The intensities of helium lines in the solar EUV spectrum

The present paper points out that the lines of neutral and singly ionized helium in the solar EUV spectrum have anomalously high intensities when compared with lines of other ions formed at similar temperatures. It is suggested that the observed absolute and relative intensities, and in addition line widths, can be accounted for if a mechanism which causes the helium atoms and ions to be excited by electrons with temperatures greater than the ionization equilibrium value is operating. Further, the observed decrease of helium line intensities in coronal holes, whilst other transition region lines decrease little if at all, can be accounted for either by the reduction of the enhancement mechanism in coronal holes and/or by the reduced temperature gradient in these regions. An essential factor that makes the helium lines so sensitive to the presence of such a mechanism is the large value of the ratio of the excitation potential of the lines to the electron temperature.