Abundance Variations in the Outer Solar Atmosphere Observed in SKYLAB Spectroheliograms

Relative abundances of neon, magnesium, silicon, and oxygen have been derived from emission measure analyses of a prominence, flare, and active region observed in the 315-625 A range on NRL Skylab spectroheliograms. The neon-to-magnesium abundance ratio changes from a value of 2.2 in the prominence to 0.64 in the active region, while the magnesium to silicon ratio does not vary, suggesting that neon is the variable element. Neon-to-magnesium abundance ratios in a larger sample of solar features have been estimated using eye estimates of line ratios in the Ne VI and Mg VI multiplets at 400 A. The abundance variations are strongly correlated with the magnetic field morphology, ranging from Ne/Mg = 0.1 or 0.2 in some diffuse, open-field structures to values of 2 or 3 in some active, closed-field structures. Large Ne/Mg ratios of approximately 3 observed in impulsive flares may be representative of the abundance distribution in the photosphere. It is suggested that the well-known weakening of helium lines in coronal holes may be caused by an abundance deficiency of helium, similar to the abundance of depletions of neon observed in open-field regions.