Latitudinal variation of chorus frequency observed by the ISIS satellites

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Latitudinal variations of chorus band frequency have been investigated to examine a model of the magnetospheric chorus obtained from OGO-1 and OGO-3 VLF observation by using VLF electric field data (50Hz-30kHz) from ISIS-1 and ISIS-2 received at Syowa Station, Antarctica in 1977 and 1978. The upper and lower limit frequencies of the dayside chorus decrease with L-value (or invariant latitude) and their latitudinal variations roughly agree with a latitudinal variation of one half of the equatorial gyrofrequency. This result supports the model of dayside magnetospheric chorus in which the chorus generated near the equatorial plane propagates along the field lines in the ducted mode or propagates inwards from the original field line in the nonducted mode. Nightside choruses were observed in disturbed periods by ISIS satellites as well as the nightside magnetospheric chorus. The upper and lower limit frequencies of choruses observed in the early morning, late evening and nighttime do not show any significant variation with L-value (invariant latitude). This may be due to different generation mechanism of the nightside chorus from the dayside one. Furthermore, we have found a new case that chorus band frequencies in latitudes beyond the plasmapause are higher than those in latitudes inside the plasmapause. Such a case has never been reported for observations of the magnetospheric chorus.
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