Abstract
Narrow-banded ELF emissions in the frequency band of 100–500 Hz have often been observed in the VLF data received at Syowa Station ( L = 6.1) in Antarctica from ISIS 1 and 2. The occurrence distributions of narrow-banded ELF emissions over the southern polar region have been studied statistically. It is found that emissions mostly occurred in the evening 17–24 M.L.T. sector and in the region of 65°–75° invariant latitude associated with V-shaped VLF hiss emissions. The emissions are correlated with disturbed geomagnetic activity and the occurrence region shifts to lower latitude in association with increased geomagnetic activity. It is also found that emissions maximize during austral winter season, from April to July. The emission frequencies are strongly dependent on the satellite's altitude. These results suggest that narrow-banded ELF emissions are excited in the topside ionosphere by upflowing ion or downflowing electron fluxes and that the growth rate strongly depends on the ambient electron density. We also discuss the relation between the emissions observed by the satellite and on the ground.
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