SPECTROSCOPIC TECHNIQUES FOR DETERMINING ELECTRON DENSITIES IN THE SOLAR ATMOSPHERE

Abstract
The determination of electron density in the solar atmosphere using diagnostic line ratios has been a field of intense activity over the past ten years. These spectroscopic techniques have given us an insight into the physical conditions of such diverse phenomena as flares, active regions, sunspots, coronal holes and the quiet Sun. In this paper, an overview will be given of the methods used for determining electron densities in the solar atmosphere. This will include a discussion of the accuracy of the atomic parameters required in such analyses. Several different approximations are used to calculate electron scattering cross-sections. These will be outlined and their accuracy for individual ions will be assessed. The use of these techniques have led to some fundamental discoveries about the nature of the solar atmosphere. The transition region was conventionally envisaged as a homogeneous layer between the low temperature chromosphere and the high temperature corona. We now know that the transition region has unresolved filamentary structures with very small "filling factors" at low temperatures. In contrast, the coronal emission seems to be more homogeneously distributed. A lot of effort has gone into the determination of the electron densities in solar flares, particularly during the impulsive phase. Such studies are crucial to distinguish between various theoretical flare models. These problems will be discussed in relation to analyses of spectral data from SKYLAB, HRTS, SMM, SOLEX and XSST and with a view to future projects such as SOHO.