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 E - and F -region field-aligned irregularities (FAIs), nighttime medium-scale traveling ionospheric disturbances (MSTIDs), and a sporadic E ( E_{\text {S}} )-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 E_{\text {S}} -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 E_{\text {S}} -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 mid- and 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 \times B effect and subsequently can evolve to spread-F like FAIs. Our observational investigation provided the first eviden