Observations of the sun at vacuum-ultraviolet wavelengths from space. Part I: Concepts and instrumentation

摘要

Studies of the high-temperature solar atmosphere are 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. Most of the VUV radiation is produced by transitions of atoms and ions. The resulting atomic and ionic spectral lines have formation temperatures between 10 000 K and 20 MK, representative of the chromosphere, transition region, corona, and solar flares. Some molecular lines and the continua originate in cooler regions of the Sun, around and below the temperature minimum between the photosphere and the chromosphere. Radiation at VUV wavelengths is strongly absorbed by the Earth's atmosphere. Consequently, it can only be detected with instruments on sounding rockets and spacecraft operating above the atmosphere. The progress in this field of research, in particular over the last 25 years, will be presented in the first part of this review by describing the concepts and instrumentation of modern spectrographs and imaging telescopes. This presentation is accompanied by some examples of high-resolution solar images and a discussion of radiometric-calibration aspects and wavelength measurements. A second part will follow in the near future, summarizing important results obtained on the plasma conditions in the solar atmosphere.