The electron temperature and fine structure of soft X-ray solar flares

I discuss the determination of the electron temperature of soft X-ray solar flares using data obtained from the iron-line Bragg Crystal Spectrometer (BCS) and Soft X-ray Telescope (SXT) instruments on the Yohkoh spacecraft. I find that there is a substantial difference in the derived temperatures obtained from the two instruments, with the highest temperatures obtained from the BCS. However, in some regions of certain flares, the SXT temperatures equal the temperatures expected from the iron-line spectra. These hot regions are almost always considerably weaker in intensity than the brightest regions in the SXT flare images. I discuss the relationship of the SXT hot regions to the brightest regions in the flare and show that, at least in some cases, the hot SXT regions appear to be related to the hard X-ray flare component. I also show that the temperature data from the two instruments can be reconciled by concluding that flare loops that appear to be single loops are actually collections of loops with different temperatures on arcsecond or less spatial scales. This result was obtained indirectly by Doschek, Strong, & Tsuneta in 1995 from an analysis of SXT flare images and is substantiated so far by flare images from TRACE. The appearance of a soft X-ray flare depends critically on the electron temperature response of the imaging instrumentation.