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Astron. Astrophys. 325, 1213-1225


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Magnetic reconnection driven by emergence
of sheared magnetic field

B. Schmieder1, G. Aulanier1, P. Démoulin1, L. van Driel-Gesztelyi1,3, T. Roudier2, N. Nitta4, and G. Cauzzi5

1Observatoire de Paris, section Meudon, F-92195 Meudon Principal Cedex, Trance
2Observatoire du Pic du Midi, F-65200 Bagnères de Bigorre, France
3Konkoly Observatory, Budapcst, PL, H-1525 Hungary
4Lockheed Palo Alto Research Laboratory, Palo Alto, CA 94304, USA
5Osservatorio Astronomico di Capodimonte, via Moiariello 16, I-80131 Napoli, Italy

Received 20 December 1996 / Accepted 10 March 1997

Abstract

Recurrent subflares (Class C) were observed in the NOAA 7608 active region on 27 October 1993. From multi-wavelength observations (white-light, magnetic field, H-alpha, X-ray), obtained during a coordinated campaign between Pic du Midi and Yohkoh, it appears that these flares were double ribbon flares caused by new flux emergence. As the flare begins, the X-ray emission observed with Yohkoh/SXT is loop-shaped with the axis almost parallel to the magnetic inversion line, while during the flare development, X-ray loops appear at the location of the emerging flux. The extrapolation of the photospheric magnetic field in a linear force-free field configuration allows identification of the magnetic configuration given by the flares. The HFORMULA flare ribbons are located at the intersections of the computed quasi-separatrice layers (QSLs) with the chromosphere. We show that the initial loop-shaped X-ray emission region is in fact formed by several smaller loops directed in a nearly orthogonal direction with their feet anchored close to or in the HFORMULA ribbons. During the flare development there are X-ray loops which represent only one foot of open or largescale magnetic loops. For the studied flares the puzzling soft X-rays observations could only be understood with the help of HFORMULA and magnetic data combined with a modeling of the coronal magnetic field. Further, from the deduced magnetic field topology, the width of the QSLs and our present knowledge of 3-D magnetic reconnection, we conclude that the flare was due to magnetic reconnection driven by emergence of sheared magnetic field impacting in the pre-existing coronal field.

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

Send offprint requests to: B. Schmieder


© European Southern Observatory (ESO) 1997

Online publication: September 9, 1997
Last change: April 28, 1998
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