DYNAMICAL RESPONSE OF MAGNETIC TUBES TO TRANSVERSE PERTURBATIONS .1. THICK FLUX TUBES

By means of a 3D numerical code we investigate the response of magneti c flux tubes to transverse perturbations. Various tubes with plasma be ta ranging from 0.1 to 10 embedded in a uniform nonmagnetic atmosphere are considered. High spatial resolution was obtained by the applicati on of a multiple nested grid strategy. Various kinds of internal longi tudinal and transverse body waves as well as surface waves were found in addition to the external sound wave. A great advantage of our 3D tr eatment is that it allows to treat energy leakage and mode conversion. We investigated the efficiency of wave energy leakage from the magnet ic tube to the external medium and found leakage rates ranging from 0. 07 for the beta = 10 tube to 0.43 for the beta = 0.1 tube. This shows that leakage is an important process particularly for low beta tubes a nd should not be ignored in studies of transverse wave propagation. As already found in 1D calculations, the purely transverse excitation ge nerates longitudinal body waves of twice the frequency. This mode conv ersion process is not very efficient. A very important result of our c omputations, however, is the efficient generation of a non-axisymmetri c longitudinal body wave which does not derive from magnetic tension f orces, but is due to an inertial pile-up effect inside the tube produc ed by the transverse motions. Particularly in low beta-tubes, the mode conversion rate for the longitudinal waves was found to be as large a s 90% of the total kinetic tube energy most of it is going into the su rface wave. This may be very significant for the heating of flux tubes and thus for the chromospheric and coronal heating.