Intensities of weak lines at the solar limb

Abstract

From the observations, made with a slitless spectrograph and which were standardized by means of the light from the centre of the Sun’s disk, the line strengths were determined in absolute units. For the absorption lines this strength means the amount of radiation missing from the continuous spectrum because of the presence of the absorption line. For emission links it has the usual meaning. All absolute intensities given here are integrated ones, as observed with the slitless spectrograph and refer to the radiation from a slice of 1 cm width, through photosphere and chromosphere, parallel to the dispersion in the spectrum, extending outwards from the level where the Moon’s limb is projected, expressed in 1014 ergs per set per sterad per Angstrom. In Fig. 1 the intensity is plotted for the line Fe 4611.295 A and in this case the equivalent width is given too. The value of the equivalent width in the centre of the Sun is indicated at 131 mA, which value is taken from the Utrecht Catalogue.(2) This line is by far the strongest line treated in this communication, but by means of the curves in Fig. 1, the absolute intensities of the lines in the next figures can be transformed to equivalent widths. This is so because the absolute intensity in the solar spectrum in the wavelength region covered, is constant within 1 per cent.‘3) In all our figures the solar limb is placed as weil as possible, in the position determined by DE GROOT’~) from photoelectric measurements during the same eclipse. The relation between the scales of r,), the number of km within the Sun’s limb and the time in which the Moon’s edge moved over a certain distance, is indicated in Fig. 7 along the abscissa. In Fig. 2 absolute intensities of a number of Fe lines are given. The decrease of the