DYNAMICAL RESPONSE OF MAGNETIC TUBES TO TRANSVERSE PERTURBATIONS .2. TOWARDS THIN FLUX TUBES

The dynamical response of magnetic flux tubes due to local transverse periodic perturbations is investigated by numerical means. Our aim was to check the applicability of the thin flux tube approximation for ve rtically oriented flux tubes with moderate magnetic field strength (pl asma beta = 1) and diameters 100 km, 50 km and 25 km immersed in an ot herwise homogeneous nonmagnetic environment. All tubes has been subjec t to the same driver. To resolve the flux tube in a 2000 x 2000 x 1000 km(3) computational domain, a multiple nested grid version of a 3D MH D code is used. We find that a description as ideally thin flux tube b ecomes more and more questionable if the diameter of the tube decrease s. For the thinnest tube we found eg. an almost complete split up into a fork-like geometry with two counterrotating legs. This can be expla ined by a more rigorous interaction of the tube with the ambient mediu m: Geometrically thinner flux tubes are less inert than corresponding thick tubes and therefore experience stronger backreaction forces beca use thinner tubes are easier to displace horizontally. As a consequenc e of this, their cross sections are significantly deformed contradicto ry to the assumptions made in the thin flux tube approximation. The en ergy loss from the internal tube motions to the surroundings by acoust ic radiation is found to be anticorrelated with the tube radius ie. th inner tubes loose more wave energy than thicker ones.