Pc1 pulsations observed by AMPTE/CCE in the Earth's outer magnetosphere

Electromagnetic ion cyclotron waves in the Pc1-2 frequency range (0.1-5 Hz) have been observed in Earth's equatorial magnetosphere over the range L=3 to 9 by the AMPTE/CCE magnetic fields experiment. The authors report detailed analysis of data from two orbits together with preliminary occurrences statistics from 31 days. One example occurred during disturbed geomagnetic conditions (maximum AE>1,000 nT) and exhibited intense Pc1 from L=5 to the magnetopause (L{approx}7.5). The second example occurred during moderate to quiet conditions (average AE{approx}300 nT) and displayed Pc1 in two distinct L ranges: low (L{approx}4) and high (L>7.5). Statistical results show that Pc1 are {approx}4 times more common for L>7 than for L<6 and occur primarily in the afternoon. The discovery of a statistically dominant outer magnetospheric source region implies that ring current interaction with the plasmasphere is not the primary source of Pc1 excitation but suggests that plasma sheet ions may be a significant energy source for Pc1. This result is consistent with the strong dependence of the linear convective growth rate on magnetic field strength.