On the rate of abundance modifications versus time in active region plasmas

Four emerging and developing active regions observed on Skylab spectrohelio grams (300-600 Angstrom) have been studied over intervals of 3 to 7 days to determine the change in their Mg/Ne abundance ratio. The Mg/Ne abundance r atio is used as a measure of the FIP effect on element abundances in the so lar upper atmosphere. Mg/Ne abundance ratios were derived from diagnostic i ntensity ratios of Mg VI lines at 400 relative to adjacent Ne VI lines. Int ensity ratios were estimated from the active region images in the AZ Skylab Atlas of Extreme-Ultraviolet Spectroheliograms. The resulting abundance ra tios expressed in terms of the FIP bias beta (see text) are plotted as a fu nction of elapsed time since emergence. After emergence the newborn region shows photospheric composition: i.e., the abundance ratio of Mg/Ne = 0.296, beta = 1. Thereafter, the developing regions show a progressive increase i n the Mg/Ne abundance ratio with the value of beta reaching after 2 days co ronal-type abundances: Mg/Ne = 1.43, beta approximate to 4.8. In the later stages, the bias increases to values between 7 and 9 after 3 to 7 days. Som e implications of the abundance modifications are briefly discussed.