Remote Sensing Measurements of the Nighttime D-Region Ionosphere Based on Very Low Frequency Tweek Observations in China

Tweeks, very low frequency (VLF) electromagnetic signals generated by lightning discharges, are a valuable tool for remote sensing of the D-region ionosphere. Propagating within the Earth-ionosphere waveguide, they encode information about the ionosphere D-region electron density and reflection height. This study analyzed nighttime tweek data from Suizhou Station (31.57°N, 113.32°E) collected between 2018 and 2021, estimating equivalent reflection heights and electron densities using the first-order cut-off frequency. The temporal and spatial variations in these parameters were examined across hourly, daily, monthly, and latitudinal scales, offering new insights into D-region dynamics. Nighttime electron densities were found to range from 20 to 32 el/cm³ in the altitude range of 85–95 km. Reflection height and electron density exhibit clear temporal and spatial patterns. Hourly variations are primarily influenced by residual solar radiation, while daily variations are less consistent, reflecting contributions from multiple ionization sources. Monthly trends display strong seasonality, with reflection heights peaking in summer and decreasing during transitional months. Electron density increases steadily from February to August and declines toward the following February. Reflection heights decrease by approximately 2 km from 5°N to 15°N and rise by 3 km toward 45°N, while electron density increases with magnetic latitude, ranging from 21 el/cm³ at low magnetic latitudes to 31 el/cm³ at higher magnetic latitudes. These findings demonstrate the effectiveness of tweeks as a method for nighttime D-region ionospheric remote sensing, providing valuable insights into D-region variation patterns and characteristics. The results hold practical significance for communication, navigation, positioning, and timing applications.