Direction‐finding measurements of magnetospheric vlf chorus emissions and analysis of their generation and propagation mechanism

Abstract Chorus is one of the most important VLF/ELF emissions in the magnetosphere, but several points are still unsolved concerning their generation and propagation mechanisms. In this paper, the near‐equatorial direction‐finding measurements made on board a satellite are studied first and it is confirmed that the rising tone chorus (frequency of the chorus increases with time) is generated at θ (the angle between the wave normal and earth's magnetic field) ˜ 0° by the cyclotron resonance interaction with the energetic electrons. These results support the previous theory of Helliwell. Moreover, the direction‐finding measurements of the rising tone chorus are carried out aboard the GEOS 1 satellite in the off‐equatorial regions. Their generation and propagation mechanism are studied with the help of three‐dimensional ray‐tracing computations and these results are compared with the corresponding results of OGO 5 direction‐finding measurements by Burton et al. As a result, the observations by OGO 5 indicate that after the generation of chorus at the equator with wave‐normal direction θ0 ˜ 0°, it reaches the satellite in ducted propagation along the magnetic field line, whereas the results of GEOS 1 suggest that the waves are generated in a wider region near the equator with comparatively smaller θ0 (30° ˜ 40°, at the most 50°) and that the subsequent propagation is nonducted in the magnetosphere. Finally, the direction finding measurements and the ray‐tracing computation are suggested to be very important in the study of generation and propagation of various emissions generated in the magnetosphere.