| Summary: | Polar Mesosphere Summer Echoes (PMSE), which are regularly observed in summer months atpolar latitudes, are strong radar backscatter from the upper mesosphere. They have been studiedwith ground-based radars whose frequency ranges from VHF to UHF. Recently, peculiar echos,which are interpreted as PMSE, were found to appear in the near range observations of Super DualAuroral Radar Network (SuperDARN). This suggests a possibility that SuperDARN becomes apowerful tool for studying PMSE as a global scale phenomenon. In this paper, we statisticallyanalysed occurrence frequency of the near backscatter echoes using two years measurements ofthe SuperDARN radars in Antarctica and Iceland. Occurrence percentage of the near rangebackscatter is enhanced significantly in summer months. Maximum is located at the summersolstice, where occurrence percentage reaches 80-90 % in Iceland and 70-80 % in Antarctica. Localtime distribution of the echo occurrence has its maximum around local noon and minimum at20LT. These characteristics of the near range echoes are quite similar to those of PMSE at VHFfrequencies, which implies that near range observation of SuperDARN contains huge amount ofPMSE. Interhemispheric difference of the echo occurrence probabilities are also estimated.Occurrence percentage averaged over summer months is 1.5 times larger in the NorthernHemisphere than that in the Southern Hemisphere, suggesting that occurrence of PMSE hasactually an interhemispheric asymmetry. However, degree of interhemispheric asymmetry is muchsmaller than that predicted from the earlier VHF radar observation. departmental bulletin paper
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