Evidence of Mid- and Low-Latitude Nighttime Ionospheric $E$ – $F$ Coupling: Coordinated Observations of Sporadic $E$ Layers, $F$ -Region Field-Aligned Irregularities, and Medium-Scale Traveling Ionospheric Disturbances

We present the observational evidence of Eand Fregion field-aligned irregularities (FAIs), nighttime medium-scale traveling ionospheric disturbances (MSTIDs), and a sporadic E (ES)-layer using the Wuhan very-high-frequency (VHF) coherent scatter radar, Wuhan Global Navigation Satellite System (GNSS) network, and Wuhan ionosonde. We observed simultaneously E and F FAIs by VHF radar and the ES-layer and spread-F by Wuhan ionosonde. We also observed MSTIDs using the Wuhan GNSS network in a southwestward direction of propagation and horizontal propagation velocity of less than 180 m/s, for a period of ~33 min. Simultaneous observations of an ES-layer and FAIs in the E-region and FAIs in the F-region suggested the existence of an electrodynamic coupling between the E and F regions in the midand low-latitude nighttime ionosphere. A polarized electric field associated with nighttime MSTIDs can generate the uplift movements of the F-region's electron density. This uplift effect consequently can excite the gradient drift instability (GDI) with an accompanying enhanced vertical gradient of the F-layer's electron density. Our results indicated that the F-region MSTIDs excited by Perkins instability might be further modulated by the E × B effect and subsequently can evolve to spread-F like FAIs. Our observational investigation provided the first evidence of the full dynamics and links among different ionospheric disturbances in a midand low-latitude nighttime region of China.