Chromospheric Interferometry

The interference spectrum of the brighter lines in the chromosphere of the uneclipsed Sun has been photographed by means of the Oxford 35 m solar telescope, used in conjunction with a Fabry–Perot etalon of high reflectivity and large range, and a low dispersion spectrograph. The very narrow spectroscope slit was set tangentially to the 32 cm solar image. Under these conditions, the emission lines of hydrogen, helium and calcium appear as a system of interference fringes, with a resolving power of nearly 10 ⁵ . A method is developed for the quantitative photometric analysis of the fringe system, from which it is possible to determine chromospheric line-profiles as a function of height, and the chromospheric intensity gradient. This technique has been employed to determine the profile of the D ₃ line of helium. It is found to be nearly independent of height over the range 1500–4250 km, and, after correction for the effect of fine structure and photometric broadening, corresponds closely to a Gaussian distribution of the form $$I = {I}_{0}\,\text{exp}\,[-(\delta \lambda /0.34)^{2}].$$ These results are interpreted as implying that there is little self-absorption, and that the atoms which give rise to the line are subject to turbulent velocities of the order of 17 km/sec.