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

Abstract

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 from dawn sector stations in both polar caps nearly simultaneously (Δt <2 s) but ∼3 s earlier in the southern hemisphere. (2) Data from the Magnetometer Array for Cusp and Cleft Studies (MACCS) in Arctic Canada, with stations from 0130 to 0600 magnetic local time, indicate dispersive propagation of the preliminary impulse (PI) at speeds decreasing from ∼150 to ∼50 km/s, directly away from a source near or slightly poleward of the cusp, rather than along the auroral oval. (3) Plots of two‐dimensional, 20‐s resolution equivalent convection vectors in the Northern Hemisphere reveal the imposition and rapid propagation and decay of a short‐lived large‐scale flow pattern opposite to the normal dawn sector flow, followed by an intensification of the original pattern. However, the perturbed flows were not dominated by the localized, tailward propagating vortices predicted in some models of SC events. In both hemispheres, observations of the PI are consistent with the temporary imposition of a field‐aligned current pair antisymmetric about local noon in the polar ionosphere and with horizontal fast mode propagation of a pulse through the ionosphere. (4) Riometer signatures do not match the magnetic variations in either time or intensity, and they propagate with a much lower velocity (∼1 km/s). We infer that the initial magnetic signatures are generated by the arrival of Alfvén waves and associated Birkeland currents at the near‐cusp ionosphere, while the riometer signatures at these high latitudes are generated predominantly by interplanetary particles associated with the causative coronal mass ejection.