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

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

Alphonse C. Sterling *

Computational Physics Inc., 2750 Prosperity Ave., Suite 600, Fairfax, VA 22031, USA (asterling@solar.stanford.edu)

Received 19 October 1998 / Accepted 18 February 1999


We deduce electron temperatures in a 2-3 month old active region from 1996 September and October, using soft X-ray SXV 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 [FORMULA] MK in SXV and at [FORMULA] MK from SXT, with emission measures of about [FORMULA] cm-3 for both instruments. Temperatures obtained from SXT are consistently lower then the SXV values, indicating a multithermal plasma. A high-temperature ([FORMULA] @ 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 SXV 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.

Key words: Sun: atmosphere – Sun: corona – Sun: X-rays, gamma rays

* Also, E.O. Hulburt Center for Space Research, Naval Research Laboratory, Washington D.C. 20375

Present address: Institute for Space and Astronautical Science, Yoshinodai 3-1-1, Sagamihara, Kanagawa 229-0022, Japan

Send offprint requests to: A. Sterling

© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999