Abstract
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. We report detailed analysis of data from two orbits together with preliminary occurrence statistics from 31 days. One example occurred during disturbed geomagnetic conditions (maximum AE>1000 nT) and exhibited intense Pc1 from L=5 to the magnetopause (L≈7.5). The second example occurred during moderate to quiet conditions (average AE≈300 nT) and displayed Pc1 in two distinct L ranges: low (L≈4) and high (L>7,5). Statistical results show that Pc1 are ≈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 Pcl 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.
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