ELECTROMAGNETIC ION-CYCLOTRON WAVES OBSERVED IN THE NEAR-EARTH PLASMA SHEET BOUNDARY-LAYER

Enhanced magnetic field fluctuations observed during an ISEE-2 crossing of the plasma sheet boundary layer at X(GSE) = 18.2 Re have the characteristics of electromagnetic ion cyclotron waves. The observed fluctuations propagate essentially along the magnetic field and are left-hand circularly polarized in the spacecraft frame, with frequencies below the proton cyclotron frequency (Omega(p)) and peak power at about 0.2 Omega(p). Plasma data indicate that these waves are associated with decreasing plasma beta and occur in the presence of an anisotropic earthward directed proton beam with velocity less than the local Alfven speed. Linear Vlasov theory indicates that these waves are most likely excited by the electromagnetic ion cyclotron instability driven by the temperature anisotropy of the proton beam.