A Symmetric Appearance of Pc5 on Dawn and Dusk Sides Associated with Solar Wind Pressure Enhancement

Simultaneous occurrence of Pc5 geomagnetic pulsations is studied using high time resolution geomagnetic data (sampled every 10 see) from the Scandinavian IMS Magnetometer Array and the North American IMS Magnetometer Network. Although the pulsation activity is longitudinally localized in most cases, an event with exceptionally coherent and similar wave form on both dawn and dusk sides was observed from 1500 to 1600 UT, on Sep. 27, 1978. The coherent oscillation was also observed at low latitude stations. We analyze this event further in detail by adding data from the ISEE-3 satellite in the interplanetary space, and from the geostationary satellites, GOES-2 and -3. These data show that this event occurred under such a peculiar solar wind condition as stable northward, anti-Parker IMF direction, and high solar wind speed (∼620 km/sec). The correspondence between the variation on the ground and that of dynamic pressure of the solar wind is found. The most likely source of this event is the magnetospheric cavity resonance generated by increases of the dynamic pressure of the solar wind.