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Astron. Astrophys. 346, 995-1002 (1999)

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1. Introduction

A perhaps simplistic, but nonetheless common categorization of the solar corona is into active regions, quiet Sun regions, and coronal holes. Active regions seem to have different phases of evolution, with some periods more active than others. We have been making spectral studies of active regions during the recent solar minimum period, using data from the Bragg Crystal Spectrometer (BCS) experiment on board the Yohkoh satellite (Sterling 1997a, 1997b; Sterling et al. 1997). Those studies involved relatively young active regions which displayed a large amount of activity. In particular, flares and weaker intensity brightenings, which we call microflares (a.k.a. active region transient brightenings; e.g., Shimizu 1995), were prevalent. Those spectral observations allowed us to determine electron temperatures in active regions both when the regions were on the solar disk, and when they were partially obscured by the solar limb. In this paper, we extend our previous studies by examining the thermal properties of a late-phase active region. In contrast to the earlier studies, this region is very diffuse and quiescent, with no indication of flaring or strong microflaring activity for nearly the entire observation period. (By "strong" microflaring, we mean microflares clearly detectable in the GOES soft X-ray channels.) In addition to BCS data, we also deduce temperatures from Yohkoh 's soft X-ray telescope (SXT).

Because the Yohkoh BCS is a full-Sun instrument, uncontaminated spectra for individual solar features can only be obtained when those features are extremely bright compared to other X-ray solar features. This is generally true for flares, except in uncommon cases when two or more flares occur simultaneously. For active regions, however, the situation is more complicated, since any other active region concurrently on the Sun will also contribute to the BCS spectra. Thus in order to study the properties of a single active region, the observations must occur at times when only a single active region is present on the Sun. Yohkoh was launched in 1991 August, when solar activity was high. At such times there was virtually always more than one active region on the solar disk. During the recent solar minimum, however, there have been opportunities to study the spectral properties of isolated active regions, as was done in the studies referred to above.

For the work here, we use data from an active region that developed at the site of a diffuse complex of activity, and became prominent on the Sun around 1996 July 6. It was designated as AR 7978 by the National Oceanic and Atmospheric Administration (NOAA), and made its first crossing of the west solar limb on about July 15. This active region was the most prominent active region on the Sun for several months. It produced a large amount of activity early in its life, including a GOES -class X2.6 flare on July 9, many M-flares, and CMEs (e.g., Dryer et al. 1998), with reduced activity after the first disk passage. In subsequent solar rotations, this same activity complex was on the Earthward side of the Sun over the approximate periods July 26-August 9 (when it was re-designated as NOAA 7891), August 22-September 4 (NOAA 7986), September 19-October 2 (too weak for a NOAA designation), and October 17-30 (too weak for a NOAA designation). This is the same region studied by Sterling et al. (1997) as it made its first passage beyond the west solar limb. Hudson et al. (1998) and Harvey & Hudson (1998) discuss other aspects of this same active region.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999