THE TEMPERATURE OF THE BRIGHT KNOTS AT THE TOPS OF SOLAR-FLARE LOOPS

The Japanese Yohkoh spacecraft has revealed the pervasive presence of confined bright regions at the tops of solar flare magnetic flux tubes . The physical parameters in the bright regions, such as temperature a nd density, are of considerable interest for attempting to understand the nature and longevity of these regions. We have found an example of such a source in spectroheliograms of a flare observed by the Naval R esearch Laboratory slitless spectroheliograph on Skylab. This instrume nt offers better temperature discrimination than possible with a broad band X-ray telescope such as flown on Yohkoh. We determine the tempera ture of the Skylab source using the most recent atomic data. From the Skylab extreme-ultraviolet images of this flare in spectral lines of F e XXII, Fe XXIII, and Fe XXIV, we find that the confined bright region can be described as an isothermal source at a temperature of about 11 x 10(6) K. However, this conclusion generally contradicts temperature s measured from uncollimated Bragg crystal spectrometer spectra from Y ohkoh and other spacecraft for similar X-ray class flares. There is al so emission at the same location from Ca XVII ions formed at about 6 x 10(6) K, implying that the source might be multithermal. A multitherm al source would indicate that the bright regions are composed of struc tures below the spatial resolution of the Skylab and Yohkoh instrument ation and could possibly resolve the discrepancies between Bragg spect rometer and imaging data.