Solar cycle dependence, seasonal and daily variations, and weekend effect deduced from long-term monitoring of ELF/VLF emissions at Syowa-Iceland geomagnetic conjugate stations

Abstract

Long-term monitoring of natural radio wave emissions in the ELF/VLF range observed at Syowa Station, Antarctica (L=6.1) and its geomagnetic conjugate point in Iceland showed dependence on solar activities for solar cycles 21 and 22. It is also found that emissions were stronger in the summer hemisphere. Moreover, the daytime maximum of the emission appeared at local noon rather than magnetic noon. These two features indicate that the emissions observed on the ground intensified under the sunlit ionosphere. We also found a weekend decrease (-8%) of the emission intensity after 7 years of data average, suggesting that electromagnetic noises in the human world calm down on weekends, and this affected the efficiency of emission generation in the magnetosphere.