| Summary: | Conjugate observation of VLF emission was carried out at Kitdalen (69.1°N, 20.3°E geographic, 65.5°N, 105.3°E geomagnetic, L=6.0) in Norway and at Syowa Station (69.0°S, 39.6°E geographic, 66.2°S, 70.5°E geomagnetic, L=6.1) in Antarctica in February 1980,and also at Andφya (69.2°N, 16.0°E geographic, 66.0°N, 102.2°E geomagnetic, L=6.1) in Norway and at Syowa in March 1982. During these campaign periods, periodic VLF emissions were often observed at both stations in opposite hemispheres. A typical periodic emission event observed at both stations was chosen for cross spectral analysis of the emission period. The results show the characteristics of "symmetrical three-phase emission" reported by BRICE (Radio Sci., 69D, 257,1965) as three wave packets, which were symmetrically spaced along the line of force, were echoing in the magnetosphere. Though the geomagnetically conjugate point of Syowa is located about 1800km far from Kitdalen or Andφya, the common nature of the emissions observed at both stations suggests that the periodic emission which originates from the common source in the magnetosphere can spread more than 1800km at the ionospheric level. This observational result is compared with the ray tracing calculation. It is concluded that if the periodic emissions are guided by a field-aligned duct and the duct is terminated at an altitude of 6000km, the periodic emissions can spread as far as the observed distance. departmental bulletin paper
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