We have studied the Pc 3, 4 wave amplitude recorded at the Air Force Geophysics Laboratory (AFGL) magnetometer network on August 19--20, 1978. During this period of time the wave amplitude was modulated coherently across the magnetometer network. From observations, three important factors affecting the wave amplitude measured on the ground have been determined as follows: (1) the intrinsic or magnetospheric magnitude of the wave events, (2) their local time dependence, and (3) the subsurface conductivity. We present a statistical method to calculate these three factors from the simultaneous observations of a longitudinal array of magnetometers. The intrinsic wave amplitude is found to be bursty. On short timescales the wave amplitudes do not follow our expectations based on long-term statistical studies. Amplitudes vary rapidly without concomitant changes in the solar wind. We also find that the amplification of the wave amplitude due to the subsurface conductivity is different from station to station. For the northern five AFGL stations it is found that the ratio between the largest amplification and the smallest amplification is close to 2. The local time dependence maximizes approximately at local noon. A possible reason due to the reflection of Alfv¿n waves in the ionosphere is discussed. ¿ American Geophysical Union 1996