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

Sporadic-E (E s ) are electron density inhomogeneities manifested in the ionospheric E region. At midlatitude area, during the daytime, E s is suggested to occur due to reinforced metallic (of meteoric origin) ionization in altitude range ∼95-120 km (Whitehead, 1989). Occasionally, E s becomes denser than the normal E-and F-layer densities, exhibiting a high correlation of occurrence with intense transionospheric signal scintillation (Maeda & Heki, 2014). Consequently, to forecast and draw patterns in the scintillation morphology, the space weather community has extensively investigated the E s structure both theoretically and experimentally. For example, information on the horizontal structures of midlatitude E s has been inferred by using ionosondes (Whitehead, 1972), radars (Miller & Smith, 1975), and rockets (Yamamoto et al., 1998. More so, S. Saito et al. ( 2006) obtained three-dimensional (3-D) structures of E s patches by using the middle and upper atmosphere (MU) radar at Shigaraki, Japan. Wu et al. (2005) deduced occurrence rates and intensities of E s at summertime midlatitudes, by using radio occultation measurements around the globe. Haldoupis (2011) and references therein have discussed an excellent tutorial review that summarizes the results from most of these studies and their affirmation on the E s structure. The literature suggests that the occurrence of E s varies with local time, altitude, latitude, longitude, and seasons, and its existence depends on the tidal wind, the Earth's geomagnetic field, and the level of meteoric depositions (Whitehead, 1989). Even with such rich literature, the E s generating mechanisms are not fully understood, although the wind shear theory driven by zonal winds has widely been accepted for the E s

Ground- and space-based Global Navigation Satellite System (GNSS) receivers can provide three-dimensional (3D) information about the occurrence of equatorial plasma bubbles (EPBs). For this study, we selected March 2014 data (during solar maximum of cycle 24) for the analysis. The timing and the latitudinal dependence of the EPBs occurrence rate are derived by means of the rate of the total electron content (TEC) index (ROTI) data from GNSS receivers in China, whereas vertical profiles of the scintillation index S4 are provided by COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate). The GNSS receivers of the low Earth orbit satellites give information about the occurrence of amplitude scintillations in limb sounding geometry where the focus is on magnetic latitudes from 20° S to 20° N. The occurrence rates of the observed EPB-induced scintillations are generally smaller than those of the EPB-induced ROTI variations. The timing and the latitude dependence of the EPBs occurrence rate agree between the ground-based and spaceborne GNSS data. We find that EPBs occur at 19:00 LT and they are mainly situated above the F2 peak layer which descended from 450 km at 20:00 LT to 300 km at 24:00 LT in the equatorial ionosphere. At the same time, the spaceborne GNSS data also show, for the first time, a high occurrence rate of post-sunset scintillations at 100 km altitude, indicating the coexistence of equatorial sporadic E with EPBs.

In computerized ionospheric tomography (CIT) with ground-based GNSS, the voxels without satellite-receiver ray traversing cannot be reconstructed directly. We present a CIT algorithm based on virtual reference stations (VRSs), called VRS–CIT, to decrease the number of unilluminated voxels and improve the precision of the estimated ionospheric electron density (IED). The VRSs are set at the nodes of grids with a 0.5° × 0.5° resolution in longitude and latitude. We generate the virtual observations with the observations from nearby six or three stations selected according to azimuths and distances. The generation utilizes multi-quadric surface fitting with six stations and triangular linear interpolation with three stations. With the virtual observations added, the IED distribution is reconstructed by the multiplicative algebraic reconstruction technique with the initial values obtained from IRI-2016. The performance of VRS–CIT is examined by using the data from 127 GNSS stations located in 24–46° N and 122–146° E to derive the IED every 30 min. The study focuses on April 29, 2014, with the adaptability of VRS–CIT analyzed by 12 days, evenly distributed around equinoxes and solstices of 2014. The accuracy of the virtual observation is about 1 TECU. Comparing to that derived from CIT with only real observations, the unsolvability of VRS–CIT declined by 4–12% for the whole region, and for the main area, the improvement can be up to 70%. Taking two IED profiles from radio occultation as reference measurements, the mean absolute error (MAE) of IED by VRS–CIT decreases by 6.88% and 8.43%, respectively. Comparing with slant total electron content (STEC) extracted from five additional GNSS stations, the MAE and the root mean square error of the estimated STEC can be reduced up to 17.24% and 33.81%, respectively.

Electrified ionosphere irregularities have extensively been an intriguing subject of research. In the summer hemisphere, at mid-latitudes during nighttime, electrified irregularities are ubiquitous in the F-region, manifested as wave-like electron density perturbations with horizontal wavelengths of several hundred kilometres and periods of 15-60 min (Bowman, 1985(Bowman, , 1990Hunsucker, 1982): typically known as medium-scale traveling ionospheric disturbances (MSTIDs). Recent studies have identified the characteristics of MSTIDs by using two-dimensional (2D) GPS-TEC (total electron content) maps (A. Saito, Fukao, & Miyazaki, 1998), three-dimensional (3-D) computerized ionospheric tomography technique (Ssessanga et al., 2015), and airglow imagers . One of the intriguing aspects of nighttime midlatitude MSTIDs is the northwest-southeast (NW-SE) (northeast-southwest; NE-SW) aligned frontal structures propagating toward equator-westward direction in the Northern (Southern) Hemisphere (Yokoyama, 2014). The Perkins instability could be the most likely mechanism to account for the directional preference of alignment and propagation of MSTID structures (Perkins, 1973). However, theoretical analysis have found that the growth rates of typical MSTIDs cannot be satisfied by a standalone Perkins instability, and have underscored a coupled effect from polarized Es structures (Es-layer instability) coupled to the Perkins instability (

Global ionospheric total electron content (TEC) is generally derived with ground-based Global Navigation Satellite System (GNSS) observations based on mathematical models in a solar-geomagnetic reference frame. However, ground-based observations are not well-distributed. There is a lack of observations over sparsely populated areas and vast oceans, where the accuracy of TEC derivation is reduced. Additionally, the modified dip (modip) latitude is more suitable than geomagnetic latitude for the ionosphere. This paper investigates the improvement of global TEC with multi-source data and modip latitude, and a simulation with International Reference Ionosphere (IRI) model is developed. Compared with using ground-based observations in geomagnetic latitude, the mean improvement was about 10.88% after the addition of space-based observations and the adoption of modip latitude. Nevertheless, the data from JASON-2 satellite altimetry and COSMIC occultation are sparsely-sampled, which makes the IRI TEC a reasonable estimation for the areas without observation. By using multi-source data from ground-based, satellite-based and IRI-produced observations, global TEC was derived in both geomagnetic and modip latitudes for 12 days of four seasons in 2014 under geomagnetic quiet conditions. The average root-mean-square error (RMSE) of the fitting was reduced by 7.02% in modip latitude. The improvement was largest in March and smallest in June.

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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