ELECTROMAGNETIC ION-CYCLOTRON WAVES OBSERVED IN THE PLASMA DEPLETION LAYER

Observations from AMPTE/CCE in the Earth's magnetosheath on October 5, 1984 are presented to illustrate 0.1 - 4.0 Hz magnetic field pulsations in the subsolar plasma depletion layer (PDL) for northward sheath field during a magnetospheric compression. The PDL is unambiguously identified by comparing CCE data with data from IRM in the upstream solar wind. Pulsations in the PDL are dominated by transverse waves with F/F(H+) less-than-or-equal-to 1.0 and a slot in spectral power at F/F(H+) = 0.5. The upper branch is left hand polarized while the lower branch is linearly polarized. In the sheath the proton temperature anisotropy, A = T perpendicular-to/T parallel-to - 1, is almost-equal-to 0.6 but it is almost-equal-to 1.7 in the PDL during wave occurrence. The properties and correlation of waves with increased anisotropy indicate that they are electromagnetic ion cyclotron waves.