Lidar Observations of Thermosphere‐Ionospheric Ca ⁺ at Beijing (116.02°E, 40.42°N), China

This study focuses on the statistical analysis of nocturnal thermosphere‐ionosphere Ca + (TICa + ) events observed at Beijing (40.42°N, 116.02°E) using an all‐solid‐state optical parametric oscillator (OPO)/amplifier (OPO/optical parametric amplifier) LiDAR system. A total of 263 nights of data collected from June 2020 to June 2023 were processed, revealing 73 nights with TICa + events. The events primarily occurred in the altitude range of 120–300 km, showing distinct seasonal and diurnal variations. The occurrence rate peaked in summer, especially in July, and TICa + was frequently observed around midnight and prior to dawn. The morphological characteristics of TICa + events were categorized into four types: “M” type, “Dawn” type, “Ca + cloud” type, and “I” type. Vertical transport induced by neutral winds and electric fields was identified as the primary formation mechanism for all types. Strong correlations between TICa + and ionospheric irregularities such as sporadic E (Es) and spread F (SF) were found, suggesting TICa + as an effective tracer for studying ionospheric dynamics. Additionally, the influence of solar and auroral activities on TICa + formation was discussed based on the correlation with the Auroral Electrojet (AE) index.