four emerging and developing active regions observed on Skylab spectroheliograms (300-600 Å) have been studied over intervals of 3 to 7 days to determine the change in their Mg/Ne abundance ratio. The Mg/Ne abundance ratio is used as a measure of the FIP effect on element abundances in the solar upper atmosphere. Mg/Ne abundance ratios were derived from diagnostic intensity ratios of Mg VI lines at 400 Å relative to adjacent Ne VI lines. Intensity ratios were estimated from the active region images in the Skylab Atlas of Extreme-Ultraviolet Spectroheliograms. The resulting abundance ratios expressed in terms of the FIP bias β (see text) are plotted as a function of elapsed time since emergence. After emergence the newborn region shows photospheric composition: i.e., the abundance ratio of Mg/Ne = 0.296, β = 1. Thereafter, the developing regions show a progressive increase in the Mg/Ne abundance ratio with the value of β reaching after 2 days coronal-type abundances: Mg/Ne = 1.43, β ≈ 4.8. In the later stages, the bias increases to values between 7 and 9 after 3 to 7 days. Some implications of the abundance modifications are briefly discussed.