Theoretical Estimation and Observation of the Wave Field Intensity on the Ground Radiated by an Electron Beam Antenna in the Ionosphere

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It has been reported recently that an electron beam modulated at VLF frequencies operates with high efficiency as a VLF transmitter in the ionosphere. The Institute of Space and Astronautical Science (ISAS) of Japan is planning a project known as Space Experiments with Particle Accelerations (SEPAC) on the US Space Shuttle in the near future. As one of many scientific experiments in SEPAC, VLF wave observations will be carried out on the ground to determine the efficiency of the electron beam antenna. The electron beam is emitted along Earth's magnetic field lines from space shuttle.This research project was to support the modulated electron beam experiments from the theoretical and observational View points listed below :(1) Calculation of the wave intensity on the ground using a horizontally stratified model of the ionosphere and assuming the electron beam is effectively a dipole transmitter of VLF waves.(2) Observation on the ground of the noise level at frequencies up to 10 kHz to determine the detection threshold of VLF waves emitted from the electron beam.The results are : The electromagnetic wave field intensities on the ground are estimated theoretically to be 3uV/m to 10 uV/m in a circular region with a radius of 200 km, under the assumptions that the beam current is 10 mA, the beam length is 100 km and the beam is modulated at a frequency of 5 kHz. On the other hand, the noise level at the frequency of 5 kHz was observed using a small loop antenna located on the ground. The intensity of the noise was in the range of 0.1uV/m to 3 uV/m. Therefore, it seems plausible that VLF waves can be detected during a SEPAC-like experiment.Ten papers related to this research project will be published in term of the project.