Case Studies of Nighttime Medium‐Scale Traveling Ionospheric Disturbances at Mid‐Latitudes: Contribution of the Sporadic E Layer to Their Development

Abstract Under the postulation of E–F coupling in generating nighttime medium‐scale traveling ionospheric disturbances (MSTIDs), the relative contributions of the E‐ and F‐regions remain poorly understood. This study investigates the generation mechanisms of nighttime MSTIDs at mid‐latitudes using dense Global Navigation Satellite System (GNSS) total electron content (TEC) data over Japan during three representative events in 2022 summer, supplemented by ionosonde and Fabry–Perot interferometer (FPI) measurements, as well as simulation outputs from the Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). FPI observations reveal a transition of thermospheric winds from southeastward to southwestward after midnight. According to Perkins instability theory, such southwestward winds should suppress instability growth. However, the TEC observations show that post‐midnight MSTID amplitudes are enhanced, accompanied by intensification of the Es layer observed by the ionosonde. Numerical calculations indicate a positive correlation between post‐midnight MSTID amplitudes and the growth rate of Es‐layer instability. These results suggest that Es layers dominate the development of nighttime MSTIDs. Importantly, MSTIDs can develop through Es‐layer instability without the Perkins instability.