Observations and theoretical analysis on the night-time mid-latitude ionospheric irregularities support the postulation of frequently coupled E and F regions. In this paper, we attempt at asserting this notion while using total electron content (TEC) measurements. The TECs are from a dense GNSS receiver network over Japan with more than 1200 stations and a mean distance of ~ 25 km between receivers; thus, ideal for analyzing small-scale perturbations in ionospheric electron density. We take an ansatz that mid-latitude night-time plasma instabilities concentrate at E and F layers. Then the integrated three-dimensional density perturbations are parameterized with a double-thin-shell model. At each shell, perturbation components are assumed identical at any point within a given grid block. Two days with events of night-time medium-scale traveling ionospheric disturbances (MSTIDs), but with different amplitudes, were investigated. Results show that the newly developed technique can infer several horizontal characteristics on E–F coupled instabilities; the coexistence of northwest–southeast (NW–SE) aligned irregular structures in E and F regions is evident. Both E- and F-region irregularities share similar propagation parameters, a shred of clear evidence of strong coupling. Graphical Abstract

The plasma density in the ionospheric F region keeps fluctuating in various temporal and spatial scales. The quasi-wavelike ionospheric disturbances are called as traveling ionospheric disturbances (TIDs), which can be further divided into small-scale TIDs (SSTIDs) (Yin et al., 2019), medium-scale TIDs (MSTIDs) and large-scale TIDs (LSTIDs) (Georges, 1968) by their scales. In terms of MSTIDs, since Sydney Radio Research Board first gave the experimental existence proof (Munro, 1950), observation methods for MSTIDs have been varying all the time (Evans et al., 1983;Jacobson et al., 1995) but the most common methods have been two-dimensional observation since 1990s due to the large observable range. One two-dimensional observation method, High Frequency (HF) radar (Samson et al., 1990), is mostly used to observe daytime MSTIDs. Another method, all-sky airglow imager was also proposed (Mendillo et al., 1997), but it can only work at nighttime because the intensity of atmospheric airglow is too low compared with that of sunlight. Besides, the completion of the first Global Navigation Satellite System (GNSS) in USA, Global Positioning System (GPS), made it possible to carry out