THE SPATIAL EXTENT OF RADIAL MAGNETIC PULSATION EVENTS OBSERVED IN THE DAYSIDE NEAR SYNCHRONOUS ORBIT

We have used simultaneous observations from the AMPTE CCE satellite, in an elliptical orbit with apogee at 8.8 R(E), and GOES 5 and GOES 6, in a geostationary orbit at 6.6 R(E), to investigate the radial and longitudinal extent of magnetic pulsation events with predominantly radial polarization. Twenty-one events were selected by visual inspection of color-coded Fourier spectrograms produced from data from all three satellites during a several month interval in fall 1984 when the apogee of AMPTE CCE was on the dayside; sixteen events were observed at all three satellites. Local time of the observed events ranged from 0900 to 1900 MLT, but the apparent longitudinal extent of the oscillation region varied considerably from event to event, ranging from the minimum resolution of 1.5 hours MLT (the local time separation of GOES 5 and GOES 6) to 8 hours MLT. Plasma wave data from AMPTE CCE indicated the waves occurred in regions of density characteristic of the outer plasmasphere (approximately 10 cm-3) and quite far outside the L shell region where densities reached 400 CM-3. These events occurred during magnetically quiet times usually after magnetic storms; interplanetary magnetic field data, when available, indicated an either roughly radial or northward orientation during the events. Wave onset often (but not always) occurred within one hour after sharp drops in the AE index to values of 100 or below. There was no apparent correlation of wave onset or amplitude with plasma beta, which ranged from 0.23 to 1.09 during the nine events presented here. Our frequent observation of the simultaneous onset of waves at different local times with considerably different frequencies reinforces the belief that the onset of these pulsations is determined by an instability that covers some longitudinal extent but that the frequencies are determined by local Alfven resonance conditions, not by the bandwidth of an external source. The data suggest that local plasma density increases associated with plasmaspheric refilling are the immediate cause of local instabilities leading to wave onset; the increase in density may alter the field line resonance conditions to allow the free energy of approximately 100-keV trapped ions to drive waves via the drift-Alfven-ballooning-mirror mode instability.