Abstract. The Active Region 8151 (AR 8151) observed in February 1998 is the site of an eruptive event associated with a filament and a S-shaped structure, and producing a slow Coronal Mass Ejection (CME). In order to determine how the CME occurs, we compute the 3D coronal magnetic field and we derive some relevant parameters such as the free magnetic energy and the relative magnetic helicity. The 3D magnetic configuration is reconstructed from photospheric magnetic magnetograms (IVM, Mees Solar Observatory) in the case of a non-constant-α force-free (nlff) field model. The reconstruction method is divided into three main steps: the analysis of vector magnetograms (transverse fields, vertical density of electric current, ambiguity of 180• ), the numerical scheme for the nlff magnetic field, the interpretation of the computed magnetic field with respect to the observations. For AR 8151, the nlff field matches the coronal observations from EIT/SOHO and from SXT/Yohkoh. In particular, three characteristic flux tubes are shown: a highly twisted flux tube, a long twisted flux tube and a quasi-potential flux tube. The maximum energy budget is estimated to 2.6 × 10 31 erg and the relative magnetic helicity to 4.7 × 10 34 G 2 cm 4 . From the simple photospheric magnetic distribution and the evidence of highly twisted flux tubes, we argue that the flux rope model is the most likely to describe the initiation mechanism of the eruptive event associated with AR 8151.

We formulate a model system suitable for the systematic numerical investigation of global aspects of the magnetorotational instability and nonlinear dynamo action in accretion disks. The model consists of a cylindrical annulus occupied by an incompressible fluid with explicit viscosity and resistivity. Boundary conditions are imposed that permit an accretion flow appropriate to the stresses acting within the fluid to develop freely through the annulus. A steady basic state is identified in which a slow, steady accretion flow is driven by the explicit viscosity. We investigate the linear theory of this state subject to different choices of boundary conditions. The choice of boundary conditions is a crucial factor in determining the nature and growth rate of the instabilities. It is found that very rapidly growing wall modes occur generically, drawing energy artificially from outside the computational domain. However, by carefully selecting boundary conditions for which the total pressure is constrained at the radial boundaries, the wall modes are found to have growth rates bounded by the local properties of the magnetorotational instability. The resulting model provides the basis for a systematic exploration of nonlinear behavior in our future work.

Motivated by the problem of the formation of active regions from a deep-seated solar magnetic field, we consider the nonlinear three-dimensional evolution of magnetic buoyancy instabilities resulting from a smoothly stratified horizontal magnetic field. By exploring the case for which the instability is continuously driven we have identified a new mechanism for the formation of concentrations of magnetic flux.