MHD WAVES IN CORONAL FLUX TUBES

The basic MHD waves of a coronal flux loop are investigated for the rectangular box model of a plasma with oblique magnetic field and line-tied at the ends. The waves found are completely different from those in a periodic box, representative for tokamaks. They consist of a mixture of Alfven components with a ballooning factor, favouring minimal field line bending, and fast components without such a factor. Pure Alfven modes are only found as singular limiting cases of cluster spectra of Alfven-fast waves, where the fast components are localised in a photospheric boundary layer which is dictated by the requirements of line-tying. This justifies the assumption of continuous spectra in coronal loops, required for the mechanism of resonant Alfven wave heating. The waves consist of large amplitude Alfven components in the corona and fast components with a small but rapidly varying amplitude in the boundary layer, so that they appear to have the right signature for effective transfer of energy from the photosphere to the corona.