Magnetospheric chorus: occurrence patterns and normalized frequency

New characteristics of VLF chorus in the outer magnetosphere are reported. The study is based on more than 400 hours of broadband (0.3 to 12.5 kHz) data collected by the Stanford University/Stanford Research Institute VLF experiment on OGO 3 during 1966-67. It is shown that most magnetospheric chorus consists of rising emissions, which are probably generated by gyroresonant electrons slightly off the equator. These emissions are often generated in two bands: an upper band at the critical frequency f/fsub(H) approximately 0.5 where focusing is maximized, and a lower band at a frequency that may perhaps correspond to a peak in the flux of gyro-resonant electrons. Modal interference may contribute to the gap between the two bands. Occasional falling tones are generated on the opposite side of the equator, and travel poleward without crossing the equator. Non-ducted emissions are refracted inward to slightly lower L shells as they propagate earthward, and may be magnetospherically reflected. In principle they can return to the equator at lower L (region C, f/fsub(H) approximately 0.2) and perhaps trigger new emissions. However the returning wave may be highly attenuated by Landau damping and to date has not been observed. Other emissions may become ducted and propagate along field lines to the lower ionosphere. (U.K.)