S XV Spectral Properties of an Active Region from the Yohkoh Bragg Crystal Spectrometer

Abstract

Using the Bragg crystal spectrometer (BCS) on board the Yohkoh satellite, we examine the spectral properties of a solar active region from 1996 March 22-24. Because the region, NOAA AR 7953, was the only one on the Sun over the 3-day period, it was possible to obtain quality spectra for that region alone despite the BCS being a full-Sun instrument. We analyzed about 150 S XV spectra with integration times ranging from about 15 to 3000 s. At least one sub-C-class flare and one C-class flare were observed during the period. Lower level transient brightenings occur nearly continuously in the region. We find average electron temperatures for the nonflaring active region ranging from 5.5 to 6.2 MK, and an average nonthermal velocity of 43.5 ± 11.8 km s-1 over the 3 days. Temporal variations of S XV values for flux and electron temperature, for electron temperature and emission measure, and for flux and emission measure are all strongly correlated both when the region is nonflaring and when it is flaring. Correlations between nonthermal velocity and flux and between nonthermal velocity and electron temperature are strong when flares are included in the analysis, but the correlation is weak when spectra from times of flares are excluded. It is, however, difficult to deconvolve the contribution of the source distribution to the nonthermal velocity estimate when flux from the region is low.