Time Variation of Solar Flare Temperatures Determined from YOHKOH BCS Spectra

Bragg Crystal Spectrometer (BCS) X-ray spectra analysis from past satellite missions indicate that it is possible to estimate temperatures in highly ionized flare plasmas to within about 12\ resonance lines in different He-like ions. This procedure is particularly valuable in cases where other temperature measuring methods are insufficient, such as during the rise phase of flares with strong X-ray spectra blue shifts. Here we examine this ratio variation in several flares using data from the Fe XXV, Ca XIX, and S XV channels of the BCS experiment onboard the Yohkoh satellite. We select flares for which we have good rise phase data, and calibrate the ratios using dielectronic-to-resonance line ratios in selected Fe XXV spectra assuming constant elemental abundances in each event. The Yohkoh BCS is about an order of magnitude more sensitive than previous X-ray flare spectrometers, and is therefore able to examine the early stages of flare development in greater detail than previously possible. For this study we select events for which we have good rise phase data, but data well into the decay phase is available for a number of the selected events. This allows us, for the first time, to follow the evolution of flare spectra from relatively cool temperatures (~ 12 MK in Fe XXV) to previously quoted ``typical" flare temperatures (~ 17 MK in Fe XXV).