Characteristics of sporadic E layer in Japan based on multi-station detection

The sporadic E (Es) layer exists at altitudes ranging from 90 to 150 km in Earth's ionosphere with electron densities exceeding 100 times the background levels of that region. This phenomenon enables long-distance HF/VHF radio communications through reflection, refraction, and scattering of radio waves. For decades, extraordinary VHF signal propagation via Es layers has been observed in aviation, maritime, and amateur radio applications. However, systematic Es-layer-based VHF communication systems remain undeveloped, primarily due to insufficient understanding of Es layer occurrence patterns and inadequate knowledge of their spatiotemporal distribution and structural characteristics. This knowledge gap highlights the urgent need for comprehensive ionospheric Es layer detection and spatiotemporal distribution studies. Japan's geographical location in a global Es layer hotspot makes it an exceptionally representative region for such investigations. In-depth research on Es layer characteristics in Japan is crucial for understanding its formation mechanisms, revealing atmosphere-ionosphere coupling processes, and developing reliable Es-based emergency communication technologies. In this paper, the intensity, spatial distribution, diurnal variation and seasonal variation characteristics of the Es layer over Japan are investigated based on the observation data from five ionosonde stations in Japan over the past 40 years, ultimately mapping their spatiotemporal patterns. The findings reveal that the intensity centers of the Es layers over Japan migrate with diurnal and seasonal cycles, displaying strong seasonal and diurnal asymmetries in intensity distribution. These patterns may originate from Japan's complex topography, land-sea configuration, and ocean current systems.