YOHKOH Observations of Fe XXVI X-Ray Line Emission from Solar Flares

We report on observations from the Bragg Crystal Spectrometer (BCS) on board the Japanese solar flare spacecraft Yohkoh showing Fe XXVI Ly alpha X-ray line emission at 1.78 Angstrom. Some 75 events over a 2 yr period between 1991 December 6 and 1993 December 31 have been analyzed. The greater sensitivity of the BCS compared with previous instruments has enabled such emission to be detected from a wider group of flares than has previously been possible. The likelihood of detecting Fe XXVI lines in a flare is found to increase sharply with the electron temperature obtained from the Fe XXV line spectrum, also observed by the BCS, and with GOES X-ray class. The width of the Ly alpha(1) line, measured after the impulsive stage, is greater than that determined by thermal Doppler broadening, but this is explained by the nonzero spatial extent of flares. Electron temperatures from the intensity ratio of a nearby feature due to Fe XXV dielectronic satellites and the Fe XXVI Ly alpha(1) line are obtained from new atomic parameters from the superstructure code, details of which are described. This revises earlier calculations that have been extensively used. Comparison of these temperatures with those from the Fe XXV spectra provides evidence for a single loose grouping of hares, with the difference between the two temperatures ranging from nearly zero to about 20 MK. A ''superhot'' component would seem to be more or less developed according to whether the temperature difference is large or nearly zero. Flares at both extremes are examined in detail. The gradually varying part of the 14-33 keV X-ray emission for these events, as observed by the Hard X-ray Telescope on Yohkoh, has a hardness ratio corresponding to temperatures and emission measures similar to those from Fe XXVI line ratios, pointing to a common origin for their emission. Many of the flares studied occurred in particular active regions with great magnetic complexity, although Fe XXVI flares do not seem to be a distinct class within large X-ray flares.