The resonance lines of M gn (A = 279.55 and 280.27 nm) are just beyond the extinction limit of the Earth’s atmosphere. Because of the high cosmic abundance of magnesium, these lines are particularly important in ultraviolet astronomy and with the extension of interference spectroscopy into the far ultraviolet (Bradley 1968), sophisticated optical techniques can now be employed at these wavelengths. On the Sun, the M gn resonance lines consist of a broad absorption with a pronounced emission core similar to the H and K lines of Ca 11, but with more prominent emission and absorption features, so that the Mg 11H and K lines are much more sensitive indicators of chromospheric phenomena. The discovery (Kachalov & Yakovleva, 1962) of structure in the emission core, giving a doubly reversed profile, confirmed the similarity with CAII. The structure of the emission core was well resolved in high resolution {ca. 3 pm) echelle spectrograms obtained with a Sun-pointed rocket (Purcell, Garrett & Tousey 1963). These echelle line profiles were, however, composite ones averaged over one third of the solar disk, so that it was not possible to distinguish between profiles from quiet and active regions, or to determine centre-to-limb variations.
Extreme and Far Ultraviolet Astronomy from Voyagers 1 and 2