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

Abstract

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 evidence of the full dynamics and links among different ionospheric disturbances in a mid- and low-latitude nighttime region of China.