The prominence-corona interface compared with the chromosphere-corona transition region

Abstract

The intensities of 52 EUV emission lines from each of 9 hedgerow prominences observed at the limb with the Harvard experiment on ATM-Skylab have been compared with intensities from the interior of network cells at the center of the disk, in order to compare the prominence-corona (P-C) interface with the chromosphere-corona (C-C) transition region. The intensity ratio Icell/Iprominence for each line varies systematically (in all of the prominences observed), with the temperature of formation of the line as ∼ T−0.6. The density sensitive C iii (formed at T ≈ 9 × 104 K) line ratio Iλ1175/Iλ977 implies an average density 1.3 × 109 electrons cm−3 in the P-C interface and ≈ 4 times this value in the C-C transition of the cells. The total optical thickness at the head of the Lyman continuum is ≲ 10 in most of the prominences studied; in two of the prominences, however, we cannot reject the possibility that τo is large. Methods of analysis of these EUV data are developed assuming both a resolved and an unresolved internal prominence structure. Although the systematic differences between the P-C interface and the C-C transition are stressed, the similarities are probably more remarkable and may be a result of fine structure in the C-C transition.