In this paper we address the problem of the non-LTE (NLTE) inversion of line profiles by means of a nonlinear least-squares minimization procedure combined with very efficient multilevel transfer methods. Our approach is based on the concept of response functions, which measure the first-order response of the emergent profiles to changes in the

First results from a high-resolution three-dimensional nonlinear numerical study of the kink oscillation are presented. We show in detail the development of a shear instability in an untwisted line-tied magnetic flux tube. The instability produces significant deformations of the tube boundary. An extended transition layer may naturally evolve as a

The nonlinear development of the kink instability of a magnetic flux tube initially lying in mechanical equilibrium in the deep convection zone is presented. The unstable evolution is started by a small-amplitude sinusoidal perturbation of the shape of the flux tube in a vertical plane. As a result of the development of the instability, the lower

IAUC 4489 available at Central Bureau for Astronomical Telegrams. IAUC 4489 available at Central Bureau for Astronomical Telegrams.

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