Identification of the broad solar emission features near 117 nm

Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA e-mail: [eavrett;rkurucz;rloeser]@cfa.harvard.edu

Received: 12 December 2005
Accepted: 1 March 2006

Abstract

Wilhelm et al. have recently called attention to the unidentified broad emission features near 117 nm in the solar spectrum. They discuss the observed properties of these features in detail but do not identify the source of this emission. We show that the broad autoionizing transitions of neutral sulfur are responsible for these emission features. Autoionizing lines of occur throughout the spectrum between Lyman alpha and the Lyman limit. Sulfur is a normal contributor to stellar spectra. We use non-LTE chromospheric model calculations with line data from the Kurucz 2004 line list to simulate the solar spectrum in the range 116 to 118 nm. We compare the results with SUMER disk-center observations from Curdt et al. and limb observations from Wilhelm et al. Our calculations generally agree with the SUMER observations of the broad autoionizing emission features, the narrow emission lines, and the continuum in this wavelength region, and agree with basic characteristics of the center-to-limb observations. In addition to modeling the average spectrum, we show that a change of ±200 K in the temperature distribution causes the intensity to change by a factor of 4. This exceeds the observed intensity variations 1) with time in quiet regions at these wavelengths, and 2) with position from cell centers to bright network. These results do not seem compatible with current dynamical models that have temporal variations of 1000 K or more in the low chromosphere.