Electron temperatures of a late-phase solar active region from Yohkoh BCS and SXT observations

We deduce electron temperatures in a 2–3 month old active region from 1996 September and October, using soft Xray S xv spectra from the Bragg Crystal Spectrometer (BCS) and images from the Soft X-ray Telescope (SXT), both on board the Yohkoh satellite. Our observations cover a full transit of the region, from before its appearance around the east limb until after it disappeared around the west limb. Over most of this transit the region is diffuse and extremely quiescent, with few strong X-ray intensity enhancements (microflares) seen in plots of the GOES flux. During the passage the region’s temperature is roughly constant at 2.5±0.2MK in S xv and at 1.9±0.1MK from SXT, with emission measures of about 1048 cm−3 for both instruments. Temperatures obtained from SXT are consistently lower then the S xv values, indicating a multithermal plasma. A high-temperature (∼ 5 MK) component, seen in younger active regions, is virtually absent in this mature active region. Our findings, combined with earlier work, provide a method for estimating S xv temperatures of structures based on their intensity in SXT, even when these structures are not isolated on the Sun and hence not directly resolvable with the full-Sun BCS instrument. Our work also suggests that old active regions form a fundamental component of the quiet-Sun corona during periods of high solar activity.