The cross-phase spectrum of the magnetic fields at two closely spaced stations located on the same meridian can be used to determine the field line resonance frequencies in the magnetosphere. The model of two harmonic oscillators previously proposed in the literature to interpret this technique, however, deviates from the traditional field line resonance model and it fails to interpret the cross-phase spectrum for the Z component. We show that the cross-phase spectrum can be interpreted by traditional field line resonance theory with consideration of ionospheric effects even in the presence of a continuum of non-interacting resonating field lines. ¿ 1998 American Geophysical Union