Morphology of VLF emissions observed with the Injun 3 satellite

Results of a study of very low frequency (VLF) emissions observed with the Injun 3 satellite are presented. Approximately 1200 hours of VLF magnetic field strength data and approximately 6000 frequency spectra samples are investigated in this study. These data cover invariant latitudes up to 82°, all local times, and altitudes from 237 to 2785 km. Contour plots as a function of invariant latitude and magnetic local time giving the frequency of occurrence of VLF emissions above a given intensity are presented. The most intense VLF emissions observed by Injun 3 are found to occur between about 55 and 75° invariant latitude and during the local day, with the maximum intensity occurring at about 65° invariant latitude and about 8 to 10 hours magnetic local time. The region of most intense VLF emissions was found to move to lower latitudes during geomagnetically active periods. The principal types of VLF emissions occurring in this region are ELF hiss and chorus, with the ELF hiss usually being the most intense.