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Astron. Astrophys. 325, 1213-1225 (1997)

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Magnetic reconnection driven by emergence of sheared magnetic field

B. Schmieder 1, G. Aulanier 1, P. Démoulin 1, L. van Driel-Gesztelyi 1, 3, T. Roudier 2, N. Nitta 4 and G. Cauzzi 5

1 Observatoire de Paris, section Meudon, F-92195 Meudon Principal Cedex, Trance
2 Observatoire du Pic du Midi, F-65200 Bagnères de Bigorre, France
3 Konkoly Observatory, Budapcst, PL, H-1525 Hungary
4 Lockheed Palo Alto Research Laboratory, Palo Alto, CA 94304, USA
5 Osservatorio 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 H [FORMULA] 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 H [FORMULA] 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 H [FORMULA] 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

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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