Dynamical response of magnetic tubes to transverse perturbations II. Towards thin flux tubes

The dynamical response of magnetic flux tubes due to local transverse periodic perturbations is investigated by numericalmeans.Our aimwas to check the applicability of the thin flux tube approximation for vertically oriented flux tubes with moderate magnetic field strength (plasma β = 1) and diameters 100 km, 50 km and 25 km immersed in an otherwise homogeneous nonmagnetic environment. All tubes has been subject to the same driver. To resolve the flux tube in a 2000×2000×1000 km3 computational domain, a multiple nested grid version of a 3D MHD code is used. We find that a description as ideally thin flux tube becomes more and more questionable if the diameter of the tube decreases. For the thinnest tube we found eg. an almost complete split up into a fork–like geometry with two counterrotating legs. This can be explained by a more rigorous interaction of the tube with the ambient medium: Geometrically thinner flux tubes are less inert than corresponding thick tubes and therefore experience stronger backreaction forces because thinner tubes are easier to displace horizontally. As a consequence of this, their cross sections are significantly deformed contradictory to the assumptions made in the thin flux tube approximation. The energy loss from the internal tube motions to the surroundings by acoustic radiation is found to be anticorrelated with the tube radius ie. thinner tubes loose more wave energy than thicker ones.