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Astron. Astrophys. 349, 927-940 (1999)


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The evolution from birth to decay of a short-lived active region

Yuan Yong Deng 1, Brigitte Schmieder 2,3, Cristina H. Mandrini 4, Josef I. Khan  *  5, Pascal Démoulin 2 and Pawel Rudawy 6

1 Beijing Astronomical Observatory, Beijing, 100080, P.R. China
2 Observatoire de Paris, F-92195 Meudon Principal Cedex, France
3 Institute of Astrophysics, N-0315 Blindern, Oslo 3, Norway
4 Instituto de Astronomía y Física del Espacio, IAFE, CC. 67, Suc. 28, 1428 Buenos Aires, Argentina
5 Mullard Space Science Lab., University College London, Holmbury Saint Mary, Dorking, Surrey, RH5 6NT, UK
6 Astronomical Institute, Wroclaw University, PL-51-622 Wroclaw, Poland

Received 26 April 1999 / Accepted 9 July 1999

Abstract

In the following paper we present results of the analysis of NOAA active region 7968, which was the target of a coordinated observing campaign involving the instruments aboard the Solar and Heliospheric Observatory (SOHO), Yohkoh , and ground-based observatories (Bialków, Pic du Midi, and Huairou). This active region was relatively short-lived, and thus provides a rare example of a region observed continuously from its birth to its decay phase. We have extrapolated the SOHO Michelson Doppler Imager (MDI) longitudinal magnetograms, and have compared the results of the modeled field with loops observed over a wide range of temperatures ([FORMULA]105-3[FORMULA]106 K). We find that, throughout the interval of disk passage, the global magnetic structure of the active region corresponds to a nearly potential-field configuration. Magnetic field shear is only seen in isolated patches along the polarity inversion line and is associated with the emergence of parasitic polarity regions. Several small GOES (B-class) X-ray flares and surge-like ejections were observed during the lifetime of the active region. We interpret both the heating of the active region loops and the flaring in terms of magnetic reconnection. In our scenario, the flares are assumed to be due to reconnection between the pre-existing field and newly emerging twisted flux tubes (as indicated by observed changes in the transverse field and inferred twisting motions of the plasma); while the heating of the plasma in quasi-static active region loops is assumed to be due to a relaxation process of the magnetic configuration. These loops appear anchored at places where the photospheric field is highly fragmented, creating a very complex connectivity pattern. This fact, together with the continual photospheric motions, provides the favorable conditions for current sheet formation and release of magnetic energy.

Key words: Sun: activity – Sun: corona – Sun: flares – Sun: magnetic fields – Sun: sunspots – Sun: X-rays, gamma rays

* Current address: Institute of Space & Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-0022, Japan

© European Southern Observatory (ESO) 1999

Online publication: September 13, 1999

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