Diurnal and Seasonal Variations in Trends in the E-Layer Critical Frequency

Abstract

Long-term changes in the E-layer critical frequency foE at three stations in the European region (Juliusruh, Slough, and Rome) are analyzed by the method described in detail in the previous paper by the authors. It is found that two former stations demonstrate a well-pronounced change in foE (a trend) during the two previous decades. At the same time, the same features of the behavior of the aforementioned trend k(foE) are obtained for both stations. The trend is positive and negative in the morning and evening hours, respectively. It is minimal near the local noon. That explains the small value of k(foE) obtained in the previous paper for 1200 LT. A well-pronounced seasonal behavior of k(foE) is detected: the trend is minimal and maximal in the summer period and at the end of fall—beginning of winter, respectively. The trend maximal amplitude in the morning hours reaches +0.04 MHz per year, whereas the minimal amplitude in evening hours is–0.06 MHz per year. No systematic changes in foE exceeding 0.01 MHz in magnitude per year are found for Rome station.