An empirical test of different ionization balance calculations in an isothermal solar plasma

By examining solar observations using the Normal Incidence Spectrometer (NIS) within the Coronal Diagnostic Spectrometer (CDS) on board SOHO, an isothermal region in the lower solar corona was chosen for analysis by three different temperature diagnostic techniques. These techniques are the line-ratio method, the Differential Emission Measure and the Emission Measure analysis. All three methods should in theory yield the same temperature. Using these powerful diagnostic methods, the reliability of all widely used ionisation balance calculations, namely those of Shull & Van Steenberg (1982), Arnaud & Rothenflug (1985) (with revisions by Arnaud & Raymond 1992) and Mazzotta et al. (1998) have been empirically tested. It has been found that the temperature obtained does not depend on the ionization balance calculation used. It is also concluded that the three ionization balance calculations give the same results, within their uncertainties. A new variant Emission Measure analysis is presented. This provides a diagnostic tool that is stable against any distorting effects arising when a few lines have large deviations from the mean.