Separating He ii and Si xi Emission Components in Off-limb 304 Å Observations

Abstract

Solar extreme ultraviolet (EUV) imaging instruments usually have a channel centered at 304 Å to observe the strong He ii 303.8 Å line, which is valuable for studying the dynamics of chromospheric and transition-region structures. In off-limb regions where He ii is weak, however, the coronal Si xi 303.3 Å line becomes significant and provides a background haze that reduces the contrast of He ii structures such as jets and macrospicules, complicating the interpretation of the observations. Generally, the separation of this background would require spectroscopic observations. In this article, we take an alternate approach by reconstructing the differential emission measure (DEM) of the quiescent corona to obtain synthetic radial emission profiles in the Si xi 303.3 Å line and show that at altitudes above 20 Mm it makes the major contribution to the background. We also find the silicon abundance to be significantly, by around 80%, lower in the quiet Sun than in the coronal hole. Based on the DEM profiles, we propose a physical model for the off-limb radial intensity profile in the Si xi 303.3 Å line. The model’s main advantage is the possibility to estimate and subtract the background of the quiescent corona using observations in the 304 Å channel alone, which may be of use for future studies of small-scale solar activity.