SOHO's guidebook to the Sun's extreme ultraviolet light
31 July 2001
The world's astronomers now have access to amazing details about the invisible ultraviolet light from the Sun, thanks to a new 'spectral atlas' prepared with the SUMER instrument on the SOHO spacecraft. It shows bright emissions at more than 1100 distinct wavelengths, more than 150 of which were not recorded or not identified before SOHO. The atlas is being presented today at an international meeting of astronomers, by Werner Curdt of Germany's Max-Planck-Institut f|r Aeronomie, the lead laboratory for the SUMER instrument."It's like reading a genetic code," Curdt explains. "But here the ingredients are chemical elements in all kinds of different ionization stages, each of them giving clues to what's going on in the Sun's atmosphere. And the same emission lines can be seen in other stars, so interest in our spectral atlas is not confined to solar physicists."
Nearly 200 years ago William Woolaston in England noticed dark lines crossing the rainbow-like spectrum of the Sun. A little later, Joseph Fraunhofer in Bavaria charted hundreds of lines. In 1859 Gustav Kirchhoff of Heidelberg realised that the spectral lines were due to absorption of visible light by atoms and ions of various elements in the Sun's atmosphere. Ever since then spectroscopy, which analyses the interactions of atoms, ions and molecules with light of different wavelengths, has played a central role in astronomy. For example, it revealed the existence of the element helium in the Sun before it was known on the Earth.
Ultraviolet light from the Sun and the stars is nearly all blocked by the Earth's air, so astronomers had to wait for rockets and satellites before they could begin ultraviolet spectroscopy in earnest. In contrast with the visible spectrum, where atoms and ions reveal their presence by absorbing particular wavelengths from the intense glow of the Sun's visible surface, the ultraviolet spectrum consists of emissions, with the lines appearing as spikes at the wavelengths characteristic of the various atoms and their states of charge.
SUMER's spectral atlas is the best-ever analysis of the ultraviolet light from the Sun, spanning wavelengths from 670 to 1609 angstroms (67 to 160.9 nanometres). The range from 670-1100 angstroms was poorly known before SOHO. Nearly all of the lines have been attributed to identified sources among the solar elements. Some of the emitting atoms or ions are at temperatures as low as 6000 degrees, whilst others are as hot as 2 million degrees. Scientists already use selected ultraviolet emissions to deduce temperatures, pressures, densities, abundances and motions of gas in the Sun's atmosphere, and changes associated with eruptive events.
SUMER stands for Solar Ultraviolet Measurements of Emitted Radiation. SOHO is the Solar and Heliospheric Observatory, a project of international cooperation between ESA and NASA. The spacecraft was built in Europe for ESA, equipped with instruments by teams of scientists in Europe and the USA, and launched by NASA in 1995.
Dr Curdt is to speak about the spectral atlas today (31 July) at a workshop on The Future of Cool-Star Astrophysics, in Boulder, Colorado. A paper, 'The SUMER Spectral Atlas of Solar-Disk Features' by W. Curdt et al., will be published in Astronomy & Astrophysics, Vol. 375, No. 2, 4 August 2001.
