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Astron. Astrophys. 357, 1056-1062 (2000)

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1. Introduction

Many observational studies have revealed that the solar magnetic field is twisted. This is manifested in various kinds of observations, such as the morphology of [FORMULA] structures (Hale 1927; Richardson 1941), the morphology of filaments (Martin et al. 1994), the morphology of coronal loops (Rust & Kumar 1996), and the signs of current helicity derived from vector magnetograms (Seehafer 1990; Pevtsov et al. 1995; Abramenko et al. 1996; Bao & Zhang 1998). All these observations demonstrate a hemispheric preference of the sense of the twist, i.e. the left-handedness in the northern hemisphere and the right-handedness in the southern hemisphere.

It is most likely that the sense of the twist is associated with the solar dynamo and physical conditions of the convection zone (Longcope & Klapper 1997; Longcope et al. 1998). Two mechanisms have been proposed to explain the formation of the twist, i.e., vortical motions of the magnetic plasma (Hofmann & Kalman 1991; Schmieder et al. 1994), and emergence of sub-surface twisted flux (Kurokawa 1987; Tanaka 1991; Wang et al. 1994; Leka et al. 1996).

Many recent investigations of the emerging flux regions have showed that magnetic flux may be already twisted below the photosphere. In the earlier studies, the emergence of twisted sub-surface structures was inferred from the temporal evolution of [FORMULA] morphology (Kurokawa 1987; Tanaka 1991). Now high resolution measurement of the photospheric magnetic field enables us to approach this problem quantitatively. Based on the evolution of the magnetic flux and electric current over 6 days, Wang et al. (1994) disclosed that a current system grew rapidly with the emergence of a magnetic loop. In the study of several bipoles that were well-observed within 12 hours since the first detection at white-light, Leka et al. (1996) showed the emerging flux and the electric current were roughly proportional in the growth. They also analyzed the morphology and proper motion of these new bipoles, and concluded that the flux bundles were twisted before their appearance.

In this paper, by using vector magnetograms observed in AR7321, which was a region with a typical EFR, we hope to check the relationship between the emerging flux and electric current, and to study the evolution of the magnetic field twist which can be characterized by the coefficient of the force-free field, [FORMULA], ([FORMULA]). These studies may provide new important constrains on theoretical models of flux emergence.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000