Recently, a transient ULF wave observed using ground-based magnetometers and an incoherent scatter radar was interpreted as a highly damped field line resonance driven by a compressional magnetospheric cavity mode. By applying a numerical model of MHD wave coupling in the magnetosphere, and choosing parameters to simulate the event, ground magnetic field time series are derived which are similar to the observed time series, provided the plasma mass density gradient in the equatorial plane is chosen to be relatively small. We consider this to be good supporting evidence for a cavity mode drive. However, our results are of direct interest in themselves, as they demonstrate for the first time the structure of a highly damped field line resonance driven by a compressional cavity mode. By analysis of the fields in the equatorial plane, it is shown firstly that the compressional magnetic field has a relatively large amplitude near the magnetopause, so that most of the cavity mode energy is stored within about 3 RE of the magnetopause; and secondly that, because of the high damping, the modeled field line resonance is poorly developed and acts to channel roughly one third of the cavity mode energy to the ionosphere. In the model, which has the same ionospheric boundary condition for both electric field components, the rest of the cavity mode energy is lost by direct Joule dissipation in the ionosphere. However, the size of the observed ionospheric electric field components suggests that a more reflecting boundary condition may be appropriate for the cavity mode electric field. If so, we speculate that only about 10% of the cavity mode energy was lost directly to the ionosphere; more than half was possibly lost by leakage from the dayside cavity down the magnetotail. ¿ American Geophysical Union 1989