The Effect of Hot Coronal Electrons on Extreme-Ultraviolet Spectral Lines of He II Emitted by Solar Transition Region Plasmas

Abstract

The cause of the lower intensities of extreme-ultraviolet (EUV) He II lines emitted by coronal hole (CH) plasmas compared with quiet Sun (QS) plasmas has been the subject of many studies dating back over half a century. In this paper, we study the effect of small amounts of "hot" electrons at coronal temperatures (T_e = 1.4 × 10⁶ K) on the intensities of EUV He II lines, as well as on the intensities of EUV lines of C III, C IV, O III, and O IV emitted by 1.5 × 10⁴ K-1.5 × 10⁵ K (4.2 <= log T_e <= 5.2) plasmas in the QS. We show that although the influence of a fraction as small as 10^-4-10^-3 of hot electrons on the intensities of the C and O lines is noticeable, the effect on the intensities of the He lines is much larger, to the extent that it could explain the excess brightness of He II lines emitted by QS regions relative to CH plasmas.