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 a lpha 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 instrument s has enabled such emission to be detected from a wider group of flare s than has previously been possible. The likelihood of detecting Fe XX VI lines in a flare is found to increase sharply with the electron tem perature 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, mea sured after the impulsive stage, is greater than that determined by th ermal 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 supe rstructure code, details of which are described. This revises earlier calculations that have been extensively used. Comparison of these temp eratures with those from the Fe XXV spectra provides evidence for a si ngle loose grouping of hares, with the difference between the two temp eratures ranging from nearly zero to about 20 MK. A ''superhot'' compo nent would seem to be more or less developed according to whether the temperature difference is large or nearly zero. Flares at both extreme s 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 Telesco pe on Yohkoh, has a hardness ratio corresponding to temperatures and e mission measures similar to those from Fe XXVI line ratios, pointing t o a common origin for their emission. Many of the flares studied occur red in particular active regions with great magnetic complexity, altho ugh Fe XXVI flares do not seem to be a distinct class within large X-r ay flares.