SOHO space probe permits better understanding of the sun and of its influence on the earth's atmosphere

On December 2, 1995, the SOHO space probe (Solar and Heliospheric Observatory) was successfully started from Cape Canaveral. On board are instruments for measuring the solar vacuum-ultraviolet (VUV) radiation, which were calibrated by the Physikalisch-Technische Bundesanstalt with the aid of the Berlin BESSY I electron storage ring.

The SOHO space probe is a cornerstone of the joint solar-terrestrial scientific program of ESA and NASA. The space probe is inserted into an orbit around a point located on the axis connecting earth and sun, at a distance of 1,5 million kilometers (i.e. about four times the distance between earth and moon). This allows the sun to be observed continually during 24 hours a day, for a minimum of two years.

It is one of the essential aims of the mission to investigate the sun's VUV radiation and to understand its connection with other characteristic solar properties such as the solar wind and the cycle of solar spots. The sun's VUV radiation and the solar wind decisively determine the processes in the upper layers of the earth's atmosphere and make themselves felt, for example, through disturbances of radio communication and the appearance of polar lights. Solar wind and solar VUV radiation develop in the sun's outer atmosphere. No sufficiently exact measurement data are so far available which allow to differentiate between competitive models of the sun's atmosphere, thus making possible a fundamental understanding of these processes.

To fill in this gap with the aid of the SOHO project, the telescope spectrometer systems on board must be able to carry out absolute measurements with high accuracy. For this purpose, the Physikalisch-Technische Bundesanstalt (PTB) in Berlin has developed special sun simulators and characterized them using the calculable radiation of the BESSY I electron storage ring. These transportable instruments allowed the telescopes' sensitivity to be determined under extreme clean room conditions which are indispensable for the storage of such instruments prior to the start. To maintain the high quality of the data during the observation time which will last at least two years, comparison measurements with sun telescopes on high-altitude sounding rockets have been planned. These will also be calibrated against the PTB's special sun simulators, on the basis of the BESSY I electron storage ring.

The calibrations carried out with great effort and outlay will allow quantitative results to be derived from the pictures of the sun obtained with SOHO, which will lead to a better understanding of the sun's atmosphere and its influence on the earth's atmosphere.