Propagation of 10-50 mHz ULF waves with high spatial coherence at high latitudes

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 activity at high latitudes. Nine of eleven events did not show evidence of field line resonance harmonics. Instead, the observed amplitudes decreased equatorward, and phase relationships indicate poleward propagation. The results suggest that the pulsations are due to incoming fast mode waves driving forced field line oscillations or are propagating direct to the ionosphere. Coherence scale lengths at the ground are of order 1×10³ km, and the pulsation frequency often appears to be predicted by the ion-cyclotron mechanism in the upstream solar wind.