Updated Expressions for Determining Temperatures and Emission Measures from GOES Soft X–ray Measurements

We investigate the conversion of the 0.5–4 and 1–8 Å soft X–ray flux measurements made by detectors on the Geostationary Operational Environmental Satellites (GOES) into temperature and emission measures of coronal plasma using modern spectral models and modern understanding of coronal abundances. In particular, the original analysis by Thomas, Starr & Crannell (1985) is updated to take into account the realization that coronal abundances may be quite different from photospheric abundances. An important result of this analysis is that the derived temperatures and emission measures depend strongly on the assumed abundances even at high temperatures where continuum rather than spectral lines dominates the Sun’s X–ray spectrum. This occurs because the higher coronal abundances mean that most of the continuum is due to free–bound emission processes, not free–free emission, and thus is abundance–dependent. We find significant differences between modern calculations of the temperature response of the flux measurements and the versions currently in use: for a typical flare, emission measures may be up to a factor of 4 smaller than the current software suggests. Derived temperatures are similar for both photospheric and coronal abundances for cool flares (e.g., 15 MK), but for hot flares (e.g., 35 MK) coronal abundances can lead to significantly (∼ 25%) lower temperatures being derived.