Relationships between Temperature and Emission Measure in Solar Flares Determined from Highly Ionized Iron Spectra and from Broadband X-Ray Detectors
We compare the electron temperature and emission measure of flares at the time of maximum soft X-ray intensity derived using two different techniques: (1) from the ratio of a dielectronic Fe XXIV line to the resonance line of Fe XXV, combined with the absolute intensity of the Fe XXV line, and (2) from the ratio of the 0.5-4.0 Å and 1-8 Å broadband X-ray fluxes, combined with the absolute flux in one of the broadband spectral regions. The high-resolution Fe spectra are obtained with the Bragg Crystal Spectrometer experiment flown on the Japanese Yohkoh spacecraft. The broadband fluxes are obtained from Geostationary Operational Environmental Satellites (GOES). A data set of 540 X-ray magnitude C2 or brighter flares, observed by both spacecraft, is used for the analysis. Both techniques assume an isothermal plasma. The broadband temperatures are substantially lower than the Fe xxv temperatures. We find that the maximum temperature of flares brighter than MS exceeds 2 × 107 K and that the maximum temperature of flares fainter than C4 is substantially lower than 2 × 107 K. We find that the Fe XXV emission measure is linearly proportional to the GOES flux in the 0.5-4.0 Å detector.