The spectral difference in ULF wave amplitude between closely spaced meridional ground stations may be used to measure the eigenfrequency of magnetospheric field lines [Baransky et al., 1985>. A more reliable technique based on the crossphase spectrum has been used to identify eigenfrequencies and study the temporal evolution of local field line resonaces. Pc3 (22--100 mHz) pulsations recorded with two pairs of low latitude ground stations have been specifically examined. Resonances and harmonics whose frequencies are in excellent agreement with model calculations of standing toroidal field line resonances, when ionospheric mass loading at low latitudes is taken into account, were identified virtually every day. This point to a diagnostic technique for monitoring temporal variations in ionospheric and magnetospheric plasma parameters.