Recent observations suggest that in some circumstances a field line resonance may be driven by discrete oscillations in the solar wind. A theoretical model of excitation of magnetohydrodynamic (MHD) field line resonances by waves originating in the solar wind is presented. Such waves occur in the solar wind although their source is not understood. The propagation of an MHD wave in the solar wind is followed through the bow shock and magnetosheath. It is shown how the properties of the magnetospheric cavity are functions of longitude. The incident wave is captured at a longitude where its azimuthal wave number has a value allowing matching to the cavity oscillation. There is significant amplification at the bow shock and at the magnetopause. It is found that the signal penetrates the cavity and excites a field line resonance on the flanks of the magnetosphere in the morning and afternoon sectors. Rather than behaving as a waveguide, the cavity behaves more like an optical filter.