A multipoint determination of the propagation velocity of a sudden commencement across the polar ionosphere

We use magnetic field and riometer data, from ground observatories in both the Arctic and Antarctic regions to characterize the high-latitude propagation of a sudden storm commencement (SC) that occurred at; 0901 UT February `21, 1994. (1) high time resolution magnetic field data. from both hemispheres indicate extremely rapid propagation of the initial part of the SC signal at high latitudes. An initial inflection point; was observed in the data front dawn sector stations in both polar caps nearly simultaneously (Delta t