Investigation into the earliest generation of medium-scale traveling ionospheric disturbances with electrodynamic signatures using dense GNSS observations over Japan

Abstract Understanding the earliest formation stages of medium-scale traveling ionospheric disturbances (MSTIDs) with electrodynamic signatures (commonly referred to as nighttime MSTIDs) is essential for comprehending their full development. To analyze the initiation of these disturbances, we utilize dense ground-based Global Navigation Satellite System (GNSS) observations over Japan, supplemented by data from ionosonde, high-frequency Doppler soundings, and the Ground-to-topside model of Atmosphere and Ionosphere for Aeronomy (GAIA). A statistical analysis from June to August in 2022 and 2023 above the Kokubunji ionosonde revealed an intensification of MSTID activity at night. However, several cases indicate that the earliest formation of these disturbances occurs before E-region (100-km) sunset, challenging the conventional view that nighttime MSTIDs develop only after F-region sunset. To further our understanding of this phenomenon, three representative cases were analyzed. Case studies and statistical results suggest that Es layers play a crucial role in the generation of MSTIDs with electrodynamic signatures. GAIA model calculations indicate that the earliest occurrence of MSTIDs with electrodynamic signatures is primarily associated with the decrease in E-region Pedersen conductance and its reduction to a level comparable to that in the F-region. The presence of Es layers, along with electric field generation in the F-region, is the necessary and insufficient conditions for the generation of MSTIDs with electrodynamic signatures. Graphical abstract