Study of structures of the sporadic E layer by using dense GNSS network observations

The sporadic E (Es) layer is an ionospheric layer with a high electron density and thin altitudinal thickness and appears at altitudes around 100 km. The density is sometimes so high that even VHF radio waves from the ground can be reflected and propagate beyond the radio horizon. It has been shown that aeronautical VHF navigation signals in the 108–118 MHz band can propagate beyond the radio horizon to cause potential interference. However, there has been little technique to monitor the Es layer in wide area. The fine structures of the Es layers which reflects the dynamics of the thermosphere has not been well understood yet. This study shows that the ROTI (rate of TEC index) map by utilizing dense GNSS observations is effective in detecting the Es layer at least in the daytime. The Es layer studies has a well-defined frontal structure, when ROTI values are mapped at the typical Es layer height (100 km). A methodology of detecting and characterizing the Es layer frontal structure without manual operation is developed by utilizing the Hough transformation, and the front direction and velocity are successfully derived. Utilizing the characteristics of the frontal structure, fine structures of the Es layer can be studied, and a couple of interesting features are found. However, this technique would not always be applicable. Application to a homogeneous Es layer and nighttime Es layer are challenging problems. Nevertheless, it is still very useful, and is suitable for automated real-time Es layer monitoring system in a wide area.