Helium line emission: its relation to atmospheric structure

Abstract

A brief review is given of observations of the resonance lines of He I and He II and their interpretation. As discussed in a previous paper, the helium lines are anomalously strong in the quiet Sun when compared with other transition region lines. The enhancement can be brought about by the transient excitation of the lines by electrons of higher temperature than that which determines the ion population. The variation in the intensity of the helium lines relative to those of other transition region lines appears to be related to variations in the temperature gradient between different parts of the atmosphere. To relate the degree of enhancement to other observable parameters, such as electron pressure and absolute line intensities, and thus to the structure of the atmosphere, a method for analysing the emission measure distribution previously developed in the context of the quiet atmosphere and active region loops is applied also to coronal holes. It is proposed that the non-thermal ion motions observed in the transition region can provide the required mechanism for transporting the helium ions across the steep temperature gradient. By making a simple model, an expression is developed which relates the helium enhancement to the non-thermal motions, the transition region temperature gradient and the electron pressure. The scaling laws implied can be tested against further observations when they become available.