Seasonal variations of over-horizon propagation characteristics of FM broadcasting waves

We have observed over-horizon FM broadcasting waves at Niijima for the purpose of clarifying some relationships between earthquake activities and the propagations of over-horizon FM waves. We have monitored 81.3MHz, 79.2MHz and 82.1MHz FM broadcasting waves, from Tokyo, Shizuoka and Hamamatsu, respectively, at Niijima. The period of the observation was for 3 years from January 2005 to December 2007. From observational results, it was found that the received levels of three FM broadcasting waves had seasonal variations in their fluctuations; the fluctuation levels in summer season were larger than those in winter. And the fluctuation levels were positively correlated with the temperature and the water vapor pressure, and were negatively correlated with the atmospheric pressure and the wind speed. In the observation period, one big earthquake with seismic intensity of 4 and three earthquakes with seismic intensity of 3 were observed near the propagation path of the FM broadcasting from Tokyo. By comparing the fluctuation levels of FM waves with the seismic intensities, it was found that there was little correlation between them. It is reasonable to consider that the received level fluctuations are mainly caused by meteorological conditions not by earthquakes. It was concluded that affections of meteorological conditions should be considered to observe seismic activities through over-horizon FM broadcasting waves.