We used the IMAGE high latitude magnetometer array from two weeks in March
1996 to study 10-50 mHz (Pc3-4) pulsations that exhibit high coherence over
an extended spatial region. These are a small sub-class of daytime Pc3 act
ivity at high latitudes. Nine of eleven events did not show evidence of fie
ld line resonance harmonics. Instead, the observed amplitudes decreased equ
atorward, and phase relationships indicate poleward propagation. The result
s suggest that the pulsations are due to incoming fast mode waves driving f
orced field line oscillations or are propagating direct to the ionosphere.
Coherence scale lengths at the ground are of order 1 x 10(3) km, and the pu
lsation frequency often appears to be predicted by the ion-cyclotron mechan
ism in the upstream solar wind.