Statistical Analysis of Ionospheric Midnight Collapse Events Observed by Arecibo Incoherent Scatter Radar

This study presents a comprehensive statistical analysis of ionospheric midnight collapse events over Arecibo, based on incoherent scatter radar (ISR) observations collected between 1971 and 2019. A total of 224 nights with valid measurements were examined to characterize the timing, intensity, and seasonal variation of these collapse events. The results showed that midnight collapses occurred on 94.6% of the nights, with the highest occurrence rate observed during spring and winter. The first collapse typically began between 22:00 and 00:00 LT, lasted for 1–4 h, initiated at altitudes between 350 and 400 km, and involved a vertical collapse of 50–100 km. A second collapse was identified on 18.8% of nights, occurring predominantly between 01:00 and 02:00 LT, with a notably higher frequency during winter. Compared to the first collapse, the second collapse tended to originate at lower altitudes and exhibited faster collapse rates. Seasonal patterns in the vertical ion drift (Vz) were also identified, with winter events characterized by a persistently downward Vz throughout the night. Further decomposition of Vz into field-aligned (Vap) and perpendicular (Vpn) components indicated that Vap played a dominant role in modulating Vz, particularly on nights with double collapses. Analysis of meridional wind variations revealed that nighttime changes in Vap were largely controlled by meridional wind, suggesting a strong coupling between thermospheric wind dynamics and field-aligned ion motion. These findings suggest that variations in Vz, primarily driven by meridional-wind-controlled changes in Vap, are a key driver of ionospheric midnight collapse events at Arecibo.