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

By examining solar observations using the Normal Incidence Spectrometer (NI S) within the Coronal Diagnostic Spectrometer (CDS) on board SOHO, an isoth ermal region in the lower solar corona was chosen for analysis by three dif ferent temperature diagnostic techniques. These techniques are the line-rat io method, the Differential Emission Measure and the Emission Measure analy sis. All three methods should in theory yield the same temperature. Using t hese powerful di agnostic methods, the reliability of all widely used ionis ation balance calculations, namely those of Shull & Van Steenberg (1982), A rnaud & Rothenflug (1985) (with revisions by Arnaud & Raymond 1992) and Maz zotta et al. (1998) have been empirically tested. It has been found that the temperature obtained does not depend on the ioni zation balance calculation used. It is also concluded that the three ioniza tion balance calculations give the same results, within their uncertainties . A new variant Emission Measure analysis is presented. This provides a diagn ostic tool that is stable against any distorting effects arising when a few lines have large deviations from the mean.