Carole Jordan

Abstract:

Observations made at the quiet Sun-centre with the Coronal Diagnostic Spectrometer (CDS) and Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instruments on the Solar and Heliospheric Observatory (SOHO) have shown that the intensities of the resonance lines of He I and He II are significantly larger than predicted by emission measure distributions found from other transition region lines. The intensities of the helium lines are observed to be lower in coronal holes than in the quiet Sun. Any theory proposed to account for the behaviour of the helium lines must explain the observations of both the quiet Sun and coronal holes. We use observations made with SOHO to find the physical conditions in a polar coronal hole. The electron pressure is found using the C III 1175-Å and N III 991.5-Å lines, as the C III line at 977.0 Å becomes optically thick in some regions at high latitudes. The mean electron pressure is a factor of ≃2 lower than that at the quiet Sun-centre. The mean coronal electron temperature is ≤9.4 × 105 K. The helium lines are enhanced with respect to other transition region lines but by factors which are ≃30 per cent smaller than at the quiet Sun-centre. The mean ratios of the intensities of the He I 537.0-and 584.3-Å lines and of the He I and He II 303.8-Å lines vary little with the type of region studied. These ratios are compared with those predicted by models of the transition region, taking into account the radiative transfer in the helium lines. No significant variation is found in the relative abundances of carbon and silicon.

Abstract:

Observations with the GHRS have made a considerable impact on the study of cool stars. Observations with the International Ultraviolet Explorer established the overall nature of the UV spectra of cool evolved stars, but only strong sources could be observed at high resolution and measurements of line widths were impeded by the lower signal-to-noise, The greater sensitivity of the GHRS has allowed many more emission lines to be observed, in particular, weak lines of C IV and fluorescent lines of Fe II, HZ and CO in a Tau (K5 III). These and other examples of how GHRS observations have led to improvements in spectroscopy and understanding cool giant atmospheres are discussed.