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Astron. Astrophys. 332, 353-366 (1998)


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Interactions between nested sunspots

II. A confined X1 flare in a delta-type sunspot

V. Gaizauskas 1, C.H. Mandrini 2, P. Démoulin 3, M.L. Luoni 2 and M.G. Rovira 2

1 Herzberg Institute of Astrophysics, National Research Council of Canada, 100 Sussex Drive, Ottawa K1A 0R6, Canada (vicgz@sdarn1.hia.sp-agency.ca)
2 Instituto de Astronomía y Física del Espacio, IAFE-CONICET, CC.67, Suc.28, 1428 Buenos Aires, Argentina * (mandrini@iafe.uba.ar; rovira@iafe.uba.ar)
3 Observatoire de Paris, DASOP, URA 2080 (CNRS), F-92195 Meudon Cedex, France (demoulin@obspm.fr)

Received 22 August 1997 / Accepted 11 November 1997

Abstract

We study the flaring activity in a nest of sunspots in which two bipolar regions emerge inside a third one. These bipolar regions belong to a large complex of activity (McMath 15314) formed by five bipoles on its May 1978 rotation. The usual spreading action during the growth of the bipoles leads to the formation of a [FORMULA] -configuration: the preceding and following spots of the two interior regions overlap (p-f collision) into a single penumbra. While [FORMULA] -configurations created in this way normally favor strong flaring activity, only very small flares occur during 5 days. Only when the following umbra in the [FORMULA] -spot breaks into pieces, accompanied by rapid photospheric motions, do intense flares occur. The largest and best observed one in this sequence, a class 1B/X1 flare on 28 May 1978, is remarkable for the absence of ejecta and for the concentration of its emission in three widely spaced sites, a pattern which holds in general over two days for lesser flares. We take this pattern as evidence that the flare is confined to the low corona. We first compute the coronal magnetic field using subphotospheric sources to model the observed magnetic data and derive the location of separatrices. In this case the magnetic field topology is defined by the link between these discrete sources. The relevant generalization of separatrices in any kind of magnetic configuration are `quasi-separatrix layers' (QSLs). We calculate them using the previous model, but also for a model obtained with a more classical extrapolation technique based on the fast Fourier transform method. We show, with both approaches, that the plage brightenings during the quiescent phase of the region and the flare kernels are located at the intersection of separatrices and QSLs with the photosphere. Moreover, they are magnetically linked. Bright and dark `post'-flare loops which form in the maximum and gradual phases of the 1B/X1 flare also highlight the location of the separatrices and the QSLs. This confirms previous studies on the importance of the magnetic field topology for flares and, with this study, we further constrain the underlying physical mechanism. We draw some conclusions about the role of magnetic reconnection in the solar corona; depending on the photospheric conditions that we identify, reconnection can lead to steady heating or flaring.

Key words: magnetic fields – MHD – Sun: corona – Sun: flares – Sun: magnetic fields

* C.H.M. and M.G.R. are Members of the Carrera del Investigador Científico, CONICET

Send offprint requests to: V. Gaizauskas

© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998
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