Variation Characteristics of the Ionospheric E Layer over the Tibetan Plateau and Surrounding Areas During a Full Solar Cycle

The ionospheric E layer (90–150 km altitude) significantly influences ionospheric dynamics and plays a crucial role in radio wave propagation. The Tibetan Plateau, as the “Third Pole,” affects E-layer morphology due to its unique topographical factors. Given the limited systematic studies in this high-altitude region, this study analyzes E-layer spatiotemporal characteristics and their controlling mechanisms over the Tibetan Plateau and adjacent regions. We analyzed foE (critical frequency of E-layer) data from six ionospheric observation stations across the Tibetan Plateau and neighboring areas during 2013–2023, covering a complete solar cycle from solar minimum to maximum. Combined with sunspot numbers as solar activity indicators, we systematically examined diurnal, seasonal, and solar cycle variations to understand regional E-layer behavior patterns. Daytime foE values significantly exceed nighttime values, demonstrating strong solar control. Spatially, Kunming shows the strongest daytime E-layer intensity with peak values reaching 3.12 MHz, while Urumqi exhibits the weakest at 2.94 MHz. Daytime foE values decrease with increasing latitude, whereas nighttime values show opposite latitudinal trends, indicating pronounced diurnal distribution asymmetry. Kunming displays the largest day-night foE variation amplitude, while Urumqi shows the smallest changes. Notably, most stations exhibit E-layer intensity peaks in July rather than June when solar zenith angles are minimum, differing from typical mid-low latitude seasonal behavior. These patterns may be related to complex vertical atmospheric coupling influenced by the region’s unique topography, which could affect the spatiotemporal distribution of the E-layer over the Tibetan Plateau.