EXCITATION OF ELECTRON-ACOUSTIC WAVES BY A GYRATING ELECTRON-BEAM

The excitation of electron acoustic waves by a gyrating electron beam in a plasma with cold and hot electron components has been examined. D ifferent instabilities are possible due to Cerenkov, normal, and anoma lous Doppler resonances. The Cerenkov and anomalous Doppler resonances lead to the excitation of intermediate- and short-wavelength parts of the electron cyclotron sound dispersion branch with frequencies well above the electron cyclotron frequency but below the electron plasma f requency, i.e., Omega(e) << omega < omega(pe). The instability due to the normal Doppler resonance leads to the excitation of intermediate- and short-wavelength parts of this branch as well. In addition, due to the normal Doppler resonance, waves with much larger wavelengths and frequencies omega greater than or similar to Omega(e) can also be exci ted. These results are applied to active beam injection in the Earth's low-altitude ionosphere during the CHARGE 2B rocket experiment, but a lso have applications to other regions of the magnetosphere where wave s are observed in the frequency range between the electron gyrofrequen cy and electron plasma frequency.