Day-To-Day Variability of Ionospheric Electron Temperature Revealed by High-Resolution SYISR Observations During April–May 2022

This study investigated the day-to-day variability of ionospheric electron temperature (T_e) over Sanya using the newly built Sanya Incoherent Scatter Radar (SYISR; 18.3°N, 109.6°E, dip latitude: 12.8°N). The 24 days of observations with transmitted beams encoded by the alternating code and directed toward the zenith provided T_e profiles during April–May 2022. The spatial-temporal resolution was as precise as 4.5 km and ∼120 s. The day-to-day variability of T_e over Sanya thus revealed, which was strong in the sunrise T_e peak region (above ∼260 km, duration increases from ∼0.5 to ∼3.5 hr with altitude) and the top of the 200–300 km T_e bulge (∼260 to ∼300 km, lasting from ∼09:00 LT to ∼18:30 LT). The day-to-day variability of T_e in the two regions could reach ∼26% and ∼16% of the average T_e, respectively. Although the day-to-day variability of T_e is negatively correlated with the variation of ionospheric electron densities (N_e) in general, a positive correlation still occurred during the daytime, especially at 400–480 km from ∼17:00 LT to ∼18:00 LT. Further Thermosphere Ionosphere Electrodynamics General Circulation Model simulations reproduced the positive correlation, which may result from the internal heat exchange between electrons, ions, and neutrals. The simulations suggested that the variation of solar radiation/geomagnetic activity contributes most to the daytime/sunrise day-to-day variability of T_e over Sanya.