Solar UV variability

Abstract

The ultraviolet part of the solar spectrum is unique. Sandwiched between the relatively stable visible radiation and the highly variable energetic radiation short of 1000Ǻ, the EUV and the X-rays, it exhibits a small but significant variation on different time scales. Further, solar UV radiation has an important influence on the earth's atmospher, being responsible for the energetics and dynamics of the so called middle atmosphere (the height region of 10 kms to about 100 kms). Part of this spectrum has an impact on the biosphere as well. Recent developments related to the ozone depletion and the global warming phenomena have made study of solar UV radiation and its variability an important topic of current day research.Measurement of solar UV radiation spectrum had to await the availability of space platforms and a reliable quantitative assessment of its variability has become possible only in the recent past. Rocket experiments and data from the Atmospheric Explorer Satellites and the SMM and SME satellites could establish the solar UV spectrum and its short term variabilities, e.g. 27 day rotation. Problems related to degradation of satellite instrumentation plagued a reliable quantification of longer term changes, such as the eleven year cycle. However, in recent years several modelling efforts and improved satellite instrumentation have overcome these difficulties to a great extent. The present paper discusses these developments.