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. Different instabilities are possible due to Cerenkov, normal, and anomalous 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 frequency, 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 excited. These results are applied to active beam injection in the Earth's low-altitude ionosphere during the CHARGE 2B rocket experiment, but also have applications to other regions of the magnetosphere where waves are observed in the frequency range between the electron gyrofrequency and electron plasma frequency.