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Astron. Astrophys. 336, 359-366 (1998)

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1. Introduction

It is commonly accepted that the energetic phenomenon of flares results from the release of free energy stored in the non-potential magnetic field. The formation of non-potential field configuration is usually associated with flux emergence (Zirin 1983; Zhang 1995; Lites et al. 1995), during which, two kinds of shear can be present. In the first case, the strong shear is concentrated on the boundary of two bipoles, where the new emerging one collides with the old one to form a [FORMULA]-structure; it is suggested that the boundary of such [FORMULA]-structures characterizes the intersection of separatrices with the photosphere (Shi & Wang 1994; Shi et al. 1995). In the second case, new bipoles are born carrying the shear already in the emergence (Tanaka 1991; Wang & Tang 1993). The former case may infer the interaction of emerging magnetic loops, while the latter case seems to mean that the nonpotential energy in magnetic loops probably pre-exists before they emerge from below the photosphere.

When a complex non-potential field is developed, the pre-existing and emerging magnetic loops may form a current-carrying system (Wang, Xu, & Zhang 1994; Leka et al. 1996). It is argued that the interaction between current-carrying magnetic loops makes an essential ingredient in trigger of the high-power energy release (Machado et al. 1988; Leka et al. 1993; Zhang & Wang 1994). Based on the evolution of vector magnetograms, Wang, Xu, & Zhang (1994) found that a large-scale new current system was established with an emerging bipole, and a series of flares were powered when it interacted with the old current system. Such scenario of flares was also found in the morphological analysis of SXR loops from Yohkoh observations (Kurokawa et al. 1992; Akioka et al. 1993; Hanaoka 1996).

From the observational point of view, the association between the magnetic configuration and flares are getting more and more confirmed. In some studies, the spatial correlation between the flare location and the topological features of the active region have been demonstrated (Démoulin, Hénoux & Mandrini 1992; Démoulin et al. 1994; Mandrini et al. 1993, 1995, 1996). On the other hand, the traditionally defined non-potential properties, like magnetic shear and vertical current system, deduced from the analysis of photospheric vector magnetograms, are observed to be spatially correlated with [FORMULA] or [FORMULA] flare kernels (Hagyard & Rabin 1986; Moore, Hagyard, & Davis 1987; Hagyard 1990; Canfield et al. 1993; Wang, Xu, & Zhang 1994; Fontenla et al. 1995; Wang et al. 1996). Knowledge of the spatial configuration of a magnetic field is an important clue to the understanding of such a relationship and of the mechanism of energy release (Priest 1981; Machado et al. 1988). However, the present instrumental capabilities are not able to observe directly the comprehensive magnetic field in the whole space but only magnetograms in the photosphere are available; instead, the coaligned multi-waveband observations offer an indirect way to access this problem. In a previous paper (Qiu et al. 1997; hereafter Paper I), we have studied in detail a flare event in AR7321, observed in chromospheric and coronal emissions, and investigated the thermal and nonthermal features involved in this event. Such a study of the co-ordinated data from the chromosphere to the corona, with very high spatiotemporal resolutions to date, also provides the opportunity to explore the possibility to recover the magnetic configuration where the flare occurred and was fueled. In this context, in the present paper, we will concentrate on the fact that the active region for this flare is a typical emerging flux region (EFR), where strong emerging flux pronouncedly changed the magnetic field from a simple bipole structure into a complex field with multiple magnetic loops (Liu et al. 1998; Wang, Wang, & Qiu 1997).

In the following, we will describe the instrumentation and observation of HSOS magnetic field in Sect. 2. In Sects. 3 and 4, the evolutions of the vector magnetic field and vertical current system are presented respectively, which, in Sect. 5, are further compared with the flare in term of their spatial relationships. In Sect. 6, we discuss the scenario of the flare in the light of the understanding on the configuration obtained in the previous sections. We come to the conclusion in the last section.

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© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998