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Astron. Astrophys. 351, 733-740 (1999)

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1. Introduction

EUV and soft X-ray observations of the solar corona, made by Skylab more than two decades ago, pointed out that it is structured by the magnetic field. This structuring has been strikingly confirmed by the spectacular soft X-ray pictures taken by Yohkoh spacecraft during recent years.

Coronal magnetic field lines are rooted in the photosphere in the form of concentrated, isolated flux tubes and, for such reason, photospheric conditions are extremely important in order to determine the coronal field. One simple but commonly used approach to model coronal magnetic fields is based on infinitely long, structureless flux sources placed on the photosphere. The coronal field is then invariant along the axis of the photospheric sources and is represented by the field lines in a vertical plane perpendicular to them. This approach has allowed to model the quasi static evolution of cancelling magnetic features (Priest et al. 1994) and magnetic configurations presumed to provide mass and magnetic flux to prominences (Priest et al. 1996; Mackay & Priest 1996). Instead of this infinitely thin (line) sources we here incorporate extense sources and investigate the effect of their width on the above processes.

To obtain the coronal magnetic configuration we solve the Grad-Shafranov equation after prescribing a suitable distribution of magnetic flux within the photospheric magnetic source. Then, using the superposition principle, we generate coronal potential magnetic field configurations related to an arbitrary combination of localised sources and sinks of magnetic field lines on the photosphere.

The analytical expressions for the computation of the coronal magnetic structure from photospheric boundary conditions are derived in Sects. 2 and 3. The particular photospheric magnetic elements used in this work and the associated coronal flux function and magnetic field components are described in Sect. 4. There results are applied to the problem of cancelling magnetic features and to the formation and magnetic flux replenishing of quiescent prominences in Sects. 5 and 6, respectively. Finally, in Sect. 7 conclusions are drawn.

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© European Southern Observatory (ESO) 1999

Online publication: November 3, 1999
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