The analysis and interpretation of solar X-ray photographs

Abstract

This paper discusses the methods used to analyse and interpret X-ray filtergrams obtained by solar soft X-ray telescopes such as the S-056 Skylab instrument. First, an appropriate definition of the line-of-sight emission measure ξ L(T) is developed, and it is shown how the X-ray data may be analysed to obtain an approximation to ξ L(T). The accuracy of this approximation is severely limited by the mathematical ill-conditioning of the problem, and additional constraints on the solutions must be imposed through the use of a specific model of the coronal region under study. Such a model is also required for the proper interpretation of the results in terms of coronal plasma processes. Examples of such models are provided and the forms of ξ L(T) derived from them compared with other, semi-empirical forms. The ‘filter ratio method’ (a simplified form of analysis in which the region under study is assumed isothermal) is discussed. It is shown that in the presence of line-of-sight temperature gradients, the values of ‘effective temperature’ and ‘emission measure’ yielded by this method cannot be directly related to the physical state of the plasma and so are of little utility in the study of coronal processes.