Detection and limb brightening of the HI n=20-19 Rydberg line in the submillimetre spectrum of the Sun

Submillimetre spectra taken near to the solar limb with a polarizing interf erometer on the James Clerk Maxwell Telescope have been compared with disk- centre spectra to reveal a limb-brightened feature whose peak intensity occ urs at the predicted frequency of the n = 20-19 Rydberg transition in Hi at 29.622 cm(-1). A shoulder on this peak, at 29.65 cm(-1), has been tentativ ely assigned to the equivalent transition in Mg I. The H I line exhibits li mb brightening of up to 9% of the disk-centre continuum intensity. The inte nsity of the Mg I line is about half of the H I line intensity across the o bserved region near to the limb. Widths of the H I line are between 0.020 a nd 0.027 cm(-1), smaller than predicted by current models of this line in t he Sun's spectrum. These measurements represent the highest-n Rydberg lines detected to date in the solar spectrum. The measured line intensity, line width, limb brightening and the relative heights of the contributions from H I and Mg I place constraints upon further modelling of the solar atmosphe re.