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Context. The solar magnetic activity and cycle are linked to an internal dynamo. Numerical simulations are an efficient and accurate tool to investigate such intricate dynamical processes. Aims. We present the results of an international numerical benchmark study based on two-dimensional axisymmetric mean field solar dynamo models in spherical geometry. The(More)
The hydrodynamic stability of accretion disks is considered. The particular question is whether the combined action of a (stable) vertical density stratification and a (stable) radial differential rotation gives rise to a new instability for nonax-isymmetric modes of disturbances. The existence of such an instability is not suggested by the well-known(More)
Received ; accepted – 2 – ABSTRACT Because of the lack of reliable sunspot observation, the quality of sunspot number series is poor in the late 18th century, leading to the abnormally long solar cycle (1784–1799) before the Dalton minimum. Using the newly recovered solar drawings by the 18–19th century observers Staudacher and Hamilton, we construct the(More)
The possibility that the magnetic shear-ow instability (alsòBalbus-Hawley' instability) might give rise to turbulence in a thin accretion disk is investigated through numerical simulations. The study is linear and the uid disk is supposed to be incompressible and diierentially rotating with a simple velocity proole with / R ?q. The simplicity of the model(More)
The important result achieved in our recent study of a large sample of HgMn stars using UVES at the VLT and FEROS at the ESO 2.2 m telescope is the finding that most HgMn stars exhibit spectral variability of various chemical elements, proving that the presence of an inhomogeneous distribution on the surface of these stars is a rather common characteristics(More)