# Longitudinal Structure in the Altitude of the Sporadic E Observed by COSMIC in Low-Latitudes
## 4 Discussion  
In Section 3, the WN4 structure is observed in the global distributions of the Es height; its local time, seasonal, and latitudinal variations are analyzed over the low latitudes. As is known, the WN4 structure has been found in different thermospheric and ionospheric parameters. An observed WN4 structure can be caused by non-migrating tides, including DW5, DE3, SW6, SE2, and SPW4. Oscillations with periods of 24 h and 12 h are referred to as diurnal (D) and semidiurnal (S) tides, respectively. The notation DWs and DEs are used to describe a westward or eastward propagating diurnal tide with zonal wavenumber \(s\). For semidiurnal tides D will be replaced by S. Stationary planetary waves with zonal wavenumber \(m\) are denoted as SPW\(m\). Within 24 h LT, a wave-4 structure caused by DW5 propagates 90\({}^{\circ}\) to the west, while DE3 propagates 90\({}^{\circ}\) to the east; SW6 displays a phase shift of 180\({}^{\circ}\) to the west, while SE2 exhibits 180\({}^{\circ}\) to the east; and SPW4 shows no phase shift at all [39]. It is generally accepted that the DE3 tide plays the most important role in the formation of the WN4 structure [20]. As shown in Section 3, the seasonal variation, eastward speed, and the symmetrical distribution of the WN4 are actually similar to those of the DE3 tide. Figure 7: LT-Lat variations of amplitudes derived from the FT for the WN4 component at the March equinox (**the top row**), June solstice (**the middle row**), and September equinox (**the bottom row**). Using a coordinate system with the \(x\)-axis horizontal and towards magnetic north, the \(y\)-axis eastward and the \(z\)-axis upward, the vertical ion velocity (\(w_{z}(z)\)) driven by horizontal winds and the electric field is expressed by the following equation [40]:  
\[w_{z}(z)=\frac{R_{i}(z)}{1+R_{i}^{2}(z)}\bigg{[}\frac{E_{z}(z)}{B_{0}(z)}+\frac{ \cos IE_{y}(z)}{B_{0}(z)R_{i}(z)}+\cos IU(z)-\frac{\sin I\cos IV(z)}{R_{i}(z)} \bigg{]} \tag{1}\]  
where \(I\) is the geomagnetic inclination angle, defined as positive (downward) in the Northern Hemisphere; \(B_{0}(z)\) is the total geomagnetic field intensity; \(E_{y}(z)\) and \(E_{z}(z)\) are the zonal and vertical electric fields; \(R_{i}(z)\) is the ratio of the ion-neutral collision frequency (\(v_{i}(z)\)) to the ion gyrofrequency (\(\omega_{i}\)); \(U(z)\) and \(V(z)\) are the components of the zonal and meridional neutral winds. It is widely accepted that the horizontal winds play the dominant role for the formation of the Es layer over the mid-latitudes. The horizontal winds also dominate the Es formation at low latitudes [41; 42; 43], although the electric field also has some influence on the formation or disruption process there [44; 45; 46]. Therefore, the effect of the electric field is ignored at this stage and will be discussed later. The Es layers will be formed where the vertical divergence of the vertical ion velocity (\(\frac{dw_{z}}{dz}\)) reaches the minimal negative value. It can be seen from Equation (1) that the \(\frac{dw_{z}}{dz}\) is controlled by the combined effect of the zonal and meridional wind/windshear, together with the geomagnetic field and the ion-neutral collision frequency. According to the windshear theory, the layer-forming efficiency of the meridional and zonal windshear depends on the height due to the altitude dependences of the ion-neutral collision frequency (\(v_{i}(z)\)). At altitudes below \(\sim\)125 km, where \(v_{i}(z)\gg\omega_{i}\), the zonal windshear mechanism plays the dominant role [47]. Thus, the WN4 structure for the Es altitude is assumed to be caused by the DE3 tide in the zonal wind through the zonal windshear mechanism. This view will be illustrated hereinafter. As mentioned above, the Es layers will be formed where \(\frac{dw_{z}}{dz}\) reaches the minimal negative value.