The helium 10830 A line in the undisturbed chromosphere

A study of the solar spectrum near helium 10830 A has shown that, where the line is very weak, the anomalous ratio of the two components is due almost certainly to faint blends. The centre-limb intensity variation over supergranule centres is in good agreement with an optically-thin law. The line is stronger over supergranule boundaries, and the ratio of the two components can be understood only if the absorbing elements have been resolved incompletely (as is probable); using centre-limb intensity variations, the authors have been unable to distinguish between unresolved horizontal platelets or inclined slabs. The integrated absorption in 1083O A over supergranule centres is double that at the boundaries. An analysis of the extension of 1083O A beyond the limb has failed to reveal the relative contributions from these two regions or their variations with height (though the supergranule-centre emission should be located relatively low). Line profiles in 1083O A, hydrogen Pa γ and Ca II 8542 A indicate that, out to at least 5000 km, line broadening is effectively non-thermal, with horizontal rms velocities of about 20 kms-1. (Auth.)