Chorus is one of the most important VLFELF 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 8 (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 I 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 O W 5 indicate
that after the generation of chorus at the equator with 73 wave-normal direction 0, -O", it reaches the satellite in ducted propagation along the magnetic field line, whereas the results of GEOS I suggest that the waves are generated in a wider region near the equator with comparatively smaller 8,, (30" -40", at the most SO") 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.