Chromospheric and transition region diagnostics using emission-line intensities

Abstract

EUV emission-line intensities from the HRTS experiment on Spacelab 2 are analyzed to determine the causes of intensity variations from point to point on the solar surface. Chromospheric lines of O I and transition region lines of C II, Si IV, and C IV exhibit saturation effects that limit the line intensities in the brighter regions. No such effect is found in lines of C I, Fe II, or Si III. The chromospheric lines saturate because the optical thickness exceeds the thermalization depth, whereas the transition region lines saturate because of the geometry of the unresolved fine structure.