How Good Are GOES XRS Temperatures and Emission Measures?

Abstract The soft X-ray (SXR) measurements made by NOAA’s GOES weather satellites are an important resource for solar physics and space weather. In particular, they are extensively used to study the energetics of solar flares via temperatures and emission measures derived from the SXR data. However, the SXR instruments measure just two channels, 0.5 – 4 Å and 1 – 8 Å: with just two data points, it is only possible to represent the flare plasma with a single temperature component, whereas it is well known that a flare can display a wide range of temperatures at any given time. In order to assess how representative the GOES SXR temperatures and emission measures are, we compare GOES measurements with EUV data for six spectral lines of Fe that cover the typical temperature range of flares, 10 – 20 MK. From a sample of 23 large flares, we find that the GOES temperatures match the emission-measure-weighted EUV temperatures surprisingly well, but (assuming photospheric abundances) the GOES emission measures are smaller than the EUV emission measures by up to 50%, with the discrepancy larger at higher temperatures.