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. Plain Language Summary: This study uses "tweek" signals—very low frequency (VLF) electromagnetic waves generated by lightning—to study the D‐region of the ionosphere, a crucial layer that influences radio signal transmission. By analyzing 4 years of nighttime data (2018–2021) collected in China using a VLF receiver developed by Wuhan University, we estimate key parameters like reflection height and electron density in the D‐region. These parameters offer deep insights into the D‐region behavior, which is typically challenging to observe. The study reveals that the electron density ranges from 20 to 32 el/cm3 at altitudes of 85–95 km, with distinct patterns based on time of day, season, and magnetic latitude. Importantly, the study significantly enhances our understanding of the nighttime D‐region dynamics in mid‐ and low‐latitude regions of China, where long‐term observational data has been scarce. The findings highlight the effectiveness and importance of nighttime VLF remote sensing for studying D‐region evolution, providing valuable information for applications like communication, navigation, and timing systems, where ionospheric conditions can affect signal quality. Key Points: The tweek dataset from 2018 to 2021 has been obtained based on the very low frequency measurement in Suizhou, ChinaWe estimated the D‐region ionosphere heights and electron densities based on the tweek datasetThe reflection heights and electron densities of tweeks showed significant seasonal and latitudinal variations [ABSTRACT FROM AUTHOR]