Magnetotail waveguide: Fast and Alfvén waves in the plasma sheet boundary layer and lobe

Numerical simulations of MHD wave propagation and coupling in a realistic magnetotail are presented. Fast mode waves are observed to disperse and couple resonantly to Alfvén waves over a broad layer rather than on an isolated field line. Indeed, the layer is likely to be so broad as to include the entire tail lobe as well as the plasma sheet boundary layer (PSBL). It appears that small k y modes (where k y is the cross‐tail wave number) will provide the most efficient coupling as they will tend to propagate along the magnetotail field lines rather than across them and out of the tail boundaries. (Moreover, it is only small k y fast modes that will be able to penetrate the lobe.) Alfvén waves in the PSBL are shown to phase mix rapidly resulting in strong field‐aligned currents with an equatorward phase motion. These properties are in agreement with observations of optical auroral emissions. The lobe Alfvén waves do not phase mix, and are not expected to excite optical emissions. They may, however, provide a significant ponderomotive force and could account for the transport of oxygen ions from the ionosphere into the distant tail lobes.