A dominant feature of high-latitude magnetic pulsations is large-amplitude irregular pulsations (Pi) which are closely correlated with the movement of the observing station under particle precipitation, producing the dayside aurora and the high-latitude expansion of nightside aurora. The dayside Pi-1 pulsation maximum centered about local magnetic noon has no strong seasonal dependence, indicating that the dayside aurora illuminates both hemispheres independent of the latitude of the subsolar point. The summer noon pulsation maximum has, however, a greater longitudinal extent than the winter noon maximum, as measured at 74¿--75¿ invariant latitude. The nightside magnetic pulsations are bursts of Pi (PiB) having an average duration of 15 min. From Defense Meteorological Satellite Program photos the auroral forms related to the high-latitude PiB can be identified as the poleward discrete arc generally having a large longitudinal extent.

If the auroral forms are very similar in both hemispheres, then the large longitudinal extent coupled with movement of the aurora could explain why 85% of the PiB events have onsets within 10 min at opposite hemisphere sites (South Pole, Antarctica, and Sondre Stromfjord, Greenland) separated in local magnetic time by about 1.5 hours. There is no seasonal dependence in the statistical occurrence of PiB, nor in its simultaneity in opposite hemispheres. Apparently, the seasonal distortion of the tail plasma sheet has little effect on the acceleration of high-latitude auroral beams. The actual several minute time difference in opposite hemisphere onsets of PiB is probably due to the westward/poleward motion of the longitudinally extended aurora. Assuming that the nonsimultaneity (within 10 min) of opposite hemisphere PiB is because one of the sites is too far removed from magnetic midnight because of the nearly 2-hour site separation, one can infer that the typical longitudinal extent of the poleward arc and/or the longitudinal extent of travel of the westward surge is from 2 hours after midnight to 4 hours before. The PiB having a sudden onset are likely the magnetic manifestation of the stepwise motion of the westward electroject current wedge poleward and/or westward. ¿ American Geophysical Union 1987