Evidence for coronal turbulence in a quiescent active region

The first evidence for nonthermal broadening of X-ray lines in a quiescent active region was based on a single observation of a limb active region by the Flat Crystal Spectrometer (FCS) on the SMM satellite, reported by Acton et al. (1981). With the renewal of SMM operations, the FCS has been used to further investigate this phenomenon. On April 28, 1984 a map of Mg XI resonance line profiles was made for a bright area in NOAA Active Region 4474 during a nonflaring period. The narrowest line profiles are consistent with the nominal instrumental width plus a thermal width equivalent to about 3 million K, the temperature derived from line ratios of O VIII, Ne IX, and Mg XI. The broadest line profiles are consistent with the instrumental width plus a thermal width equivalent to about 7 million K, but a substantial amount of plasma at this temperature would result in much greater flux in the FCS higher-temperature channels than was seen. If the excess width is attributed solely to plasma turbulence, the corresponding velocity would be about 40 + or - 10 km/s.