HYDROMAGNETIC WAVE-PROPAGATION AND COUPLING IN A MAGNETOTAIL WAVE-GUIDE

For some time the magnetotail has been considered as a possible region where hydromagnetic waves can propagate as waveguide modes. Recently, attention has turned to the magnetospheric flanks as waveguides, and much useful insight has been gained into propagation of fast waveguide modes there, and the structure of the field line resonances they can drive. We return to the magnetotail and investigate hydromagnetic wave propagation and coupling in a magnetotail waveguide. This problem is significantly different from the flank waveguide as the ambient magnet ic field is directed along the waveguide rather than across. Field lin e resonances of the flank type are not possible in the lobe waveguide. We describe a numerical simulation of a model waveguide in which the Alfven speed decreases across the waveguide to the central plasma shee t. The waveguide is stimulated by a short compressional perturbation l ocated in the far tail. The dress-tail spatial structure is chosen to give relatively weak coupling between fast and Alfven modes so that ph ase and group velocities of uncoupled fast modes can be used to interp ret the results. We find that the perturbation propagates dispersively down the waveguide in the form of fast waveguide modes. Fourier compo nents with small parallel wavenumber contain most of the energy, and p ropagate relatively slowly toward the ''Earth.'' These act as moving s ources which launch Alfven waves continuously earthward. The wave disp ersion relations are such that the waveguide modes couple with Alfven waves only in a limited region of the transverse Alfven speed gradient . The Alfven waves travel at the local Alfven speed along each field l ine, so that as they travel the wave on a given field line becomes inc reasingly out of phase with waves on adjacent field lines. The phase m ixing in our model is novel in that it includes the effects of transve rse gradients in both Alfven frequency and parallel wavenumber which t end to cancel each other out. Nevertheless, the phase-mixing process l eads to increasingly fine transverse structure as the waves progress d own the waveguide. The results are likely to be applicable in regions such as the plasma sheet boundary layer and the plasma mantle.