Seasonal dependence of the vertical distributions of auroral kilometric radiation sources and auroral particle acceleration regions observed by the Akebono satellite

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Seasonal variations in the vertical distributions of occurrence probabilities of auroral kilometnc radiation (AKR) sources and auroral acceleration regions indicated by upward-flowing ion (UFI) events were compared based on statistical analyses of plasma waves and energetic particles data observed by the Akebono satellite. The peak altitude in the vertical distribution of occurrence probability of AKR sources whose emission intensities were larger than -150dBW/m^2-Hz occurred at 5000-6000 km in the summer polar region and at 3000-4000 km in the winter polar region. The analyses have also clarified that the vertical distributions of occurrence probabilities of auroral acceleration regions also show seasonal variations that are quite similar to those of the AKR sources. Based on the observation that intense AKR emissions can be generated even in high-density plasma, we suggest that processes other than cyclotron maser instability (CMI), such as mode conversions, may be dominant under conditions of dense ambient plasma in the summer polar regions. The contri-bution of the emissions generated by these mechanisms to the seasonal variations in the vertical distributions of AKR sources is thought to be significant.
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