Excitation of VLF Waves by an Electron Beam Injected into the Ionosphere

We propose a scenario for radiation of electromagnetic whistlers as a result of the injection of a modulated electron beam into the ionosphere. We show that for plasma parameters typical of the ionosphere and electron beam parameters typical of electron guns carried into space aboard rockets or spacecraft, the interaction can result in the instability of electrostatic waves driven by Cherenkov resonance between the waves and the beam. A linear stability analysis was carried out for two limiting cases: a wide and a narrow electron beam; the wave excitation is shown to develop at typical distances of the order of 1 km. Nonlinear evolution of the instability was investigated by a combination of analytical and numerical methods. Electron beam energy is efficiently transformed into the electrostatic mode. The mechanism of electromagnetic mode generation is then dipole radiation from the electron beam, which is strongly bunched as a result of the electrostatic instability. The efficiency of radiation is a strong function of the ratio of the electron beam velocity to electron Alfven velocity (v(ae) = c omega(ce)/omega(pe)); the possibility of increasing the efficiency of radiation is discussed.