Further Measurements of Emission Line Profiles in the Solar Ultraviolet Spectrum

A further flight of a high resolution echelle spectrograph has been carried out on a Skylark rocket to measure solar line profiles in the region 1400–2200 Å. Improvements in reflective coatings and better thermal stability resulted in an increase in both the number and quality of profiles recorded. Microdensitometer traces for 37 emission lines are presented. Analysis of the profiles leads to a non-thermal mechanical velocity component which increases with temperature over the range 6. $${10}^{3}-{10}^{5}$$ K, always remaining subsonic. Interpretations are considered in terms of a progressive mechanical energy flux to the corona. These include acoustic and MHD modes for both spherically symmetric and network models of the atmosphere. Neither mode can be eliminated, although there are some difficulties in fitting the observations. It is also possible that the observed broadening is caused by non-propagating modes.