Past and recent observations of the solar upper atmosphere at vacuum-ultraviolet wavelengths

Abstract

Our understanding of the high-temperature solar atmosphere is to a large extent based on spectroscopic observations of emission lines and continuum radiation in the vacuum-ultraviolet (VUV) wavelength range of the electromagnetic spectrum. In addition, important contributions stem from soft X-ray measurements. The VUV radiation is produced by transitions of atoms and ions, or to some extent, of molecules. The atomic and ionic emission lines have formation temperatures between 10 000 K and several million kelvin, representative of the chromosphere, the transition region and the corona. The molecular lines and the continua originate in cooler regions of the Sun. Radiation at VUV wavelengths is strongly absorbed by the Earth's atmosphere leading to important geophysical processes at high altitudes. In our context it means that this radiation can only be detected with instruments on sounding rockets and spacecraft above the atmosphere. Detailed studies of the spectral radiances together with atomic physics data furnish information on the electron density and temperature of the solar atmosphere, as well as on elemental abundances, whereas Doppler line-shift measurements show bulk plasma motions, turbulence, and ion temperatures. Highlights of the research in this field will be presented from the first solar ultraviolet spectra to Skylab observations and the modern measurements in the SOHO and TRACE era, and their relevance to solar physics investigations will be outlined. Special emphasis will be put on imaging telescopes and spectrographs, which provide insight both into the morphology and the dynamics of the solar plasma structures.