GROUND-SATELLITE CORRELATION OF LOW-LATITUDE PI-2 PULSATIONS - A QUASI-PERIODIC FIELD LINE OSCILLATION IN THE MAGNETOSPHERE

Magnetometer data at dip-equator and at geosynchronous altitudes (GOES 5 and 6 satellites) and energetic particle data from geosynchronous s pacecraft (1982-019, 1984-037, 1984-129) are studied to investigate fi eld line and particle signatures associated with ground Pi 2 onset. Pi 2 pulsations observed at the ground station are discussed as field di sturbances that are launched in the midnight sector of the magnetosphe re in association with field reconfiguration at substorm onset. In the geosynchronous orbit, both quasi-periodic field line oscillations and particle injections are often observed in the nightside sector in ass ociation with the onset of ground Pi 2 pulsations. Of 216 Pi 2 events selected in this report, 16% showed associated quasi-periodic field li ne oscillations. The dominant periods of the oscillations in space are similar to but not equal to those of the corresponding ground Pi 2 pu lsations. The oscillations in space are polarized primarily in the azi muthal direction with a de offset that develops to form a field-aligne d current signature, namely, eastward (westward) directed in the premi dnight (postmidnight) sector. Simulation of forced field line oscillat ions has been carried out by adding the sudden increase in plasma pres sure, in the equatorial midnight sector region that is expected to com e from particle injection at substorm expansion onset. We discuss the relevance of the quasi-periodic oscillations to plasma injections at g eosynchronous altitudes with reference to the result of the model calc ulations.